GENERAL BOTANY 



and the air. Fig. 1, a, illustrates this placing of the organs 

 of a young, growing plant in the positions usually assumed 

 by them, and we shall find that these positions are, in part at 

 least, directly traceable to the effects of environmental forces 

 acting on the plant body. We may note in the figure that 

 the main taproot grows vertically downward toward the center 



of the earth, but 

 that the lateral, 

 or secondary, roots 

 assume quite a 

 different position, 

 growing at more 

 or less definite an- 

 gles from the pri- 

 mary taproot. The 

 main stein likewise 

 grows in a direc- 

 tion exactly oppo- 

 site to that of the 

 main root, as it 

 should do in order 

 to perform its as- 

 signed task of dis- 



FIG. 1. Two seedlings of the scarlet-runner bean 



a, a seedljng growing in a normal position with its organs 

 properly adjusted to light and soil ; b, the stem (hypocotyl) 

 has adjusted itself hy curvature in response to gravity act- 

 ing as a stimulus 



playing the leaves 

 to the sunlight 

 and the flowers to 

 wind and insects. 

 The student has doubtless noticed also that the leaves of plants 

 may assume the normal horizontal position illustrated in the 

 figure, or, in the case of plants before a window, the leaf blades 

 may take an oblique position, so that they face the maximum 

 light. If the plant represented in Fig. 1, a, had been grown 

 in a flowerpot laid on its side, as in 6, or if it had started its 

 growth on a steep hillside, the roots, stem, and leaves would 

 have turned so as finally to assume the most favorable relation 

 to sunlight, air, and soil. Botanists have demonstrated the fact, 

 which we shall explain more in detail later, that growing roots, 



