708 COMPARATIVE ELECTRO-PHYSIOLOGY 



responses, of which the first were negative, the second part 

 diphasic, and the last portion reversed positive (fig. 213). 



Looking at the phenomenon of digestion, we see that it 

 consists first of a secretory process, by which certain solid 

 substances are dissolved, and secondly of the absorption of 

 these dissolved substances. Similar functions are subserved 

 in vegetable life by the root, by which solid inorganic food- 

 materials are first dissolved by secreted acids, and then 

 absorbed. The proof of the former is seen in the well- 

 known corrosion-figures produced by growing rootlets on a 

 marble surface. I have also been able to demonstrate the 

 phenomenon of excitatory secretion in young roots by allow- 

 ing them to absorb dilute salt solution, and then under exci- 

 tation to secrete it into highly dilute silver nitrate solution' 

 This last was attended by the visible formation of a white 

 precipitate. The electrical response of young roots of 

 Colocasia, moreover, I found to be by induced galvanometric 

 negativity (fig. 214), which, under long-continued stimulation, 

 was apt to show reversal to positivity. The older roots, on 

 the other hand, under the same intensity of stimulation, gave 

 response by galvanometric positivity (fig. 215). The former 

 of these responses, there is every reason to believe, is as- 

 sociated with secretion, and the latter with absorption. 



This question of the absorption of inorganic food materials 

 by the plant is naturally connected with the subject of the 

 Ascent of Sap, which is regarded as one of the most difficult 

 problems in plant physiology. The non-physiological theories 

 advanced are admittedly inadequate to the explanation of 

 this phenomenon. That the ascent, nevertheless, could not 

 be due to physiological action was held to have been proved 

 by the facts (i) that water-conduction takes place pre- 

 ferentially through sap-wood, assumed to be dead ; and (2) 

 that poisonous solutions, such as would kill a living tissue, 

 have been found to be transported through the roots, or the 

 cut ends of their trunks, to the tops of trees. 



I have, however, been able to show that these objections 

 are not valid. For in the first place, the supposed dead 



