558 



COMPARATIVE ELECTRO-PHYSIOLOGY 



as sensitive as those outside. The transmission of stimulus 

 by the plant, in such a way as effectively to maintain such 

 complex life-activities as motility and growth, even in the 

 absence of direct stimulation, is thus fully demonstrated, 

 And we may gather an idea from this fact of the funda- 

 mental importance, to the life of the plant, of those nervous 

 elements by which this is rendered possible. 



One of the most important functions of the venation ot 

 the leaf, not hitherto suspected, is now made clear to us- 



Among external stimuli, none 

 perhaps is so essential, or so 

 universally and easily available 

 to green plants, as energy ot 

 light. And we now see that 

 the fine ramification of fibro- 

 vascular elements over as wide 

 an area as possible in the leaf, 

 provides a virtual catchment- 



basin for the reception of stimu - 



lus. The expanded lamina is 

 thus not merely a specialised 

 structure, for the purpose of 

 photo-synthesis, but also a sen- 

 sitive area for the absorption 

 of stimulus, the effect of which 



is gathered into larger and larger nerve-trunks, in the course 

 of its transmission downwards into the body of the plant. 



And even in the interior of the plant the distribution or 

 these is such that no mass of tissue is too remote to be ex- 

 cited by the stimulus conducted through the nervous ele- 

 ments buried in them. How reticulated they may be, even 

 in the trunk, is seen in the accompanying photograph of the 

 distribution of fibro-vascular elements in the main stem of 

 Papaya (fig. 335). This network, of which only a small 

 portion is seen in the photograph, girdles the stem through- 

 out its whole length, and in this particular case there were 

 as many as twenty such layers, one within the other. 



F IG - 335- Distribution of, Fibro- 

 vascular Elements in Single 

 Layer of Stem of Papaya 



There are at least 20 such layers 

 engirdling the stem. 



