Law of Proenvironment 217 



has wholly or largely been canceled, by the chemotropic or 

 food searchmg stimulus. Thus, whether a seed be placed 

 with the embryonic radicle directed toward the bark or toward 

 the earth, the radicle in elongating bends toward and pene- 

 trates the bark, while showing no perceptible gravic response. 

 In this case the apoheliotropic and hydrotropic may possibly 

 aid the chemotropic stimulus in determining the proenvironal 

 pathway that is most satisfying. 



It would be impossible in the present work to enter into 

 a detailed description or discussion of the perception of gravity 

 by root tips and the propagation of the stimulus to the re- 

 sponsive region further back where growth and bending occurs, 

 in cases where the root has been placed transversely or ob- 

 liquely downward toward gravity. The papers by Charles 

 and Francis Darwin, by Czapek, Haberlandt, Noll, Newcombe, 

 and others, are all quoted in recent works on plant physiology. 

 While the writer believes that mechanic stimulation by sliding 

 starch-grains in cells of the root cap or tip may aid in geo- 

 perception, histological studies he has made cause him to regard 

 fundamental changes in constituents of the chromatin, that 

 he has termed geoenergids, as the prime determining cause 

 (p. 120). The study is one of especial interest from the stand- 

 point of cell energetics. 



Stems and branches of vascular plants have been minutely 

 studied in their tropistic responses by Sachs, Darwin, Wiesner, 

 and other more recent authors. On page 191 a short refer- 

 ence was made to responses in seedling stems of bean or pea. 

 The rapidily growing extremities of stems of many flowering 

 plants are exceptionally favorable for proving that linked up 

 resultant responses are very frequent, almost invariable for 

 them. Only a few added cases can now be cited briefly. 



If the growing end of a French bean, a sensitive plant, or 

 Cuphea be grown in a pot in optmium and generally difl'used 

 light, it will grow vertically upward under the apogeotropic 

 and directly overhead heliotropic stimuli (Fig. 7, a). But, if the 

 pots be placed in a dark box into which light enters by a narrow 

 slit from a position to the right of Figure 7, b, the for- 

 merly cooperating stimuli that linked up to give a resultant 



