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

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Year of Pollination: Stamen Movement in the Flowers of Prickly Pears

Last week I made an effort to convince you to add a prickly pear or two to your water-wise gardens. One standout reason to do this is their strikingly beautiful flowers. Apart from being lovely to look at, many prickly pear flowers have a distinct feature that makes them quite fascinating. A demonstration of this feature can be seen in the following video.

 

Stamen movement in response to touch is a characteristic of many species in the genus Opuntia. It isn’t exclusive to Opuntia, however, and can also be seen in Berberis vulgaris, Portulaca grandiflora, Talinum patens, among others. Knowing this makes me want to touch the stamens of any flower I can find just to see what will happen.

The response of stamens to touch has been known for at least a few centuries, but recent research is helping us gain a better understanding of how and why this phenomenon occurs. In general, this movement is thought to assist in the process of cross-pollination. In some cases it may also aid in self-pollination. Additionally, it can have the effect of protecting pollen and nectar from “robbers” (insects that visit flowers to consume these resources but that do not provide a pollination service). Quite a bit of research has been done on this topic, so to simplify things I will be focusing on a paper published in a 2013 issue of the journal, Flora.

In their paper entitled, Intriguing thigmonastic (sensitive) stamens in the plains prickly pear, Cota-Sanchez, et al. studied the flowers of numerous Opuntia polyacantha individuals found in three populations south of Saskatoon, Saskatchewan, Canada. Their objective was to “build basic knowledge about this rather unique staminal movement in plants and its putative role in pollination.” They did this by conducting two separate studies. The first involved observing flower phenology and flower visitors and determining whether the staminal movement is a nasty (movement in a set direction independent of the external stimulus) or a tropism (movement in the direction of the external stimulus). The second involved using high-powered microscopes to analyze the morphology of the stamens to determine any anatomical traits involved in this movement. While the results of the second study are interesting, for the purposes of this post I have chosen to focus only on the findings of the first study.

An important note about the flowers of O. polyacantha is that they are generally protandrous, meaning that the anthers of a single flower release pollen before the stigmas of that same flower are receptive. This encourages cross-pollination. An individual flower is only in bloom for about 12 hours (sometimes as long as 30 hours), however flowering doesn’t occur all at once. The plants in this study flowered for several weeks (from the second week of June to the middle of July).

To determine whether the staminal movement is a nasty or a tropism, the researchers observed insects visiting the flowers. They also manually stimulated the stamens with various objects including small twigs, pencils, and fingers, touching either the inner sides of the filaments (facing the style) or the outer sides (facing the petals). In every observation, the stamens moved in the same direction, “inwards and towards the central part of the flower.” This “consistent unidirectional movement, independent of the area stimulated” led the researchers to categorize the staminal movement of O. polyacantha as thigmonastic. They also observed that staminal movement slowed as the blooming period of an individual flower was coming to an end – “and finally when all the anthers had dehisced, the anthers rested in a clustered position, marking the end of anthesis.” Furthermore, it was observed that “filaments move relatively faster in sunny, warm conditions as opposed to cloudy, cold and rainy days.”

The researchers went on to discuss unique features of the stamens of O. polyacantha. Specifically, the lower anthers contain significantly more pollen than the upper anthers. When the stamens are stimulated, their movement towards the center of the flower results in the lower anthers becoming hidden below the upper anthers. They also noted that small insects less than 5 millimeters in size did not trigger stamen movement. Further observations of the insect vistors helped explain these phenomena.

SAMSUNG

A “broad diversity of insects” was observed visiting the flowers, from a variety of bees (bumblebees, honeybees, sweat bees, and mining bees) to bee flies, beetles, and ants. The large bees  were determined to be the effective pollinators of this species of prickly pear. Their large weight and size allows them to push down through the upper anthers to the more pollen-abundant anthers below. After feeding on pollen and nectar, they climb out from the stamens and up to the stigma where they take off, leaving the flower and depositing pollen as they go. Because the bees are visiting numerous flowers in a single flight and the flowers they visit are protandrous, pollen can be transferred from one flower to another and self-pollination can be avoided.

Beetles were observed to be the most common visitors to the flowers; however, they were not seen making contact with the stigma and instead simply fed on pollen and left. Ants also commonly visit the flowers but largely remain outside of the petals, feeding from “extranuptial nectaries.” In short, beetles and ants are not recognized as reliable pollinators of this plant.

