Page:Popular Science Monthly Volume 28.djvu/83

Rh variegated with convergent lines of different colors, which guide the bee toward the exact spot where the nectaries are engaged in elaborating honey for his benefit. The two next in order, called the wings, are generally shorter and smaller, and in most advanced types they possess two little indentations, one on each side, specially adapted to afford a foothold for the legs of the visiting bee, in the exact position that will enable him at once to reach the honey and to brush off the pollen against the sensitive surface. The two lowest petals of all are usually united by their under edge, so as to form a single organ, known as the keel, and closely inclosing the stamens and pistil. As a rule, too, all ten stamens are united into a single tube or sheath; or else the nine lower ones are so united, while the upper one is free. In spite of the general uniformity of floral type, however, many special modes of insect fertilization prevail among the various pea-flowers. Sometimes the blossom bursts open elastically when the bee lights upon it, dusting him all over with the ripe pollen; sometimes a small quantity is pumped out from the sharpened point of the keel by the weight of the insect's body; sometimes the pollen is deposited from his breast on the spirally curled summit of the pistil; sometimes it is swept off by a little brush of hairs, situated close beside the sensitive surface of the embryo pod. All that it is here necessary to bear in mind, however, is the general fact that the papilionaceous type of flower has gained its present high position as a dominant floral pattern by its beautiful and varied adaptation to insect fertilization.

Such being the general nature of the pea-flowers as a whole, we have next to inquire what are the special peculiarities which have enabled the clovers in particular to fill their peculiar niche in the existing economy of Nature. Clearly, the positions which clovers are adapted to adorn are not the high places in the hierarchy of vegetal life. They are not tall forest-trees or bushy shrubs; they are not long, creeping trailers or climbers; they are herbs of low and procumbent character, best fitted for filling up the interspaces of taller vegetation, and for vying with the grasses as elements of the close, tender, delicate greensward. The points which have enabled them to survive, therefore, are just those which allow a plant to thrive under such special conditions; and we must ask briefly what those points may be before we proceed to consider the specific characteristics of the various individual clovers.

In foliage the clovers are distinguished by their graceful trefoil leaves which are an adaptation of the typical papilionaceous pattern to the special necessities of their humble situation. For the common form of pea-leaf consists of a long leaf-stalk, with one terminal leaflet, and with several pairs of lateral leaflets, arranged opposite each other along a central line. In the clovers, however, and in most other small field forms of papilionaceous plants, only one pair of lateral leaflets is developed; and this arrangement allows the leaf-stalk to be elevated