ABSORPTION OF FOOD BY PLANT 357 



no vital actions take part in it, then it is obvious that we 

 have only an incomplete knowledge of the causes at work 

 and of the relationship of the different factors concerned.' l 



The inadequacy of these theories to explain the ascent 

 of sap has, then, been freely admitted. With special refer- 

 ence, further, to that of osmosis, I shall myself be able to 

 show that the movement of water often takes place in the 

 plant, in a direction contrary to what it would be if osmosis 

 alone were involved. The fact that the absorption of water 

 is not a merely passive process, but a phenomenon connected 

 with irritability, will be further shown in the depression of 

 the rate of water-movement by such conditions as depress 

 irritability, whereas the opposite circumstance will be found 

 to enhance it. 



We are thus driven to examine the possibility of a 

 physiological explanation of the ascent of sap. On such 

 a theory it must be supposed to be brought about by the 

 action of stimulus, inducing reactions expressed in the re- 

 sponsive movement of water. The objections made to the 

 physiological explanation have already been recapitulated. 

 They are (i) that the movement of water is known to take 

 place rapidly, and by preference, through wopdy {issues 

 which are supposed to be dead ; and f2lj:hat when the roots 

 have been killed by hot water, or when poison is supplied, 

 the transport of sap continues to take place. I shall now 

 proceed to examine these arguments, and to show that the 

 objections raised, though apparently so strong, are not really 

 valid. 



We shall first refer to the argument which has been 

 based upon the fact that in trees conduction takes place 

 very rapidly through woody portions which are regarded as 

 dead. It is not, it must be noted, implied by this that 

 the presence of wood is essential to the ascent of sap, in- 

 asmuch as even in trees there are tracts of living cortical 

 tissue in the roots which have to be traversed before the 

 water can reach the woody tissues. In seedlings of Gramince 



1 Pfeffer, Physiology of Plants (English translation), 1903, p. 224. 



