Maize Anatomy and the Anatomy of a Maze

Commonly known as corn throughout much of North America, maize is a distinctive emblem of the harvest season. It is one of the most economically important crops in the world (the third most important cereal after rice and wheat) and has scads of uses from food to feed to fuel. The story of its domestication serves as a symbol of human ingenuity, and its plasticity in both form and utility is a remarkable example of why plants are so incredible.

The genus Zea is in the grass family (Poaceae) and consists of five species: Z. diploperennis, Z. perennis, Z. luxurians, Z. nicaraguensis, and Z. mays. Maize is the common name of Zea mays subsp. mays, which is one of four Z. mays subspecies and the only domesticated taxon in the genus. All other taxa are commonly and collectively referred to as teosintes.

The domestication of maize, apart from being an impressive feat, has long been a topic of research and a challenging story to tease apart. The current understanding is that maize was first domesticated around 9000 years ago in the Balsas River valley in southern Mexico, the main progenitor being Zea mays subsp. parviglumis. It is astonishing how drastically different in appearance teosintes are from modern day maize, but it also explains why determining the crop wild relative of maize was so difficult.

Teosinte, teosinte-maize hybrid, and maize - photo credit: wikimedia commons

Teosinte, teosinte-maize hybrid, and maize – photo credit: wikimedia commons

Teosintes and maize both have tall central stalks; however, teosintes generally have multiple lateral branches which give them a more shrubby appearance. In teosinte, each of the lateral branches and the central stalk terminate in a cluster of male flowers; female flowers are produced at the nodes along the lateral branches. In maize, male flowers are borne at the top of the central stalk, and lateral branches are replaced by short stems that terminate in female flowers. This is where the ears develop.

Ears – or clusters of fruits – are blatantly different between teosintes and maize. To start with, teosinte produces a mere 5 to 12 fruits along a short, narrow cob (flower stalk). The fruits are angular and surrounded in a hard casing. Maize cobs are considerably larger both in length and girth and are covered in as many as 500 or more fruits (or kernels), which are generally more rounded and have a softer casing. They also remain on the cob when they are ripe, compared to teosinte ears, which shatter.

Evolutionary biologist, Sean B. Carroll, writes in a New York Times article about the amazing task of “transform[ing] a grass with many inconvenient, unwanted features into a high-yielding, easily harvested food crop.” These “early cultivators had to notice among their stands of plants variants in which the nutritious kernels were at least partially exposed, or whose ears held together better, or that had more rows of kernels, and they had to selectively breed them.” Carroll explains that this “initial domestication process which produced the basic maize form” would have taken several hundred to a few thousand years. The maize that we know and love today is a much different plant than its ancestors, and it is still undergoing regular selection for traits that we find desirable.

Female inflorescence (or "ear") of Zea mays subsp. mays - photo credit: wikimedia commons

Female inflorescence (or “ear”) of Zea mays subsp. mays – photo credit: wikimedia commons

To better understand and appreciate this process, it helps to have a basic grasp of maize anatomy. Maize is an impressive grass in that it regularly reaches from 6 to 10 feet tall and sometimes much taller. It is shallow rooted, but is held up by prop or brace roots – adventitious roots that emerge near the base of the main stalk. The stalk is divided into sections called internodes, and at each node a leaf forms. Leaf sheaths wrap around the entirety of the stalk, and leaf blades are long, broad, and alternately arranged. Each leaf has a prominent midrib. The stalk terminates in a many-branched inflorescence called a tassel.

Maize Anatomy 101 - image credit: Canadian Goverment

Maize Anatomy 101 – image credit: Canadian Government

Maize is monoecious, which means that it has separate male and female flowers that occur on the same plant. The tassel is where the male flowers are located. A series of spikelets occur along both the central branch and the lateral branches of the tassel. A spikelet consists of a pair of bracts called glumes, upper and lower lemmas and paleas (which are also bracts), and two simple florets composed of prominent stamens. The tassel produces and sheds tens of thousands of pollen grains which are dispersed by wind and gravity to the female inflorescences below and to neighboring plants.

Female inflorescences (ears) occur at the top of short stems that originate from leaf axils in the midsection of the stalk. Leaves that develop along this reduced stem wrap around the ears forming the husk. Spikelets form in rows along the flower stalk (cob) within the husk. The florets of these spikelets produce long styles that extend beyond the top of the husk. This cluster of styles is known as the silk. When pollen grains land on silk stigmas, pollen tubes grow down the entire length of the silks to reach the embryo sac. Successful fertilization produces a kernel.

The kernel – or fruit – is known botanically as a caryopsis, which is the standard fruit type of the grass family. Because the fruit wall and seed are fused together so tightly, maize kernels are commonly referred to as seeds. The entire plant can be used to produce feed for animals, but it is the kernel that is generally consumed (in innumerable ways) by humans.

There is so much more to be said about maize. It’s a lot to take in. Rather than delve too much further at this point, let’s explore one of the other ways that maize is used by humans to create something that has become another feature of the fall season – the corn maze.

Entering the corn maze at The Farmstead in Meridian, Idaho

Exploring the corn maze at The Farmstead in Meridian, Idaho

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Bat Pollinated Flowers of a Mexican Columnar Cactus

Pollination syndromes – suites of floral traits used to determine potential pollinators and routes of pollination – have been informative in studying plant-pollinator interactions, but are generally too simplistic to tell the full story. Most flowering plants are generalists when it comes to pollinators, whereas pollination syndromes imply specialization. Not all pollinators are created equal though, and some may be more effective at pollinating particular plants than others. In fact, occasionally pollination syndromes ring true and a predicted plant-pollinator combination turns out to be the most effective and reliable interaction.

