214 



The Living Plant 



expressed in this form in order to bring the matter home to my 

 students. Now, strangely enough, the botanists are not yet 

 agreed either as to the source of the energy or the precise physical 

 method by which this considerable work is accomplished; and in 

 default of precise information I can only present to the reader a 

 synopsis of such data as we possess, along with some comments 

 on their probable bearing. And here follow the principal explana- 

 tions which have been offered for the physics of sap ascent. 



Fig. 72. — A generalized drawing of the tissues of a typical stem, showing the water- 

 carrying ducts (the three larger tubes), and a food-carrying sieve-tube (the single 

 dot-lined tube), with the associated tissues. (Copied from Kerner's Pflanzenlcben.) 



1. Root pressure. — In the preceding chapter it was shown that 

 roots absorb water osmotically and forcibly start it up the ducts. 

 But this pressure, which, in some greenhouse plants has been 

 found sufficient to raise water 40 to 50 feet, and in trees up to 

 80 feet, is wholly insufficient to explain the ascent when trees 

 reach 400 feet, as they do in some kinds of Australian Eucalyptus ; 

 and therefore this cannot be the explanation. 



