46 GERMINATION 



Among external stimuli, none perhaps is so essential, or so 

 universally and easily available to green plants, as energy 

 of 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 stimulus. The expanded lamina is thus not merely a 

 specialised structure, for the purpose of photo-s3Tithesis, 

 but also a sensitive 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." 



That hypothesis, however, is not supported by the fact 

 that the venation of the leaf is of negative sign, whereas 

 light energy cannot be otherwise than of positive sign ; 

 the air sign. 



Professor Bose favours the absorption theory, animal 

 and plant are accumulators, dependent upon some outside 

 source of energy for their activating force. Acceptance of 

 that theory would not only involve complete disregard of 

 the electrical interchanges between atmosphere and earth 

 which are constantly taking place, and their unchanging 

 polarities, but would enforce the belief that Nature places 

 sole dependence upon a form of energy which not only varies 

 in intensity almost from moment to moment, but which, for 

 an appreciable part of each twenty-four hours practically 

 ceases to exist. 



Upon the testimony of the galvanometer the venation of 

 the leaf of any earth-grown plant is always negative, and 

 it is much more reasonable to postulate that if light-energy 

 is absorbed, as I have no doubt it is, in combination with 

 air, absorbed, it is a function of the aerolae and not of the 

 venation of the leaf. The earth-current may be shut off 

 by the leaf being removed from the plant, or the tree, and 

 light may be temporarily shut off without the leaf djing. 

 But so long as it is not deprived of air it will continue to 

 live until, by reason of loss of moisture, there is complete 

 cessation of conductivity. Up to that moment the aerolae 

 take in positive charge and the venation remains, by induc- 

 tion, negative. 