According to a study published in American Journal of Botany by Ibarra-Cerdeña, et al., Stenocereus queretaroensis, a species of columnar cactus endemic to western Mexico, adheres to this scenario. Stenocereus is a genus in a group of columnar and tree-like cacti called the Pachycereeae tribe. Cactus in this group are generally bat pollinated; however, their flowers are typically visited by various species of birds and insects as well, and in some cases, bats are not the primary pollinator. In their introduction, the authors note that specialization appears to be more common in tropical latitudes, and chiropterophilic (bat pollinated) columnar cacti that occur in temperate regions can be comparatively more generalized. This is because “extratropical chiropterophilic cacti appear to be faced with unpredictable seasonal year-to-year variation in pollinators,” while “cacti in tropical regions” experience “highly reliable seasonal availability of nectar-feeding bats, thereby leading to a temporally stable pollination system.”

Stenocereus queretaroensis is a massive cactus, reaching up to ten meters tall. Several vertical stems rise from a short, stocky, central trunk. Each stem has up to eight distinctive ribs and averages around 15 centimeters in diameter. Groupings of white to grey spines up to four centimeters long appear along the ribs. Flowers are light-colored, around 10 to 14 centimeters in length, and occur along the upper half of the stems, extended well beyond the spines. Flowers open at night – producing abundant nectar – and close by the afternoon the following day. Floral characteristics led the authors of this study to predict bats to be the main pollinator, and they set up a series of experiments to test this.

Stenocereus queretaroensis - photo credit: wikimedia commons

Stenocereus queretaroensis – photo credit: wikimedia commons

Part of their experiment consisted of five treatments involving 130 flowers on 75 plants. One group of flowers was bagged and allowed to self-pollinate naturally, while another group was bagged and self-pollinated manually. A third group was left exposed during the night but bagged in the morning, while a fourth group was bagged during the night and exposed during the daytime. The final group was left alone. For each of these five treatments, aborted flowers and mature fruits were counted and seed set was determined. Nectar samples were taken from a separate group of flowers at two hour intervals from 8:00 PM to 8:00 AM, after which no nectar was produced. A camera was also used to document floral visits. Visits were deemed “legitimate” when the “visitor’s body came in contact with anthers and/or stigma” and “illegitimate” when “no contact with anthers or stigma” was made.

The researchers found S. queretaroensis to be “incapable of self-pollination,” as no fruit set occurred for the first two treatments. The control group and the nocturnally exposed group had nearly identical results, producing significantly more fruits with greater seed set compared to the nocturnally bagged group. During the day, flowers were visited by four species of birds (two hummingbirds, a woodpecker, and an oriole) and several species of bees (mainly honey bees). During the night, apart from illegitimate visits from a nectar robbing hawkmoth, one species of bat was the dominant floral visitor, and the majority (93.8%) of the visits were legitimate. This bat species was Leptonycteris curasoae, the southern long-nosed bat.

Leptonycteris curasoae - photo credit: wikimedia commons

Leptonycteris curasoae – photo credit: wikimedia commons

The abundance of nectar-feeding bats was monitored in the study area over a four year period, and L. curasoae was by far the most abundant species throughout the study period. Nectar produced in the flowers of S. queretaroensis was at its maximum around midnight, which seemed to correlate with observations of bat visits. Even though daytime visitors appeared to contribute to fruit and seed set, the nocturnal treatment produced significantly more fruit with significantly higher seed set, suggesting that bats are the more efficient pollinator. Insects visiting during the daytime, when nectar was decreasingly available, were most likely robbing pollen.

The authors acknowledge that for most plant species, “a wide array of taxonomically diverse fauna such as insects, birds, and mammals usually serve as potential pollinators,” and that “generalized pollination systems are more frequent than specialized ones.” However, in this case, “a close association between L. curasoae and S. queretaroensis [suggests] that the chiropterophilic syndrome is still a useful model.”

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Bats As Pollinators – An Introduction to Chiropterophily

Most plants that rely on animals to assist in pollination look to insects. In general, insects are abundant, easy to please, and efficient at transferring pollen. Because insect pollination is such a common scenario, it is easy to overlook pollination that is carried out by vertebrates. Birds are the most prominent pollinator among vertebrates, but mammals participate, too. The most common mammal pollinator is the bat.

About a fifth of all mammal species on the planet are bats, with species estimates numbering in the 1200-1300 range. Bats are the only mammals that can truly fly. They are not blind, nor are they flying rodents, and they are not going to suck your blood (except in very rare cases!). Most bats eat insects, but a small, significant group of them are nectarivorous. Their main food source is the nectar produced within flowers. In the process of feeding, these bats pollinate plants.

Out of 18 families in the order Chiroptera, only two include species with morphologies that set them apart as nectar-feeders. The family Pteropodidae, known commonly as Old World fruit bats or flying foxes, occurs in tropical and subtropical regions of Africa, Asia, Australia, Papa New Guinea, and the Pacific Islands. The family Phyllostomidae, known commonly as American leaf-nosed bats, occurs in tropical and subtropical regions of the Americas. For simplicity’s sake, the former are referred to as Old World bats, and the latter as New World bats. While both groups are similar in that they consist of species that feed on nectar, they are only distantly related, and thus the nectar feeding species in these families have distinct behavioral and morphological differences.

Grey headed flying fox photo credit: wikimedia commons

Grey headed flying fox (Pteropus poliocephalus), a floral visiting bat from Australia (photo credit: wikimedia commons)

More than 500 species of plants, spanning 67 plant families, are pollinated by bats. This pollination syndrome is known as chiropterophily. In general, flowers that use this approach tend to be white or dull in color, open at night, rich with nectar, and musty or rotten smelling. They are generally tubular, cup shaped, or otherwise radially symmetrical and are often suspended atop tall stalks or prominently located on branches or trunks. In a review published in Annals of Botany, Theodore Fleming, et al. state “flower placement away from foliage and nocturnal anthesis [blooming] are the unifying features of the bat pollination syndrome,” while all other characteristics are highly variable among species. The family Fabaceae contains the highest number of bat-pollinated genera. Cactaceae, Malvaceae, and Bignoniaceae follow closely behind.

