360 COMPARATIVE ELECTRO-PHYSIOLOGY 



besides the water coming from the roots, an additional 

 supply available from the lateral reservoirs, The loss of 

 water thus sustained by the cortex during the day is made 

 up again at night, when it will suck water outwards from the 

 central reservoir. We have here a case analogous to the 

 action of the excitatory tissue of the pulvinus of Mimosa 

 expelling water into the wood on excitation, and re- 

 absorbing it on the cessation of excitation. The occurrence 

 of these reactions in the cortex explains the observation 

 made by Kraus that the organs of the plant diminish in bulk 

 from morning to afternoon, the reverse process taking place 

 from afternoon to morning. 



We have thus seen how important a factor is excitatory 

 reaction in the observed movements of water, even on the 

 supposition that the woody tissue, being dead, is a merely 

 passive agent. The question has still to be attacked, 

 however, whether this assumption, so generally made, is 

 correct, that the wood used for conduction of water is dead. 

 This supposition has arisen from the chemical transformation 

 undergone by the protoplasm in woody vessels. We have 

 seen, however, in the case of the epidermal cells of the skin, 

 that it is possible for chemical transformation to occur, 

 without necessarily being accompanied by the death-change. 



Before proceeding to inquire whether the conducting 

 woody channels are really dead, it is desirable to say a few 

 words as to the particular tissues in the wood, which are 

 most effective for this purpose. Many experiments have 

 been carried out to determine this. Among other things 

 various staining fluids have been employed. But an objec- 

 tion raised in the case of some of these has been that the 

 water of such solutions travels faster than the dye dissolved 

 in it. For my own part, I have found the employment of 

 dilute solution of phenolpthaline to be exceedingly delicate 

 and useful for the purpose of this investigation. It is 

 perfectly colourless, and the staining appears only after 

 appropriate development. The cut end of the stem is placed 

 in this dilute solution and left for some time. Transverse 



