When I was growing up, I thought of plants simply as green things that we sometimes ate, or mowed down every week or so, or from which we picked cherries and apples. At some point, I realized that they also provided perches for birds and nesting places for gray squirrels. But, essentially, they were static entities, useful no doubt, but incapable to doing anything interesting. Was I ever ignorant!
Gradually I came to understand that plants can do about as many things of interest as animals; it’s just that they often do them in different ways. To illustrate this, let’s consider the entire process of reproduction, from the getting together of male and female to the raising of offspring. And let’s divide this process into three phases: what happens before mating, what happens between mating and fertilization of eggs, and what happens after fertilization. Events at any point in the process may affect reproductive success, which is the ultimate measure of fitness: individuals that make more surviving offspring contribute more genes to future generations, and their genes come to dominate the population.
In the animal kingdom, it is usually males that compete for females (with some notable exceptions) and females commonly choose their mates more selectively than males. Males may cruise around looking for receptive females (in bears, for example) or set up display areas and wait for interested females to visit (as in grouse, for example). Sometimes they set up territories that exclude other males, and advertise their real estate and potential nest sites (as in blackbirds). Males fight each other physically or establish superiority by means of visual and acoustic displays. Basically, males show off their looks, strength, and endurance by engaging in strenuous displays, or they defend a territory with suitable feeding and breeding sites. Females cruise around looking for males or territories that they like and settling down for breeding with a chosen male. Both males and females often engage in extracurricular sexual activity, usually as a way of increasing the number of offspring (for males) or the diversity of offspring.
In the plant kingdom, things are seemingly quite different, because plants don’t move around; they can’t dance and sing. Nevertheless, there is often intense competition for mates. This is especially clear in plants that use animals to pollinate flowers (think of wild iris, or fireweed, or blueberries, or skunk cabbage…). Flowers are the plant’s way of attracting an animal that will bring pollen to female flower parts and carry pollen from male flower parts to a receptive female. Flowers provide nectar to reward flower visitors, some of which (such as bees) also collect pollen for food. There may be competition to have pollen delivered to female parts (female competition), or competition to deliver that pollen (male competition). In either case, flowers that offer more nectar or pollen or flowers that attract more visitors because of aroma or the size or configuration of petals would tend to have higher reproductive success. It is important to keep in mind that reproductive success is measured not only in terms of seed production (maternal function) but also in terms of seeds fathered.
Post-mating and pre-fertilization events
In the animal kingdom, males compete with each other indirectly in what is called sperm competition. Sometimes sheer numbers of sperm produced by one male can overwhelm the sperm of other males that mated with the same female; in fact, among primates, sperm production is greater in species in which multiple mating is common than in species that tend to mate with only one or perhaps two females. The timing of copulation can be part of this process; in certain ducks, if a male observes that his mate has been sexually accosted by an outsider, the paired male will immediately copulate with his mate, in an attempt to flood out the rival’s sperm. Some dragonflies go this one better: after a female has mated, a subsequent male can remove the first male’s sperm from the female’s reproductive tract. In a few animals, there is evidence that females themselves can eliminate sperm from an unwanted male. Many bats have delayed fertilization, in which the sperm do not penetrate the eggs immediately, but rather float around in the female for some time. One can speculate that this might give the females time to sort out the sperm of different males.
In the plant kingdom, the analogous process is pollen competition. Pollen donors (males) may increase their chances of fertilizing eggs (to make future seeds) by delivering lots of pollen grains that cover the receptive surface of female flower parts, occupying the surface and reducing the space for the pollen of other males. Males could also increase the number of eggs they fertilize by rapid germination of pollen grains and rapid growth of the sperm-delivery tube through the style (see the illustration) to the eggs in the ovary.
Female plants have some opportunity to select among the pollen grains because both germination and growth of pollen are based not only on characteristics of each male’s pollen but also on interaction with female tissue. In some species, it is a long way (possibly several inches in ornamental hibiscus, for example) from the receptive surfaces where pollen is deposited down the style to the eggs in the ovary, so there is ample opportunity for interactions between pollen and female tissue. Some plants have very delayed fertilization, for several weeks or more, and researchers have suggested that prolonged delays enhance the opportunity for females to accomplish more sorting of pollen from different pollen donors.
Post-fertilization and rearing of offspring
In the animal kingdom, where multiple paternity of litters is quite common (e.g., mink, bears, many birds), half-siblings within a litter compete with each other for parental care and nutrition. When food is scarce, some offspring receive less food and eventually die. From the fathers’ perspectives, having offspring with good competitive abilities is an advantage. From the mothers’ perspectives, there is the opportunity to selectively tend to certain young, thus effectively choosing among potential fathers.
In the plant kingdom, exactly the same process may occur. If the growing seeds in the same or even in different ovaries on a plant have different fathers, some may be better than others in capturing nutrients from the maternal plant. And the maternal plant may have the chance to be selective of which immature seeds will reach maturity.
I don’t mean to imply that all organisms use all of these tactics all of the time. Indeed, it is interesting to speculate about why some species use one tactic and not another. It should be clear that the reproductive process in plants and animals can involve both competition and choice of partners in the parenting process. I hope it is also clear by now that plants are anything but inert sticks of green. They can be little ‘peyton places’ of their own.