The characteristics of bat pollinated flowers vary widely partly because the bats that visit them are so diverse. Between the two bat families there are similarities in their nectar-feeding species, including an elongated rostrum, teeth that are smaller in number and size, and a long tongue with hair-like projections on the tip. Apart from that, New World bats are much smaller than Old World bats, and their rostrums and tongues are much longer relative to the size of their bodies. New World bats have the ability to hover in front of flowers, while Old World bats land on flowers to feed. Old World bats do not have the ability to use echolocation to spot flowers, while New World bats do. Fleming, et al. conclude, “because of these differences, we might expect plants visited by specialized nectar-feeding [New World bats] to produce smaller flowers with smaller nectar volumes per flower than those visited by their [Old World bat] counterparts.”

Pollination by bats is a relatively new phenomenon, evolutionarily speaking. Flowers that are currently pollinated by bats most likely evolved from flowers that were once pollinated by insects. Some may have evolved from flowers that were previously bird pollinated. The question is, why adopt this strategy? Flowers that are bat pollinated are “expensive” to make. They are typically much bigger than insect pollinated flowers, and they contain large amounts of pollen and abundant, nutrient-rich nectar. Due to resource constraints, many plants are restricted from developing such flowers, but those that do may find themselves at an advantage with bats as their pollinator. For one, hairy bat bodies collect profuse numbers of pollen grains, which are widely distributed as they visit numerous flowers throughout the night. In this way, bats can be excellent outcrossers. They also travel long distances, which means they can move pollen from one population of plants to an otherwise isolated neighboring population. This serves to maintain healthy genetic diversity among populations, something that is increasingly important as plant populations become fragmented due to human activity.

Pollinating bats are also economically important to humans, as several plants that are harvested for their fruits, fibers, or timber rely on bats for pollination. For example, bat pollinated Eucalyptus species are felled for timber in Australia, and the fruits of Durio zibethinus in Southeast Asia form after flowers are first pollinated by bats. Also, the wild relatives of bananas (Musa spp.) are bat pollinated, as is the plant used for making tequila (Agave tequilana).

Durio sp. (photo credit: wikimedia commons)

The flowers of durian (Durio sp.), trees native to Southeast Asia, are pollinated by bats (photo credit: wikimedia commons)

There is still much to learn about nectarivorous bats and the flowers they visit. It is clear that hundreds of species are using bats to move their pollen, but the process of adopting this strategy and the advantages of doing so remain ripe for discovery. Each bat-plant relationship has its own story to tell. For now, here is a fun video about the bat that pollinates Agave tequilana:

Attract Pollinators, Grow More Food

It seems obvious to say that on farms that rely on insect pollinators for crops to set fruit, having more pollinators around can lead to higher yields. Beyond that, there are questions to consider. How many pollinators and which ones? To what extent can yields be increased? How does the size and location of the farm come into play? Etc. Thanks to a recent study, one that Science News appropriately referred to as “massive,” some of these questions are being addressed, offering compelling evidence that yields grow dramatically simply by increasing and diversifying pollinator populations.

It is also stating the obvious to say that some farms are more productive than others. The difference between a high yield farm and a low yield farm in a given crop system is referred to as a yield gap. Yield gaps are the result of a combination of factors, including soil health, climate, water availability, and management. For crops that depend on insects for pollination, reduced numbers of pollinators can contribute to yield gaps. This five year study by Lucas A. Garibaldi, et al., pubished in a January 2016 issue of Science, involving 344 fields and 33 different crops on farms located in Africa, Asia, and Latin America demonstrates the importance of managing for pollinator abundance and diversity.

The study locations, which ranged from 0.1 hectare to 327.2 hecatares, were separated into large and small farms. Small farms were considered 2 ha and under. In the developing world, more than 2 billion people rely on farms of this size, and many of these farms have low yields. In this study, low yielding farms on average had yields that were a mere 47% of high yielding farms. Researchers wanted to know to what degree enhancing pollinator density and diversity could help increase yields and close this yield gap.

By performing coordinated experiments for five years on farms all over the world and by using a standardized sampling protocol, the researchers were able to determine that higher pollinator densities could close the yield gap on small farms by 24%. For larger farms, such yield increases were seen only when there was both higher pollinator density and diversity. Honeybees were found to be the dominant pollinator in larger fields, and having additional pollinator species present helped to enhance yields.

These results suggest that, as the authors state, “there are large opportunities to increase flower-visitor densities and yields” on low yielding farms to better match the levels of “the best farms.” Poor performing farms can be improved simply by managing for increased pollinator populations. The authors advise that such farms employ “a combination of practices,” such as “sowing flower strips and planting hedgerows, providing nesting resources, [practicing] more targeted use of pesticides, and/or [restoring] semi-natural and natural areas adjacent to crops.” The authors conclude that this case study offers evidence that “ecological intensification [improving agriculture by enhancing ecological functions and biodiversity] can create mutually beneficial scenarios between biodiversity and crop yields worldwide.”

photo credit: wikimedia commons

photo credit: wikimedia commons

A study like this, while aimed at improving crop yields in developing nations, should be viewed as evidence for the importance of protecting and strengthening pollinator populations throughout the world. Modern, industrial farms that plant monocultures from one edge of the field to the other and that include little or no natural area – or weedy, overgrown area for that matter – are helping to place pollinator populations in peril. In this study, after considering numerous covariables, the authors concluded that, “among all the variables we tested, flower-visitor density was the most important predictor of crop yield.”

