FUNCTIONS OF VEGETAL NERVE 553 



a responsive movement is induced. These are facts which 

 can be demonstrated by shielding the lamina and pulvinus 

 alternately from the action of light. When the lamina alone 

 is exposed there is no action ; but when it is the pulvinus 

 to which light is admitted, there is an immediate responsive 

 movement. 



The lamina, it is true, is provided with a fine fibro-vascular 

 network containing the nervous strands. But the stimuli 

 received by this extensive system are ultimately conducted 

 along the thicker channels of which it forms the terminal 

 ramification, and serve to stimulate the plant as a whole. 

 When such stimuli reach the petiole, then, they cannot act in 

 :hat direct and unilateral manner in the motile organ which 

 is required for the responsive movement of the leaf. The 

 petiole, it is true, from its dorsiventral character, is unequally 

 excitable on its two sides. But since, in the process of the 

 transference of stimulus from the nervous to the ordinary 

 elements there is a great loss, and since, moreover, the motile 

 tissues in the case of most petioles are very sluggish, the 

 diffusely transmitted stimulus induces practically no directive 

 effect. Nor qould there, in any case, have been any com- 

 parison between the effective strength of an external 

 stimulus acting directly and unilaterally, and a transmitted 

 stimulus acting diffusely. 



It will thus be seen that conduction through specialised 

 nervous elements is by no means the essential factor in 

 bringing about those numerous directive curvatures which 

 subserve so many important functions in the life of the plant. 

 The question, therefore, as to what is the part in the economy 

 of the organism played by the vegetable nerve still remains 

 to be answered. 



One very obscure problem in connection with Vegetable 

 Physiology is that of Correlation. Thus various complex 

 activities may be set up in one part of the plant, when 

 another part, more or less distant from it, is subjected to the 

 variation of some excitatory influence. In this way every 

 part of the plant-organism would appear to be en rapport with 



