All living creatures are so constructed as to be capable of reproducing their kind. In fact, with all the lower Orders, that is the one object in their life. We have seen that flowers have stamens and pistils. These are the reproductive organs; the calyx, petals, nectar and fragrance are for other purposes as we shall see later.
The simple pistil is composed of three most important parts: At the summit is a stigma, this usually being sticky; just below is a slender tube called the style; at the base, the pistil is enlarged or swollen and contains the ovules or undeveloped seeds. The simple stamen is composed of a slender filament supporting at its end a little case or enlargement containing a fine powder-like substance called pollen.
The relationship between the pistil and the stamens was first discovered by a botanist named Grew, in the 17th century. His discovery, later confirmed by Linnaeus, was that in order for the seed to be developed, pollen from the anther must come in contact with the stigma, thence being transmitted through the style to the seeds below and quickening them to life.
Many puzzling propositions occurred, that these scientists were unable to unravel, - as, - "if the stamens were shorter than the pistil, how could the pollen ever reach the stigma?" It remained for Sprengel, late in the 18th century to declare that pollen was carried from the anthers to the stigma by insects while feeding on the flowers. He also discovered that the corolla, often brightly colored, was for the purpose of attracting insects and the nectar was for their food; fragrance also was a factor in drawing about the useful insects and, often, lines on the corolla or petals directed the visitor to the supply of food at the base.
It remained for the great Darwin to discover the exact truth about the many complicated methods of fertilization. Whereas Sprengel had supposed insects simply transferred pollen from the anther to the stigma on the same flower, Darwin claimed that it was of vital importance that the pollen from one blossom should be left at the stigma of a different one, and that many flowers were so constructed that they were incapable of being fertilized by their own pollen.
Nature's plan is to disperse families in order to prevent interbreeding, the continuance of which decreases vitality. All plants are slowly developing schemes for insuring cross-fertilization. Many flowers now are self-pollenized, but all first offer the opportunity to insects of various kinds to perform that office for them, and flowers so cross-pollenized will be stronger and healthier than the others. In ages to come, we may expect that, through the gradual elimination of the weaker, all species will be incapable of self-pollenization.
Methods for the preventing of self-pollenization are numerous and varied. The simplest is in having the anthers or stigma mature, one before the other. Many ingenious devices locate these members where they may not come in contact with one another, and so that an incoming insect will first touch the stigma and then, as he is departing, be showered with or have masses of pollen attached to some portion of his anatomy.
Many plants, usually annuals, whose roots die each year and that are dependent entirely upon setting of seed for their existence, have what are called cleistogamous flowers. These are bud-like flowers that never open, but fertilize themselves in the bud. These plants also have flowers that do open and that try to woo insect visitors; the others are for protection in case of several years shortage of useful insects.
Just as some flowers are so careful in adapting themselves to certain useful insects, they must be just as careful in protecting themselves against useless ones. In general, smooth bodied insects are of little value to plants, but they all like nectar. The most useless visitors are crawling ones, especially ants and it is against these that plants must erect their barriers.
The most frequently used preventative is a downy stem. The small hairs impede upward progress and often exhaust the tiny insects before they can reach the flower; often the calyx is sticky and the marauder finds a barrier from which he must turn or risk being caught on its surface. Some flowers have their nectar in long slender tubes so that only moths, butterflies or long-tongued bees can reach it. Others have the entrance closed with a palate, to open which the weight of a bee on the platform outside is necessary. Still others, like the Closed Gentian, are always closed, but the petals can be forced apart by the strong bumblebee.
Many flowers are very highly specialized, - adapted to be fertilized by but one species of insect. This specialization is carried to the highest degree in the great Angraecum Orchid of Madagascar, that has a nectar tube eleven inches in length. But one insect, a huge sphinx-moth, has a tongue of sufficient length to drain this nectary and to this insect alone, the plant owes its existence. It is related that Darwin, being confronted with the evidence of this flower against one of his theories, claimed that such a moth lived, even before it had been discovered.
The seeds having been matured must be dispersed. The simplest method is simply allowing them to drop to the ground; a better one is to provide for their spreading, this constantly widening the range of a species and making a stronger race by bringing together widely distant families.
We have all seen the rough-coated milkweed pods that burst open in the Fall and release quantities of silky-winged seeds. This flossy subtance is not for ornament; neither is it designed to amuse children. Each seed has an airy parachute that often carries it miles from the scenes of its birth before it finally comes to earth. This method is perhaps the simplest and best for wide dispersal; many flowers have adopted it, - thistles, dandelions, etc. Others have seeds in pods, like touch-me-not, that explode when they are matured and scatter the seeds over an area of several square yards. Still another method is of having tiny hooks, like the seeds of the genus Bidens, or in burs, like the burdock, that attach themselves to the hair of passing animals or the clothing of persons, and travel, perhaps, miles before they are shaken or brushed off.