Back to stating the obvious, if pollinators aren’t present yields decline, and as far as I’m aware, we don’t have a suitable replacement for what nature does best.

This study is available to read free of charge at ResearchGate. If you are interested in improving pollinator habitat in your neighborhood, check out these past Awkward Botany posts: Planting for Pollinators, Ground Nesting Bees in the Garden, and Hellstrip Pollinator Garden.

Year of Pollination: Botanical Terms for Pollination, part two

“The stage is set for reproduction when, by one means or another, compatible pollen comes to rest on a flower’s stigma. Of the two cells within a pollen grain, one is destined to grow into a long tube, a pollen tube, that penetrates the pistil’s tissues in search of a microscopic opening in one of the ovules, located in the ovary. … The second of a pollen grain’s cells divides to become two sperm that move through the pollen tube and enter the ovule.” – Brian Capon, Botany for Gardeners

“Once pollination occurs, the next step is fertilization. Pollen deposited on the sticky stigma generates a fine pollen tube that conveys the sperm through the style to the ovary, where the ovules, or eggs, have developed. After fertilization, the rest of the flower parts wither and are shed as the ovary swells with seed development.” – Rick Imes, The Practical Botanist

Pollination tells the story of a pollen grain leaving an anther by some means – be it wind, water, or animal – and finding itself deposited atop a stigma. As long as the pollen and stigma are compatible, the sex act proceeds. In other words, the pollen grain germinates. One of the pollen grain’s cells – the tube nucleus – grows down the length of the style, forming a tube through which two sperm nuclei can travel. The sperm nuclei enter the ovary and then, by way of a micropyle, enter an ovule. Inside the ovule is the female gametophyte (also referred to as the embryo sac). One sperm nucleus unites with the egg nucleus to form a zygote. The remaining sperm nucleus unites with two polar nuclei to form a triploid cell which becomes the endosperm. The sex act is complete.

The illustration on the left includes the cross-section of a pistil showing the inside the ovary where pollen tubes have made their way to the ovules. The illustration on the right shows pollen grains germinating on a stigma and their pollen tubes begining to work their way down the style. (photo credit: wikimedia commons)

The illustration on the left includes the cross section of a pistil showing the inside of the ovary where pollen tubes have made their way to the ovules. The illustration on the right shows pollen grains germinating on a stigma and pollen tubes as they work their way down the style. (image credit: wikimedia commons)

The zygote divides by mitosis to become an embryo. The endosperm nourishes the development of the embryo. The ovule matures into a seed, and the ovary develops into a fruit. During this process, the remaining parts of the flower wither and fall away. In some cases, certain flower parts remain attached to the fruit or become part of the fruit. The flesh of an apple, for example, is formed from the carpels and the receptacle (the thickened end of a flower stem – peduncle – to which the parts of a flower are attached).

As the seed matures, the endosperm is either used up or persists to help nourish the embryonic plant after germination. Mature seeds that are abundant in endosperm are called albuminous. Examples include wheat, corn, and other grasses and grains. Mature seeds with endosperm that is either highly reduced or absent are called exalbuminous – beans and peas, for example. Certain species – like orchids – do not produce endosperm at all.

The cross section of a corn kernel showing the endosperm and the embryo (image credit: Encyclopedia Britannica Kids)

The cross section of a corn kernel showing the endosperm and the embryo (image credit: Encyclopedia Britannica Kids)

It is fascinating to consider that virtually every seed we encounter is the result of a single pollen grain making its way from an anther to a stigma, growing a narrow tube down a style, and fertilizing a single ovule. [Of course there are always exceptions. Some plants can produce seeds asexually. See apomixis.] Think of this the next time you are eating corn on the cob or popcorn – each kernel is a single seed – or slicing open a pomegranate to reveal the hundreds of juicy seeds inside. Or better yet, when you are eating the flesh or drinking the milk of a coconut. You are enjoying the solid and liquid endosperm of one very large seed.

Much more can be said about pollination and the events surrounding it, but we’ll save that for future posts. The “Year of Pollination” may be coming to an end, but there remains much to discover and report concerning the subject. For now, here is a fun video to help us review what we’ve learned so far:

 

Also, take a look at this TED talk: The Hidden Beauty of Pollination by Louie Schwartzberg

And finally, just as the “Year of Pollination” was coming to an end I was introduced to a superb blog called The Amateur Anthecologist. Not only did it teach me that “anthecology” is a term synonymous with pollination biology, it has a great series of posts called “A Year of Pollinators” that showcases photographs and information that the author has collected for various groups of pollinators over the past year. The series includes posts about Bees, Wasps, Moths and ButterfliesFlies, and Beetles, Bugs, and Spiders.

Year of Pollination: Botanical Terms for Pollination, part one

When I began this series of posts, I didn’t have a clear vision of what it would be. I had a budding interest in pollination biology and was anxious to learn all that I could. I figured that calling 2015 the “Year of Pollination” and writing a bunch of pollination-themed posts would help me do that. And it has. However, now that the year is coming to a close, I realize that I neglected to start at the beginning. Typical me.

What is pollination? Why does it matter? The answers to these questions seemed pretty obvious; so obvious, in fact, that I didn’t even think to ask them. That being said, for these last two “Year of Pollination” posts (and the final posts of the year), I am going back to the basics by defining pollination and exploring some of the terms associated with it. One thing is certain, there is still much to be discovered in the field of pollination biology. Making those discoveries starts with a solid understanding of the basics.

Pollination simply defined is the transfer of pollen from an anther to a stigma or – in gymnosperms – from a male cone to a female cone. Essentially, it is one aspect of plant sex, albeit a very important one. Sexual reproduction is one way that plants multiply. Many plants can also reproduce asexually. Asexual reproduction typically requires less energy and resources – no need for flowers, pollen, nectar, seeds, fruit, etc. – and can be accomplished by a single individual without any outside help; however, there is no gene mixing (asexually reproduced offspring are clones) and dispersal is limited (consider the “runners” on a strawberry plant producing plantlets adjacent to the mother plant).

