The Forest Unseen: A Year's Watch in Nature by DavidGeorge Haskell

the stigma assesses it, rejecting any from different species. The plant also shuns its own pollen and that from close relatives, preventing self-fertilization and inbreeding. In a few species, this rule against self-fertilization is broken if no other suitable pollen arrives. Such self-fertilization is a strategy of last resort used by
Hepatica
and other species that bloom in the early spring. For these species, desperateself-love is better than no love at all when inclement weather grounds their pollinating insects.
    If the biochemical matchmaking goes well, the stigma’s cells release water and nutrients to melt the pollen’s tough armor. The pollen grain’s shell cracks open, ruptured by the swelling pair of cells within. The larger of these two cells grows, amoeba-like, out of the ruptured pollen coat and starts to burrow down between the enveloping cells of the stigma, forming a tube. Each stigma is at the tip of a stalk known as the style, and the pollen tube works its way down the style, either pushing between the cells or, if the style is hollow, flowing down the style’s inner wall like a drop of oil. The second, smaller pollen cell divides and forms two sperm cells. These float down the pollen tube, carried along like rafters in a flowing river. Unlike the sperm cells of animals, mosses, and ferns, these rafters have no oars; their movement is entirely passive.
    The style’s length is caused by the need to hold the stigmas up where bees will bump against them. This creates a challenging odyssey for the pollen tube and, therefore, a convenient testing ground on which the plant can assess her suitors. Bees dump many pollen grains onto each stigma, so the style may have several tubes growing at once. If so, the style becomes the Kentucky Derby of plant love. The sperm cells jockey their tubes toward the ovule, which contains the plant’s egg; the price of failure is the annihilation of the rider’s genes. There is some evidence that vigorous plants produce fast pollen tubes, so the style’s length allows the flower to select mates with a history of success. Perhaps the styles are a little longer than strictly necessary for intercepting bees, just to give the pollen stallions a good hard run.
    When the pollen tube reaches the base of the style, it burrows into the fleshy ovule. Here the pollen tube releases its two sperm cells. One sperm cell joins with the egg to make an embryo; the other joins with the DNA from two other tiny plant cells to make a larger cell with a triple complement of DNA. This triply endowed cell divides, fattens up, and becomes a food storage area for the developing seed, a store thathumans have put to use as wheat flour and cornmeal. Such double fertilization is unique to flowering plants; sexual union in all other creatures requires only one sperm cell and one egg cell.
    The chickweed in front of my hand lens is a hermaphrodite, producing both pollen and eggs, male and female, in each blossom. Every flower contains all the necessary reproductive apparatus: pollen grains; anthers to make and store the grains; filaments to hold up the anthers, stigmas, styles; and an ovary to contain the eggs. These parts are all crowded within the cup of the flower, ringed by colored petals designed to appeal to animal eyes. Such tiny, tidily arranged complexity makes for a compelling display.
    All the mandala’s spring ephemeral flowers are hermaphrodites, a strategy well suited to these tiny plants that produce just a few flowers during a short, unpredictable season. By combining male and female into one flower, the plants leave open the door to self-breeding. They also spread their investment between male and female functions, increasing the chance that at least some of their genes will pass to the next generation. Other species, such as many wind-pollinated trees—oaks, walnuts, elms—use a different strategy, producing great numbers of unisexual flowers. In this case, each flower has a specialized