THE ORGANIZATION OF THE INDIVIDUAL PLANT 



141 



Thus, regardless of the position in which the seed may happen to lie, 

 the developing root will always tend to penetrate the soil instead of 

 growing upward or sideways. As the radicle pushes downward, the 

 elongating hypocotyl, bearing the cotyledons and plumule, grows up- 

 ward, 1 its reactions being just opposite to those of the radicle. Upon 

 emerging into the light, the cotyledons, together with the rest of the free 

 parts of the plant, become green. 

 This enables them to assist the 

 plumule and stem in manufacturing 

 food, besides yielding the store that 

 they already possess. As the de- 

 veloping leaves take over food 

 manufacture, the cotyledons waste 

 away and are ultimately shed. 



The principal parts of the plant 

 body are already differentiated in 

 the seed — root and stem (which to- 

 gether form the axis) and leaves. 

 These parts are analogous to the 

 organs of a higher animal, since 

 each consists of many types of spe- 

 cialized cells grouped into tissues, 

 and since the arrangement of the 

 tissues is such as to fit the entire 

 structure for the performance of 

 specific tasks. The roots anchor the 

 plant, take in soil water and its dis- 

 solved substances, and store manu- 

 factured food. The stem supports 

 the leaf spread and transports 

 water and other raw materials to, 

 and manufactured food away from, 

 the leaves; it is often also a stor- 

 age place for food. The leaves capture the energy of sunlight and 

 use it to manufacture food, act as the chief organs of respiration, and 

 largely control the evaporation of water. The flower and the fruit are 

 organs adapted to the purposes of reproduction. Buds are embryonic 

 leaf and flower shoots in a state of arrested development, awaiting the 

 advent of favorable conditions; in this respect, they resemble the resting 

 embryo in the seed. Like the seed, they are enclosed in a resistant coat, 

 in this instance composed of overlapping scales. 



1 In many plants, however, only the plumule emerges from the ground, the hypo- 

 cotyl and cotyledons remaining buried. 



Fig. 9.4. Terminal and lateral buds of the 

 lilac, showing bud scales. (Photo by Prof. 

 E. B. Mains.) 