To simplify things, we will consider only pollination that occurs among angiosperms (flowering plants); pollination/plant sex in gymnosperms will be discussed at another time. Despite angiosperms being the youngest group of plants evolutionarily speaking, it is the largest group and thus the type we encounter most.

A flower is a modified shoot and the reproductive structure of a flowering plant. Flowers are made up of a number of parts, the two most important being the reproductive organs. The androecium is a collective term for the stamens (what we consider the male sex organs). A stamen is composed of a filament (or stalk) topped with an anther – where pollen (plant sperm) is produced. The gynoecium is the collective term for the pistil (what we consider the female sex organ). This organ is also referred to as a carpel or carpels; this quick guide helps sort that out. A pistil consists of the ovary (which contains the ovules), and a style (or stalk) topped with a stigma – where pollen is deposited. In some cases, flowers have both male and female reproductive organs. In other cases, they have one or the other.

photo credit: wikimedia commons

photo credit: wikimedia commons

When pollen is moved from an anther of one plant to a stigma of another plant, cross-pollination has occurred. When pollen is moved from an anther of one plant to a stigma of the same plant, self-pollination has occurred. Cross-pollination allows for gene transfer, and thus novel genotypes. Self-pollination is akin to asexual production in that offspring are practically identical to the parent. However, where pollinators are limited or where plant populations are small and there is little chance for cross-pollination, self-pollination enables reproduction.

Many species of plants are unable to self-pollinate. In fact, plants have evolved strategies to ensure cross-pollination. In some cases, the stamens and pistils mature at different times so that when pollen is released the stigmas are not ready to receive it or, conversely, the stigmas are receptive before the pollen has been released. In other cases, stigmas are able to recognize their own pollen and will reject it or inhibit it from germinating. Other strategies include producing flowers with stamens and pistils that differ dramatically in size so as to discourage pollen transfer, producing separate male and female flowers on the same plant (monoecy), and producing separate male and female flowers on different plants (dioecy).

As stated earlier, the essence of pollination is getting the pollen from the anthers to the stigmas. Reproduction is an expensive process, so ensuring that this sex act takes place is vital. This is the reason why flowers are often showy, colorful, and fragrant. However, many plants rely on the wind to aid them in pollination (anemophily), and so their flowers are small, inconspicuous, and lack certain parts. They produce massive amounts of tiny, light-weight pollen grains, many of which never reach their intended destination. Grasses, rushes, sedges, and reeds are pollinated this way, as well as many trees (elms, oaks, birches, etc.) Some aquatic plants transport their pollen from anther to stigma via water (hydrophily), and their flowers are also simple, diminutive, and produce loads of pollen.

Inforescence of big bluestem (Andropogon gerardii), a wind pollinated plant - pohto credit: wikimedia commons

Inflorescence of big bluestem (Andropogon gerardii), a wind pollinated plant – photo credit: wikimedia commons

Plants that employ animals as pollinators tend to have flowers that we find the most attractive and interesting. They come in all shapes, sizes, and colors and are anywhere from odorless to highly fragrant. Odors vary from sweet to bitter to foul. Many flowers offer nectar as a reward for a pollinator’s service. The nectar is produced in special glands called nectaries deep within the flowers, inviting pollinators to enter the flower where they can be dusted with pollen. The reward is often advertised using nectar guides – patterns of darker colors inside the corolla that direct pollinators towards the nectar. Some of these nectar guides are composed of pigments that reflect the sun’s ultraviolet light – they are invisible to humans but are a sight to behold for many insects.

In part two, we will learn what happens once the pollen has reached the stigma – post-pollination, in other words. But first, a little more about pollen. The term pollen actually refers to a collection of pollen grains. Here is how Michael Allaby defines “pollen grain” in his book The Dictionary of Science for Gardeners: “In seed plants, a structure produced in a microsporangium that contains one tube nucleus and two sperm nuclei, all of them haploid, enclosed by an inner wall rich in cellulose and a very tough outer wall made mainly from sporopollenin. A pollen grain is a gametophyte.”

A pollen grain’s tough outer wall is called exine, and this is what Allaby has to say about that: “It resists decay, and the overall shape of the grain and its surface markings are characteristic for a plant family, sometimes for a genus or even a species. Study of pollen grains preserved in sedimentary deposits, called palynology or pollen analysis, makes it possible to reconstruct past plant communities and, therefore, environments.”

Scanning electron microscope image of pollen grains from narrowleaf evening primrose (Oenothera fruticosa) - photo credit: wikimedia commons

Scanning electron microscope image of pollen grains from narrowleaf evening primrose (Oenothera fruticosa) – photo credit: wikimedia commons

Year of Pollination: Scarlet Gilia and Its Pollinators

Flowers that are visited and/or pollinated by hummingbirds typically fit the following description: petals are brightly colored, often red; petals are fused to form a long, narrow tube; a “landing pad” is absent; abundant nectar is produced deep within the flower; and fragrance is weak or nonexistent. Scarlet gilia (Ipomopsis aggregata) is a typical example of such a flower, and hummingbirds are indeed among its most common visitors. But there is so much more to the story.

Scarlet gilia (also commonly known as skyrocket) is a wildflower in the phlox family (Polemoniaceae) that occurs in many parts of western North America. It is considered a biennial or short-lived perennial. It spends the first year or so of its life as a compact rosette of fern-like leaves. Later it sends up a branched, flowering stem that can reach 5 feet tall or more. The flowers are slender, trumpet-shaped, and composed of five fused petals that flare outward creating five prominent, pointed lobes. They are self-incompatible and require a pollinator in order to set seed. The stamens of an individual flower produce mature pollen before the stigma of that flower is ready to receive it – this is called protandry and is one mechanism of self-incompatibility.