Similar results involving two other Opuntia species were found by Clemens Schlindwein and Dieter Wittmann. You can read about their study here.

There are lots of flower anatomy terms in this post. Refresh your memory by visiting another Awkward Botany post: 14 Botanical Terms for Flower Anatomy.

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14 Botanical Terms for Flower Anatomy

I like to know the names of things. Certainly I don’t have to know what everything is called in order to appreciate it for what it is, but that appreciation deepens when I understand it better. Scientific exploration helps us discover the workings of the world around us, and through that exploration comes the naming and describing of things. The names are largely arbitrary apart from the fact that they help us keep track of the descriptions associated with the discoveries. Calling things by name and knowing how to describe them not only increases our awareness of the natural world but can also give us greater appreciation for the larger picture and our place in it all. With that I introduce a new series of posts concerning botanical terms.

It’s mid-summer now (at least in the northern hemisphere) and flowers abound, so this first Botanical Terms post will help us become better familiar with flower anatomy. [I’m also releasing this post while the Botanical Society of America convenes for its annual conference in my current hometown – Boise, Idaho – so it seems fitting]. Of course, as soon as I began looking into the subject of flower anatomy, I realized very quickly that, like so many other things, it is incredibly complex. First of all, in the larger world of plants, not all produce flowers. Non-vascular plants don’t. And within the category of vascular plants, non-seed producing plants don’t make flowers either. Within the category of seed producing plants, there are two groups: gymnosperms and angiosperms. Angiosperms produce flowers; gymnosperms don’t. Even though that narrows it down quite a bit, we are still dealing with a very large group of plants.

The complexity doesn’t stop there, of course. Memorizing the names of flower structures and recognizing them on each flowering plant would be easy if every flowering plant had all of the same structures and if all structures existed on each flower. However, this is not the case. Depending on the flower you are looking at, some structures may be absent and some may have additional structures that are not common ones. Also, some plants have inflorescences that appear as a single flower but are actually a collection of many smaller flowers (or florets), like plants in the sunflower family (Asteraceae) for example. Regardless, we are going to start with basic terms, as there are a large number of flowering plants that do exhibit  all or most of the following basic structures in their flowers.

flower anatomy

Pedicel and Peduncle: These terms refer to the stem or stalk of the flower. Each individual flower has a pedicel. When flowers appear in groups (also known as an inflorescence), the stalk leading up to the group of flowers is called a peduncle.

Sepal and Calyx: Sepals are the first of the four floral appendages. They are modified leaves at the base of the flower that protect the flower bud. They are typically green but can be other colors as well. In some cases they may be very small or absent altogether. The sepals are known collectively as the calyx.

Petal and Corolla: Petals are colorful leaf-like appendages and the most familiar part of a flower. They come in myriad sizes, shapes, and colors and are often multi-colored. Their purpose is to attract pollinators. Many plants are pollinated by specific pollinators, and so their petals are designed to attract those pollinators. The petals are known collectively as the corolla.  

Stamen, Anther, and Filament: Pollen is produced in a structure called an anther which sits atop a filament. Collectively this is known as a stamen. Stamens are considered the male portion of the flower because they produce the pollen grains that fertilize the egg to form a seed. Flowers often have several stamens, and on flowers that have both male and female structures, the stamens are found surrounding the female portion.

Pistil, Carpel, Stigma, Style, and Ovary: The female portion of a flower consists of a stigma (where pollen grains are collected), a style (which raises the stigma up to catch the pollen), and an ovary (where pollen is introduced to the ovules for fertilization). Together this is known as a carpel. A collection of carpels fused together is called a pistil. Just like with stamens, flowers can have multiple pistils.

Start learning to identify floral structures on flowers like rugosa rose (Rosa rugosa). (photo credit: eol.org)

Start learning floral anatomy on flowers with easily recognizable structures like the flowers of rugosa rose (Rosa rugosa). (photo credit: eol.org)

Flowers are small art pieces worthy of admiration in their own right. However, recognizing and exploring the different floral structures can be just as enthralling. The structures vary considerably from species to species, each its own piece of nature’s artwork. So, I encourage you to find a hand lens (or better yet a dissecting microscope) and explore the intimate parts of the flowers around you.