Active Causes of Organic Evolution 



191 



of all three enmronal forces, acting on or stimulating the young 

 cells of the radicle. The three radicles proenviron (to coin 

 a suitable verb) exact and yet different pathways, before even 

 the slightest external indication is shown. As every plant 

 and animal physiologist knows, the length of time needed 

 for proenvironing determination has been termed the excitation 

 or latent period. The future "fate for weal or woe" of the 

 plant may largely depend on the proenvironed pathway pro- 

 jected by the protoplasmic or chromatin molecules that con- 

 stitute the energids. 



But let us pursue the investigation further. As the plumule 

 or young stem disengages itself from the seed coat, in an open 

 exposed situation, it at once plots out a proenvironal course 



Fig. 4. — Seedlings of pea. a, plumule uniformly exposed to light; h, plu- 

 mule of plant placed transversely in dark box Avith light admitted by slit s; c, 

 plumule of upright plant in box. The shoots of b and c show resultant re- 

 sponse, in varying degree, to apogeotropic, apohydrotropic and heliotropic 

 stimuli. 



that ordinarily is apogeotropic, positively heliotropic, and 

 apohydrotropic. So the actual pathway pursued is as seen 

 in Fig. 4, a. But the second, that was placed under a bell- 

 jar, pursues the course of Fig. 4, b, while the third devel()])s 

 a direction as in Fig. 4, c. Further, when the fully formed 