The rosette of scarlet gilia (Ipomopsis aggregata)

The rosette of scarlet gilia (Ipomopsis aggregata)

The flowering period of scarlet gilia can last several months. Depending on the location, it can begin in mid-summer and continue through the fall. During this period, it produces dozens of flowers. It is also at this time that it runs the risk of being browsed by elk, mule deer, and other animals. This doesn’t necessarily set it back though, as it has the potential to respond by producing additional flowering stalks and more flowers. Its flowers are visited by a variety of pollinators including bumblebees, hawkmoths, butterflies, syrphid flies, solitary bees, and of course, hummingbirds. But hummingbirds, in many parts of scarlet gilia’s range are migratory, and that’s where things get interesting.

Early flowers of scarlet gilia are usually red. As the season progresses, flowers slowly shift from red to pink. In some cases, they lose all pigmentation and become white. In the early 1980’s, pollination biologists Ken Paige and Thomas Whitman set out to determine the reason for this shift in flower color. They spent three years observing a population of scarlet gilia on Fern Mountain near Flagstaff, Arizona. They noted that the change in flower color corresponded with the migration of hummingbirds and that the now lighter colored flowers continued to be pollinated by hawkmoths until the end of the flowering season.

ipomopsis aggregata

A series of experiments and observations led them to conclude that hummingbirds prefer darker colored flowers and hawkmoths prefer lighter colored flowers. By shifting to a lighter flower color, scarlet gilia appeared to be taking advantage of remaining pollinators after hummingbirds had migrated. They also concluded that the color change was not the cause of hummingbird migration since other flowers with nectar-rich, red, tubular flowers (specifically Penstemon barbatus) remained available in the area throughout their migration. It was also noted that the flowers of scarlet gilia shifted the timing of nectar production, presumably to better match the behavior of hawkmoths which are more active in the evenings.

No plants were observed shifting from light colored flowers to dark colored flowers, which further supported their conclusion. They also compared the population they studied to populations that do not lose their hummingbird pollinator and noted that when hummingbirds remain, the flowers of scarlet gilia don’t change color.

Scarlet gilia (Ipomopsis aggregata) with white flowers

Scarlet gilia (Ipomopsis aggregata) with white flowers

But just how effective are hummingbirds as pollinators of scarlet gilia? A seperate study carried out by a different group of researchers determined that, while hummingbirds were “the most common floral visitor,” long-tongued bumblebees were the more effective pollinator when it came to pollen deposition and seed set. The study involved observations of a scarlet gilia population in Colorado over a 5 year period. Considering how well the floral traits of scarlet gilia match up with the hummingbird pollination syndrome, it is surprising to learn that long-tongued bumblebees are comparatively more effective at pollinating them.

This study provides further evidence against strict adherence to pollination syndromes and the most effective pollinator principle, both of which imply specialized plant-pollinator interactions. (I wrote about these topics here, here, and here in earlier Year of Pollination posts.) In their discussion, the authors propose two possible explanations as to why scarlet gilia, despite its phenotypic floral traits, does not appear to be specialized. One explanation is that “natural selection favors a specialized [floral] morphology that excludes all but a single type of visitor, but there are constraints on achieving this outcome.” Perhaps the pollinators aren’t cooperating; their opportunism is leading them to “exploit flowers on which they can realize an energetic profit, even if they do not mechanically ‘fit’ very well.” The “sensory abilities” of the pollinators may be “broadly tuned,” making it difficult for plants to develop flowers with “private signals detectable only by specific types of pollinators.”

The second explanation proposed by the authors is that “selection favors some degree of floral generalization, but that flowers can retain features that adapt them to a particular type of pollinator in spite of generalization.” In the case of scarlet gilia, specialization could be detrimental because after they send up their flower stalks, they are doomed to die. This gives them only one season to set seed, and if hummingbirds are either not available that year or only available in limited numbers, a scarlet gilia population can lose the opportunity to reproduce. As the authors put it, “the fact that individual plants enjoy only a single season of reproduction, suggests the value of ‘backup’ pollinators.” This may also explain why flower color shifts in order to take full advantage of hawkmoth pollination after hummingbirds are gone.

Scarlet gilia is not only a beautiful and widespread wildflower, but also a plant with a very interesting story. Follow the links below to learn more about this fascinating plant:

Year of Pollination: Hand Pollinating Cucurbits

Because of their large, open, unisexual flowers, plants in the gourd family are perfect for practicing hand pollination. There are several species in this family that are commonly grown in gardens, and all can be hand pollinated. Hand pollination of cucurbits is most often done when there are problems with pollination (lack of pollinators, etc.) or for seed saving purposes (i.e. to ensure that a variety breeds true). It can also be done just for fun, and that’s mostly what this post is about.

But first, if your goal is to save seeds and maintain the integrity of the varieties you are growing, there are a few things to keep in mind. Cucumbers, melons, and watermelons are all different species (Cucumis sativus, Cucumis melo, and Citrullus lanatus respectively), so you won’t have to worry about crosses between these crops. You will, however, have to worry about crosses between different varieties within individual species. So, for example, if you are growing multiple varieties of cucumbers – or if your close neighbors are also growing cucumbers – you should hand pollinate. Summer squash, winter squash, pumpkins, and some gourds are members of at least four species in the genus Cucurbita (C. pepo, C. maxima, C. mixta, and C. moschata). There is a possibility of hybridization between some of these species as well as between varieties within the same species, so precautions should definitely be taken when saving seeds for these crops. This can mean, along with hand pollination, placing bags over flowers so that bees are unable to bring in pollen from “the wrong” plants.

