| 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.
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.
The fruit of the field geranium ([Fig. 133]) is a long capsule composed of five carpels arranged round a central column (A). When the seeds are ripe, the carpels suddenly spring from the rod, remaining attached only at their upper ends (B), and fling the seeds into the air. The method may be watched by stroking the fruits very gently with a small brush, when they open in the manner described. The experiment is most likely to succeed in dry, sunny weather, about the middle of the day.
The fruit of the poppy.—The pistil of the poppy swells during ripening into a large, globular capsule known as the poppy head. The top of the fruit ([Fig. 134]) is the persistent stigma. Just below this, a line of small holes like windows runs round the head. As the fruit hangs inverted on the top of the flower stalk it is shaken about by the wind, and the tiny seeds fall out through the windows.