1. The fruit of the wallflower.—Examine wallflower fruits and make out that each consists of the ripened pistil. Does the fruit open of itself? How many chambers does it consist of? Where are the seeds attached? Are they blown off at last by the wind? Draw the fruit. It is called a siliqua.
2. Compare with this the fruit of shepherd’s purse ([Fig. 62]), and penny cress ([Fig. 132]), and notice that they are of the type of the wallflower fruit, but are much broader in proportion to the length. Such a fruit is called a silicula. Draw.
3. The fruit of the pea.—Examine a ripe pea-pod and compare it with (a) the pistil of an unfertilised flower, (b) a half-ripe pod. How many carpels have taken part in forming the pod? How many seeds (peas) does the pod contain? Leave a pod on the plant until the shell becomes dry, to find out how the fruit opens. Does it open along one edge only, or along both? How are the seeds attached? Such a pod is called a legume. Draw it.
Compare and draw the legumes of the broad bean, French bean, scarlet runner, laburnum (remember its seeds are poisonous), and bird’s foot trefoil. The legume of the bird’s foot trefoil bursts open suddenly and scatters the seeds in the air. Is the scattering of the seeds any advantage? Why?
4. The fruit of the field geranium.—Make out that five carpels are grouped around a central rod. Examine fruits which have opened. About noon on a bright, sunny day gently touch a ripe fruit with a small brush, and watch the carpels spring back from the rod and jerk the seeds into the air. Compare and contrast this fruit with a siliqua, silicula, and legume respectively.
5. The fruit of the poppy.—Examine a poppy head. The top of the fruit is the stigma. Observe below this a line of small holes running round the fruit. Draw. Shake the fruit, and notice that seeds fall out through the holes. Cut the fruit across to see the large number of small seeds inside. How does the fruit hang on the growing plant? Does the wind shake it and liberate the seeds?
6. The fruit of the pansy and violet.—Watch the ripening of the fruits on the plants. Observe that the ovary swells up into an egg-shaped body which afterwards splits into three boat-shaped valves containing seeds. Try to make out why the seeds are one by one shot out as the sides of the valves dry. Put a ripe fruit before the fire and watch the process. Imitate it by placing a pea between two flat rulers and pressing the rulers together.
The origin of a fruit.—When the ovules of a flower have been fertilised ([p. 92]) by the pollen tubes they change into seeds which have the remarkable power of growing up—in favourable circumstances—into plants resembling that which produced the seeds. This is not, however, the only result of fertilisation. Whilst the ovules are changing into ripe seeds, those parts of the flower—the stamens, corolla, and calyx—which have finished their work wither and fall off, though the calyx sometimes remains. Other parts—the pistil and sometimes the receptacle ([p. 90])—take on new duties, and become gradually modified in order to protect or scatter the seeds.
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 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.