There are plenty of great resources about saving seeds that offer much more detail than I have gone into here, one of which is a book by Marc Rogers called Saving Seeds. Consult such resources if you would like to try your hand at seed saving. It’s easier than you might think, and it’s very rewarding.

Regardless why you are hand pollinating your cucurbits, the first step in the process is differentiating a male flower from a female flower. This is simple. Female flowers in the family Cucurbitaceae have inferior ovaries, meaning that the ovary sits below the area where the petals and other flower parts are attached. The ovaries are quite pronounced and resemble a miniature fruit. The male flowers lack ovaries, so instead are simply attached to a slender stem. You can also observe the sex organs themselves – male flowers have stamens, female flowers have carpels. Male and female flowers may also be located on different areas of the plant and may open at different times of the day. All that being said, the most obvious indication is the “mini-fruit” at the base of the flower or lack thereof.

Cucurbit flowers: male (top) and female (bottom) - photo credit: wikimedia commons

Cucurbit flowers: male (top two photos) and female (bottom two photos) – image credit: wikimedia commons

Once you have identified your flowers, you have a limited amount of time to hand pollinate them. It’s best to find flowers that are just starting to open, as the female flowers may only be receptive for as little as 24 hours. You can use a cotton swab to gather pollen from the male flower, or you can simply pluck the flower from the plant, remove the petals, and touch the pollen-loaded anthers to the stigmas of a female flower. Either way, you must get the pollen from the male parts of a flower to the female parts of a flower as that is the essence of pollination. Simply put, it’s plant sex. Play some soft jazz while you do it if you want to.

A honeybee in a squash flower

A female squash flower with honeybee inside

A honeybee covered in pollen drinking the nectar of a female squash flower

Honeybee covered in pollen drinking the nectar of a female squash flower

As with saving seeds, there are a lot of resources out there explaining the details of hand pollinating cucurbit flowers, including this guide from Missouri Botanical Garden and the following You Tube Video.

 

While we are on the subject of cucurbit flowers, it should be noted that squash flowers are edible and can be prepared in a variety of ways, as described in this post at The Kitchn. Just another reason to be impressed by this amazing group of plants.

Year of Pollination: Bumblebees and Climate Change

Bumblebees, generally speaking, are having a rough time. In a world increasingly dominated by humans, some bumblebee species continue to thrive while many others are seriously struggling. Several are nearing extinction. A recent study involving 67 species of European and North American bumblebees concluded that climate change is having a major impact. Bumblebees do not appear to be migrating north in response to warming climates – a hesitation that could spell disaster.

There are over 250 species of bumblebees worldwide (46 are found in North America north of Mexico). Unlike other bees, whose diversity is greatest in Mediterranean climates, bumblebee diversity is highest in cool, temperate climates and montane regions. The majority of bumblebee species are native to the Northern Hemisphere; a few species are native to South America, and a handful of species from Europe have been introduced to New Zealand and Tasmania. Some species of bumblebees, such as the polar bumble bee (Bombus polaris) and the forest bumble bee (Bombus sylvicola), can be found in extreme cold climates and are among a select group of pollinators found in such areas.

The field guide, Bumble Bees of North America, by Paul Williams, et al. provides this description:

“Bumble bees are very hairy bees with combinations of contrasting bright colors, mostly black and yellow, sometimes with various combinations of red or white. They have two pairs of wings that are usually folded back over the abdomen while they are foraging on flowers, or hooked together as a single unit when in flight. Bumble bees also have slender elbowed antennae, and females of the pollen-collecting species have the hind tibia expanded, slightly concave, and fringed with long hairs to form a pollen basket or corbicula.”

Most bee species are solitary insects; bumblebees, like honeybees, are social insects. Unlike honeybees, bumblebee colonies begin with a new queen each year. New queens, after mating in late summer, overwinter in a protected area and emerge in the spring. They then search for food and a nesting site. Suitable nests include abandoned rodent dens,  the bases of bunchgrasses, hollow logs, and human-made structures. They build up a colony of workers which maintain the nest and forage for food and other resources. As the season comes to a close, the queen produces males and new queens. The new queens mate, go into hibernation, and the rest of the bumblebee colony dies off.

Brown-belted Bumblebee (Bombus griseocollis) - photo credit: wikimedia commons

Brown-belted Bumblebee (Bombus griseocollis) – photo credit: wikimedia commons

Bumblebees face numerous threats, both natural and human-caused. Despite their defensive sting, they are regularly eaten or attacked by various mammals, birds, and invertebrates. They are also host to a variety of pests, parasites, and pathogens, some of which have been introduced or exacerbated by human activities. The commercial bee industry is particularly at fault for the spread of certain maladies. Other major threats include loss of habitat and excessive and/or poorly timed use of insecticides. One looming threat that new research suggests is especially concerning is climate change.

A group of researchers from various institutions looked at the historical ranges of 67 species of bumblebees in Europe and North America over a 110 year period. They “measured differences in species’ northern and southern range limits, the warmest or coolest temperatures occupied, and their mean elevations in three periods relative to a baseline period.” They found that on both continents bumblebees are not tracking climate change by expanding their northern range limits and that their southern range limits are shrinking. They also observed that within the southern range limits, some bumblebee species have retreated to higher elevations. They investigated land use changes and pesticide applications (in the US only) to determine the effect they had on the results. While these things certainly affect populations on an individual level, climate change was determined to be the most important factor that lead to nearly universal range contractions of the bumblebees in this study.

The question then is why are they not tracking changing climates the same way that many other species of plants and animals have already been observed doing? Bumblebees evolved in cooler climates, so shrinking southern range limits is not as surprising as the bumblebees’ delay in moving north. Many factors may be contributing to this phenomenon including lack of specialized habitats beyond their historical ranges, daylength differences, and population dynamics. The researchers call for further investigation in order to better evaluate this observed “range compression.” They also suggest experimenting with assisted migration of certain bumblebee colonies, which in general is a controversial topic among conservation biologists. (Read more about this study here.)

