18 



THE PLANT: ITS STRUCTURE, LIFE - PROCESSES AND ENVIRONMENT 



of flowers in rainy weather and at night, and 

 numerous contrivances for shedding water, all 

 serve to keep the pollen dry. 



In a series of classical experiments, Darwin 

 showed that self-pollination, or the placing of the 

 pollen on the stigma of the plant that produced it, 

 does not give as vigorous offspring as cross-polli- 

 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 (dioecious 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 fldwer 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 fiowers 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. 



When the supply of oxygen gives out, carbon 

 dioxid continues to be produced for a time, at the 

 expense of oxygen, which is in loose combination 

 with the tissues. This is accompanied by the for- 

 mation of alcohol. This process is known as intra- 

 molecular respiration. Respiration takes place in 

 every living cell, since every such cell has need of 

 energy to perform its work. In plants, each cell 

 absorbs its oxygen for the most part from the air 

 that enters the stomata, lenticels and cracks in the 

 bark, and penetrates everywhere into the spaces 

 between the cells. 



(2) The food is used to build tissues, cell-walls 

 and other parts. 



(3) It is stored in various special storage or- 

 gans, principally as starch, fats, oils and proteids. 

 In the germination of seeds we can see very clearly 

 that the stored food, before it can be used, must be 

 digested just as in the animal body. Starch is 

 changed to sugar and proteids are converted into 



