FUNCTIONS OF VEGETAL NERVE 557 



maintenance of life. How, then, is the external stimulus 

 conveyed to these ? It is evident that this can only be 

 accomplished through the agency of the nervous elements. 



This fact, of the transmission of the excitatory effect of 

 an external stimulus from one part of the plant to another at 

 a distance, there to maintain the tonic condition, is again 

 still more clearly seen in the well-known experiment on the 

 sensitiveness of Mimosa when partially kept in the dark. If 

 one branch of this plant be covered by a dark box, while the 

 rest of it is exposed to light, it is found that the leaves of the 



I 



FIG. 334. Photographic Record showing Enhancement of Excitability 

 under Action of Light in Nerve of Fern 



First series, normal mechanical responses to electrical stimuli, in dark ; 

 second series, the same, taken under light ; third series, taken after 

 withdrawal of light. 



first undergo no loss of motile sensitiveness. It is thus 

 evident that the photo-tonic stimulus has been transmitted 

 from the illuminated to the unilluminated portions of the 

 plant, through conducting channels, in order to maintain the 

 normal excitatory condition. 



Next comes the very interesting experiment of Sachs, in 

 which a long shoot of Cticurbita was made to grow inside a 

 dark box, the" rest of the plant being exposed to light. The 

 covered part of the plant, under these circumstances, showed 

 normal growth of stem and leaves. Normal flowers and a 

 large fruit were also produced in the same confinement. 

 The tendrils inside the box, moreover, were found to be fully 



