Thus, the tender wall of the pistil often becomes a woody, leathery, or juicy seed-case; while the receptacle, or top of the flower stalk, may [Pg 167] become fleshy and swollen with sugary pulp, as a bait for birds and other animals. In any case we give the name of fruit to all such altered and persistent parts together with the seeds which accompany them. A pea pod, for example, is as truly a fruit as a plum, and a poppy-head as a strawberry. What a fruit is like depends to a great extent upon the characters of the pistil which gave rise to it. If, for example, the pistil consists of several separate carpels, the ripened carpels or fruits will also be separate. When, on the other hand, the pistil is composed of united carpels, these will remain united, at least until the fruit is ripe. Then in some cases they come apart. Fig. 131.—A, fruits of wallflower (× ⅕); B, siliqua (× 1); C, siliqua open. Fig. 132.—Fruits (siliculae) of Penny Cress. (Nat. size.) The part of a fruit which is derived from the walls of the pistil is called the pericarp. The fruit of the wallflower.—After fertilisation the stamens, petals, and sepals of the wallflower drop off, leaving the pistil alone on the top of the flower stalk. The pistil increases greatly in size (Fig. 131, B) during the ripening of the seeds. At last its wall (the pericarp) splits into two flaps. These become free at the bottom, exposing a central plate which bears the rows of seeds. When the seeds are quite ripe, a slight breeze is sufficient to shake them off, and they fall to the ground to take their chance of finding a place favourable for germination. [Pg 168] A fruit with a dry pericarp, which opens of itself when the seeds are ripe, is called a capsule. This particular kind of capsule—consisting of two carpels which come apart at maturity, leaving a central partition bearing seeds—is known as a siliqua. When it is short in proportion to its length, as in the shepherd’s purse (Fig. 62) and penny cress (Fig. 132), it is distinguished as a silicula. The fruits of the pea tribe.—The pod of the pea (Fig. 3) and its relatives is a capsule of another kind. It consists of one carpel only, and opens, when ripe, along both back and front margins to liberate the seeds. Such a fruit is called a legume. In the young fruit the pericarp is somewhat fleshy and succulent, but it becomes dry during ripening. The legume of the bird’s foot trefoil bursts open suddenly, and throws the seeds to a considerable distance.