1. Shepherd’s purse.—Compare the shepherd’s purse ([Fig. 60]) with the wallflower. The flower is very much smaller, and white, but the parts have the same arrangement as in the wallflower, viz., four sepals, four petals arranged in the form of a cross, six stamens (two short and four long), and a central pistil, all arranged separately on the receptacle. Look down the plant, and notice that in the oldest (lowest) flowers, everything but the pistil has dropped off, and that this has become greatly enlarged to form a fruit. Cut some fruits open, both lengthwise and crosswise, and observe that each consists of two pocket-like chambers, separated by a thin partition on which the seeds are borne. Notice the manner in which the oldest fruits have opened naturally.
2. Other relatives of the wallflower.—Compare also the flowers and fruits of the stock and candytuft, and also cress, mustard, radish ([Fig. 61]), cabbage, and turnip, which have been allowed to “run to seed.” Examine the roots of the turnip and radish.
Make a note of the earliest dates on which you see the above plants in flower.
The wallflower family.—The plants of the family to which the wallflower belongs are of very great importance to mankind; for while not one of them is poisonous, many are extremely valuable as food-crops. They are all dicotyledons; that is, their seeds contain two cotyledons or “makeshift leaves,” as has already ([Chapter I.]) been seen in the case of the mustard. The net-like venation of the leaves of the full-grown plant also indicates this. There is a great family likeness between the flowers of this group, and they are easily recognised ([Fig. 61] a, c) by the cross-shaped corolla and the six stamens (two short and four long). All the parts of the flower—sepals, petals, stamens, and pistil—are fixed separately on the top of the flower stalk or receptacle. The cross-arrangement of the petals has led to these plants being called Crucifers (cross-bearers). The shepherd’s purse ([Fig. 60])—so-called from the shape of its fruit—is a common weed with small, white flowers. All stages of the flower may generally be found on the same plant. While at the top the buds may still be unopened, the flowers below have been fertilised (in this case generally self-fertilised); the sepals, petals, and stamens—having fulfilled their duties—have fallen off; the ovules have become seeds, and the ovule-box or ovary has become a seed-box or fruit, consisting of two bags separated by a partition which bears two rows of seeds on each side ([Fig. 62]).
Fig. 61.—Wild Radish. a, flower (nat. size); b, petal; c, stamens and pistil (× 2); d, pistil (× 2); e, fruit (× 1); f, cross section of fruit; g and h, embryo (mag.)
