The fertilized egg soon be gins to develop and eventu ally forms a tiny plant with rudimentary root, stem and leaf, as we find it in the seed. The coats of the seed develop from those of the ovule ; some times the ovary wall or a part of it remains permanently at tached to the seed. The endo sperm of the seed comes from two endosperm nuclei (Fig. 42, end), which fuse with a nuc leus from the pollen-tube (s pn). The endosperm may thus show the characters of both parents. In corn, in which the endosperm deter mines the color of the grain, an ear of yellow corn that re ceives pollen partly from yel low and partly from blue corn may show, on the same ear, both blue and yellow grains side by side.
Since pollen is easily in jured by rain or dew, various devices exist for keeping it dry. The closing or drooping of flowers in rainy weather and at night, and numerous contrivances for shedding water, all serve to keep the pollen dry.
In a of classical experiments, Darwin showed that self-pollination, or the placing of the p 'lien on the stigma of the plant that produced it, d not give as vigorous offspring as nation, or the transfer of pollen from another indi vidual. In plants we find numerous devices to pro mote cross-pollination and to prevent self-polli nation. It is common to find the stamens and pistils in separate flowers on the same plant (moncecious plants, as squash and corn), or produced on separate plants (dimcious plants, as the hop). When the organs are not thus separated they may mature at different times, or otherwise promote cross-pollination.
Pollen is carried from one flower to another through the agency of wind (as in corn), or water (as in many aquatics), or by insects. Whether the insects are attracted by the color or by the odor of flowers is to some extent still an open question.
The fruit.—The fruit is the ripe or ripening ovary, with its contents and any surrounding parts that remain attached to it. The first work of the fruit is to convey nourishment to the young seeds and protect them during their development. The great importance of the food supply is evident from the fierce struggle that takes place, not only between flowers and fruits on the same plant but between the developing seeds in the same fruit. Usually many fruits fall because of lack of nourish ment, and this is aided by the grower, who thins the fruit to secure a few large ones rather than many small fruits. In a number of fruits many seeds in the ovary fail to develop from lack of sufficient food. In the majority of cases the plant gives its whole store of food to the fruit and then dies. The stalks of grain, for example, are almost completely emptied of nutriment during the ripening period, leaving the stalks dry and taste less. This occurs even if the grain be cut before the seed is fully ripe. On reaching the seed the food is often transformed, as from starch to oil. During the ripening process many changes in the food substances occur, as when the acrid taste in apples gradually gives place to sweetness and agreeable flavor ; and at the same time various gelatinous substances are produced that render the ripe fruit suitable for jelly-making. Such changes
take place after the fruit is removed from the tree, as is illustrated by the familiar practice of allowing pears to ripen in drawers.
In order to insure abundant fruit, there must be vigorous and healthy development of foliage early in the season, followed later by a decrease in water supply and increase of light and heat. The tendency to produce wood instead of fruit is checked by decreasing the water supply, as evi denced in the practice of pruning or laying bare the roots, and breaking or notching the branches to increase productiveness.
An important function of the fruit is to scatter the seeds so that the plant may be reproduced in abundance. Some fruits float long distances on water, as the coconut ; others, as the dandelion, develop wings, or parachutes, so that they may be carried far by the wind. Some stick to the rough coats of animals ; others, by their pleasant taste and bright color, attract birds, which scatter the seeds.
Some seeds can germinate as soon as ripe, while others require long periods of rest before they germinate. A sufficient supply of water, warmth and air are necessary for germination. If these are not furnished the seed remains dormant, often retaining its vitality for many years.
General properties of plants.
Nutrition and respiration. —The formation of elaborated food has already been described. Such food is disposed of in three ways : (1) It is oxidized or burned just as in the animal body, producing heat, chemical energy, and so on. In this process, called respiration, carbon dioxid is pro duced and given off to the air, to be again decom posed and built up into food. This food is burned in turn, forming more carbon dioxid ; and so the process goes on in a never-ending cycle. It is evi dent that the chief object of producing food is to have energy stored in convenient form, so that it can be utilized whenever needed. The constructive work of the plant separates carbon from oxygen, which is given off into the air, and stores energy ; the destructive work of the plant unites (burns) carbon with oxygen and sets energy free. The amount of energy set free may be estimated from the amount of carbon dioxid given off. When an organism has produced its own weight of carbon dioxid, it has set free sufficient energy to raise itself about 600 miles. Some bacteria give off twice their weight of carbon dioxid in 24 hours, while a man in the same time exhales about 1.2 per cent of his weight. Green plants consume much more carbon dioxid than they produce. The consumption of carbon dioxid stops at night, while its production goes steadily on. The amount pro duced is small, and a hundred plants in a room at night would not " vitiate " the air so much as a single candle.