The effect of fertilization extends beyond the ovule, the ovary and often other parts of the flower being affected, enlarging and often becoming bright-colored and juicy, forming the various fruits of the angiosperms. These fruits when ripe may be either dry, as in the case of grains of various kinds, beans, peas, etc.; or the ripe fruit may be juicy, serving in this way to attract animals of many kinds which feed on the juicy pulp, and leave the hard seeds uninjured, thus helping to distribute them. Common examples of these fleshy fruits are offered by the berries of many plants; apples, melons, cherries, etc., are also familiar examples.
The seeds differ a good deal both in regard to size and the degree to which the embryo is developed at the time the seed ripens.
Classification of the Angiosperms.
The angiosperms are divided into two sub-classes: I. Monocotyledons and II. Dicotyledons.
The monocotyledons comprise many familiar plants, both ornamental and useful. They have for the most part elongated, smooth-edged leaves with parallel veins, and the parts of the flower are in threes in the majority of them. As their name indicates, there is but one cotyledon or seed leaf, and the leaves from the first are alternate. As a rule the embryo is very small and surrounded by abundant endosperm.
The most thoroughly typical members of the sub-class are the lilies and their relatives. The one selected for special study here, the yellow adder-tongue, is very common in the spring; but if not accessible, almost any liliaceous plant will answer. Of garden flowers, the tulip, hyacinth, narcissus, or one of the common lilies may be used; of wild flowers, the various species of Trillium ([Fig. 83], A) are common and easily studied forms, but the leaves are not of the type common to most monocotyledons.
The yellow adder-tongue (Erythronium americanum) ([Fig. 81]) is one of the commonest and widespread of wild flowers, blossoming in the northern states from about the middle of April till the middle of May. Most of the plants found will not be in flower, and these send up but a single, oblong, pointed leaf. The flowering plant has two similar leaves, one of which is usually larger than the other. They seem to come directly from the ground, but closer examination shows that they are attached to a stem of considerable length entirely buried in the ground. This arises from a small bulb (B) to whose base numerous roots (r) are attached. Rising from between the leaves is a slender, leafless stalk bearing a single, nodding flower at the top.
The leaves are perfectly smooth, dull purplish red on the lower side, and pale green with purplish blotches above. The epidermis may be very easily removed, and is perfectly colorless. Examined closely, longitudinal rows of whitish spots may be detected: these are the breathing pores.