Buff-tailed Bumblebee (Bombus terrestris) - photo credit: wikimedia commons

Buff-tailed Bumblebee (Bombus terrestris) – photo credit: wikimedia commons

The loss of bumblebees is concerning because they play a prominent role in the various ecosystems in which they live. They are prolific and highly effective pollinators of both agricultural crops and native plants, and they are also a major component in the food web. Some species of plants “prefer” the pollination services of bumblebees, such as those in the family Solanaceae. Many plants in this family benefit greatly from buzz pollination – a process in which a bumblebee (or occasionally bees of other species) grabs hold of the flower and vibrates its body, dislodging the pollen.

Participating in bumblebee conservation is simple. It’s similar to any other kind of pollinator conservation. Just learning about the pollinators in your region and being mindful of them can make a big difference. If you own or rent property and have space for a garden (even if its just a few containters on a patio), choose plants that provide food for bumblebees, including spring and summer bloomers. If you live in North America, this Xerces Society publication and the field guide mentioned above are great resources that can help you determine which plants are best for your region. Additionally, if you are working in your yard and happen upon a hibernating queen or a bumblebee nest, do your best not to disturb it. It may disrupt your gardening plans for a season, but the bumblebee sightings and the pollination service they provide will be worth it.

One family of plants in particular that you should consider representing in your yard is the legume family (Fabaceae). Bumblebees are commonly seen pollinating plants in this family, and because these plants have the ability to convert nitrogen in the air into fertilizer, their pollen is especially rich in protein. In his book, A Sting in the Tale, Dave Goulson describes the relationship between bumblebees and legumes:

“From a bumblebee’s perspective, legumes are among the most vital components of a wildflower meadow. Plants of this family include clovers, trefoils and vetches, as well as garden vegetables such as peas and beans, and they have an unusual trick that allows them to thrive in low-fertility soils. Their roots have nodules, small lumps inside which live Rhizobium, bacteria that can trap nitrogen from the air and turn it into a form usable by plants. … This relationship gave legumes a huge advantage in the days before artificial fertilizers were widely deployed. Ancient hay meadows are full of clovers, trefoils, vetches, meddicks and melilots, able to outcompete grasses because they alone have access to plentiful nutrients. Most of these plants are pollinated by bumblebees.”

More information about bumblebees and bumblebee conservation:

Bumblebee Conservation Trust

Bumble Bee Watch

BugGuide (Bombus)

The Xerces Society – Project Bumble Bee

Year of Pollination: Figs and Fig Wasps

This post originally appeared on Awkward Botany in November 2013. I’m reposting an updated version for the Year of Pollination series because it describes a very unique and incredibly interesting interaction between plant and pollinator. 

Ficus is a genus of plants in the family Moraceae that consists of trees, shrubs, and vines. Plants in this genus are commonly referred to as figs, and there are nearly 850 described species of them. The majority of fig species are found in tropical regions, however several occur in temperate regions as well. The domesticated fig (Ficus carica), also known as common fig, is widely cultivated throughout the world for its fruit.

common fig

Common Fig (Ficus carica) – photo credit: wikimedia commons

The fruit of figs, also called a fig, is considered a multiple fruit because it is formed from a cluster of flowers. A small fruit develops from each flower in the cluster, but they all grow together to form what appears to be a single fruit. The story becomes bizarre when you consider the location of the fig flowers. They are contained inside a structure called a syconium, which is essentially a modified fleshy stem. The syconium looks like an immature fig. Because they are completely enclosed inside syconia, the flowers are not visible from the outside, yet they must be pollinated in order to produce seeds and mature fruits.

This is where the fig wasps come in. “Fig wasp” is a term that refers to all species of chalcid wasps that breed exclusively inside of figs. Fig wasps are in the order Hymenoptera (superfamily Chalcidoidea) and represent at least five families of insects. Figs and fig wasps have coevolved over tens of millions of years, meaning that each species of fig could potentially have a specific species of fig wasp with which it has developed a mutualistic relationship. However, pollinator host sharing and host switching occurs frequently.

Fig wasps are tiny, mere millimeters in length, so they are not the same sort of wasps that you’ll find buzzing around you during your summer picnic. Fig wasps have to be small though, because in order to pollinate fig flowers they must find their way into a fig. Fortunately, there is a small opening at the base of the fig called an ostiole that has been adapted just for them.

What follows is a very basic description of the interaction between fig and fig wasp; due to the incredible diversity of figs and fig wasps, the specifics of the interactions are equally diverse.

First, a female wasp carrying the pollen of a fig from which she has recently emerged discovers a syconium that is ready to be pollinated. She finds the ostiole and begins to enter. She is tiny, but so is the opening, and so her wings, antennae, and/or legs can be ripped off in the process. No worries though, since she won’t be needing them anymore. Inside the syconium, she begins to lay her eggs inside the flowers. In doing so, the pollen she is carrying is rubbed off onto the stigmas of the flowers. After all her eggs are laid, the female wasp dies. The fig wasp larvae develop inside galls in the ovaries of the fig flowers, and they emerge from the galls once they have matured into adults. The adult males mate with the females and then begin the arduous task of chewing through the wall of the fig in order to let the females out. After completing this task, they die. The females then leave the figs, bringing pollen with them, and search for a fig of their own to enter and lay eggs. And the cycle continues.

But there is so much more to the story. For example, there are non-pollinating fig wasps that breed inside of figs but do not assist in pollination – freeloaders essentially. The story also differs if the species is monoecious (male and female flowers on the same plant) compared to dioecious (male and female flowers on different plants). It’s too much to cover here, but figweb.org is a great resource for fig and fig wasp information. Also check out the PBS documentary, The Queen of Trees.