THE PLANT BODY 25 



As the embryo emerges from the seed the plumule is curved 

 and the leaves are seen to be borne on the first internode of the 

 stem above the cotyledons, which is known as the epicotyl. The 

 hypocotyl has meanwhile elongated and is continued as the pri- 

 mary root. The curved condition of the plumule protects its deli- 

 cate leaves from being broken off as it pushes up through 

 the soil. The curvature is not due to gravity but to unknown 

 internal causes. 



As soon as the embryo emerges from the soil, the plumule, 

 now composed of several nodes and internodes, straightens and 

 finally assumes an erect position. As the leaves expand they 

 respond to the stimulus of light and take up a favorable posi- 

 tion for the reception of the maximum amount of light for the 

 manufacture of starch. 



The roots, meanwhile, respond to gravity in such a manner 

 that the lateral roots grow out at an angle from the vertical 

 taproot and so permeate a large area of soil from which to draw 

 water and soil salts. 



The mature seedling (Fig. 13,/), through these various ad- 

 justing movements, is thus admirably adapted to securing from 

 the environment the energy, gases, water, and salts necessary 

 to its growth and development. 



It is quite evident that the above movements and changes 

 in the position of the various organs of the growing seedling 

 are quite independent of the division of the stem into nodes 

 and internodes and of the cyclic or spiral arrangement of the 

 leaves. These structural arrangements are, however, closely cor- 

 related to the adjusting movements, so that the architectural 

 plan and the unfolding movements work together for a better 

 final adjustment of the organs in the older seedling. 



Mature plants do not usually manifest such definite and obvi- 

 ous movements as seedlings, and yet there is abundant evidence 

 in adult plants of all kinds that the ultimate position of their 

 growing organs, and their more perfect adjustments to the 

 environment, are brought about by a correlation of body plan 

 with adjusting movements which are caused by external and 

 internal stimuli. 



