ABSORPTION OF FOOD BY PLANT 351 



\ (2) whether or not the ascent of sap is fundamentally due to 

 similar excitatory reactions. 



With regard to the latter of these questions it may be 

 stated here that the nature of the efficient cause of the 

 ascent of sap is universally regarded, in plant physiology, 

 as constituting a problem of the greatest obscurity. The 

 various non-physiological theories which have hitherto been 

 advanced are admitted to be inadequate, as we shall see later. 

 We are thus confronted either with an insoluble problem or 

 with the necessity of finding physiological reactions which' 

 will account for the ascent of sap. 



As regards the latter of these alternatives, however, ob- 

 jections apparently very serious have been brought forward. 

 Against the physiological character of the action it has 

 been urged (a) that wood, being supposed to be dead, 

 could take no part in the ascent of sap. It is known 

 moreover (b) that killing the roots with boiling water does 

 not prevent the ascent of sap. And, lastly (c) y in the well- 

 known experiments of Strasburger it was found that strongly 

 poisonous solutions can be carried to the tops of trees. From 

 these facts it has been held to be proved that the ascent of 

 sap cannot be dependent on the livingness of the tissues 

 concerned. 



But if, on the other hand, it could be shown that these 

 objections were not valid, and if, further, some crucial experi- 

 ment were devised to demonstrate that excitatory action was 

 attended by a concomitant responsive movement of water in 

 the tissue, it might then be claimed that the physiological 

 theory of the ascent of sap had been established on a firm 

 basis. The attempt to do this will form the subject of the 

 next chapter. 



The first question that falls within the scope of our 

 investigation, then, is as to whether the reactions of the root 

 are or are not similar to those of digestive organs in general. 

 We have seen, in the case of the latter, that as there are two 

 opposite activities, of secretion and absorption, so also there 

 are two opposite responsive phases, negative and positive, the 



