The Science of Botanical Art

Pollination

By Dick Rauh

Originally appeared in The Botanical Artist,  Number 29

 

Pollination is the business of flowers. The reason why there is such variety in the form and color and scent of blossoms is basically because of each plant’s need to beat the competition. When flowering plants first arose the tremendous advance of the enclosed and protected ovule required some outside help to get the sperm to the egg. That outside help was to come fortunately, or more accurately inevitably, with the burst in speciation in the animal world. Flowering plants paralleled a similar burst in speciation. The dependence on water, and wind had suddenly vastly widened, and the vectors enabling the pollen (containing the sperm, and the tube cell to deliver the sperm) to reach the stigma, included insects, birds and mammals. The ovary, containing the ovules that were to become the seeds developed into fruit, another advance of flowering plants, designed to solve the additional problem of how to disperse the seeds (but that discussion will come in another piece). What I want to focus on in this article is the morphological diversity of flowers based primarily on adaptations made for pollination. 

This may be stating the obvious, because almost if not all the adaptations in flowers are made for no other purpose. Flying insects are the primary vectors for pollination from the animal world, although they are by no means the only pollinators, and consequently the form of flowers is designed to serve their needs. Blossoms are shaped to allow them access to the pollen from the anthers, and the receptive stigmas on which to deposit these vital links in the chain of floral reproduction. 

The provision of landing sites of varying degrees of accessibility often determines form. Cones and tubes of different lengths and widths entice, or discourage various lengths of tongue or beak. Long distance advertising, using color, size and petal position, or more intimate advertising by scent informs the insect when and where to visit and is another of the duties of flowers. Beards and surface hairs act as ladder rungs for crawling, nectar or honey guides (those enticing patterns that make so many flowers a joy to paint) are there to direct the pollinators to hidden rewards. Rewards are part of the equation -”Do this job for me and I’ll make it worth your while.” Nectar or pollen or sometimes food in the form of fleshy perianth or food bodies, more specialized compounds for specific insect needs or even the provision of temporary housing, all of these factors determine the shape of flowers and the shapes of the components that make up a blossom. 

As painters, what are the special features that we should look for? For one thing how are the flowers normally positioned - are they facing up, hanging down, or held out perpendicular to the stem? Pollinators largely determine this. Most insects- especially those not particularly adept at flying like some beetles, appreciate a dish-shape upright flower so they have ample space to land. Often bell shaped flowers, pollinated by crawling insects will be pendant (and in the process of hanging down they are able to protect anthers from rain. Hovering insects or birds with long tongues prefer horizontal tubular flowers. Bat-pollinated flowers tend to be large, light-colored or white, and are often brush-like or wide pendant cones. In the competition for pollinators some genera have enclosed their reproductive apparatus and the insect rewards in bilabiate or two-lipped corollas, as in the mints or the scrophs, or in a keellike fusion of two lower petals as in the peas, that require a strong bee to force entry. 

The interesting difference of these two forms is that in the gullet, or bilabiate form we find the reproductive organs in the upper part of the corolla tube, set to dust the head and upper body of the bee (or to receive the pollen on a receptive stigma), while in the so called Banner form of the pea flower the organs are located in the keel, formed from the two lowest petals. The pea flower in anthesis sits horizontally from its stem, the upper petal (the banner or flag) upright and spread, advertising its ripeness while the two side wing petals partially hide the keel and form a platform for the bee who then needs to force open the slit at the top of the keel to reach the nectar, and incidentally perform the vital service of pollination. 

The shape of flowers like Nicotiana is called salverform- combining a dish-shaped landing platform with a narrow corolla tube - the butterfly or moth can land and then thrust it’s long coiled proboscis into the tube to gain it’s reward. Not always apparent, but something to look for is the open ushaped channel that is present on the inner surface of the petal-derived tepal of many lilies. Probably for support as well, this acts as a guide for the proboscis of the pollinating butterfly, leading it to the nectar at the base of the perianth. One of the problems faced by most flowers is the need to avoid selfing. One of the major advantages of the reproductive process is the opportunity for variation and the encouragement of dominant over recessive genes provided by crossing, or out breeding. Even though a flower is apt to be pollinated from another flower in the same population, there is enough difference to strengthen the offspring. When a flower pollinates itself (and this is a necessity in some cases - a weaker offspring is better than none) the result- ing generation is a clone of the parent. 

Plants that invade a newly disturbed and barren habitat have to resort to selfing. One of the reasons why the common violet is so successful a colonizer is because it produces two distinctly different forms of blossoms. The familiar open five-petaled flower of early spring, with all the normal apparatus for attraction and reproduction, is replaced later in the season by cleistogamic flowers that never open but produce seeds when their own pollen fertilizes their own pistil. There are a number of devices used by plants to avoid selfing. One of the more obvious is the separation of blooms into staminate (male) and pistillate (female) flowers. No chance here for pollen to find its way to its own stigma. This is true whether the plant is dioecious (on two separate plant bodies) or monoecious (one plant with imperfect flowers of both sexes). With most perfect flowers, provisions to avoid selfing are usually temporal - either the stamens or the stigmas ripen first. Rarely will you find the stigmas receptive when the anthers are dispensing their pollen, but when this happens there is usually a positional difference. The style will have pushed the stigmas above the anthers, or the releasing mechanism of the stamens will be facing away from the pistil. When you are painting a population of flowers check to see if this temporal or positional effect isn’t apparent, that you will spot some flowers with loaded anthers and no evident style, or some that have obvious receptive stigmas, and the anther sacs are withered. 

Color is another factor that caters to specific pollinators. Bees, by far the largest genera to perform this function are known to have a vision range that sees from yellow to ultraviolet, so you can be reasonably certain that red flowers are intended for hummingbirds or butterflies. Often the nectar guides on bee flowers are invisible to the human eye, as in the Marsh Marigold, because they are ultraviolet. Light, white, or other pale colored flowers are aimed at nocturnal visitors, bats (primarily in the tropics) or moths. 

Then there are the flowers that depend on wind for pollination. In flowering plants this is considered a secondary adaptation - or a reversion to a more primitive condition after the species has passed through an animal- pollinated stage. The genus Salix is an example of a group that has its feet in both camps - there are examples that rely on insects, and others that depend on wind. Because plants (getting back to the business metaphor of the beginning of the article) are soundly economic organisms, and flourish best when the valuable energy they manufacture is not squandered, wind pollinated plants have eliminated the unnecessary luxury of petals, or the production of nectar, or scent, but instead spend more on the production of pollen, since the wind is a much more fickle vector than the more singleminded bee. 

There is much that I haven’t covered, but many books exist on the subject if you become intrigued. Pollination Ecology by Faegri and Van der Pijl, is technical reading but something of a classic on the subject. The Sex Life of Flowers by Meeuse and Morris is aimed at a lay audience and is very readable and filled with beautiful and informative photographs.  

  • Papilo troilus on Buddleja davidii © Dick Rauh
  • Vespula maculifrons on Allium tuberosum, © Dick Rauh