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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