THE TRANSFERENCE OF STIMULI 181 



may possibly have importance in this respect, whereas the light and heat 

 produced by plants exert a negligible formative influence. 



Chemical influences. It has already been mentioned that the distribution 

 of food directly and indirectly affects metabolism and growth, although it is 

 doubtful whether the correlation between shoot and root is maintained in 

 this manner. Specific metabolic products might be employed for this 

 purpose, or the chief part might be played by stimuli transmitted by the 

 interprotoplasmic connexions. 



That growth can be influenced by special substances is strikingly 

 evidenced in the formation of galls, and probably tropistic and localized 

 chemical stimuli are frequently employed for the attainment of specific 

 shape. It is, however, incorrect to suppose that a specific stimulatory 

 substance is required for each form of growth, one for a leaf, one for a stem, 

 another for a root, and so on. 



Mechanical influences may also act directly or as stimuli, and the 

 internal needs as well as the external demands may contribute towards 

 inducing the development of mechanical tissue. It is further possible that 

 self-regulation involves a power of discrimination between stationary tension 

 and pressure and strains due to movement. 



The supply of water affects turgidity, which again exercises me- 

 chanical or stimulating influences. Special stimuli may also arise from 

 transpiration, the movements of water, and related factors. 



SECTION 52. The Transference of Stimuli. 



Symbiotic organisms show that pronounced formative results may be 

 produced without the existence of living continuity, but nevertheless the 

 protoplasmic connexions existing in all vascular plants appear to be of 

 primary importance for the transmission of correlating stimuli, and to be of 

 little or no use for translocation. It can in fact be shown that a naked 

 non-nucleated mass of protoplasm forms a cell-wall around itself if connected 

 with the nucleated portion by a fine protoplasmic thread, whereas the 

 closest contact is insufficient for this purpose when the thread has been 

 broken. The precise importance of living continuity in maintaining cor- 

 relation during growth and adult life has not yet been determined, but 

 it is at least certain that these cytoplasmic threads may convey stimuli 

 of internal as well as of external origin. The passage of stimuli may be 

 -due to either (i) the transference of special stimulating substances, (2) the 

 transference of living particles of protoplasm (pangens), or (3) to the propa- 

 gation of physical or chemical changes. 



A transference of protoplasmic particles is probably of common 

 occurrence, and may take place in the absence of permanent protoplasmic 



