THE WORK OF STEMS 241 



It is of course impossible to estimate from the re- 

 sults of the above experiment how much pressure is 

 required to force water to the top of a Pine tree fifty 

 or one hundred feet high, but we know that it must be 

 a much greater amount than is needed to force it 

 through a short branch such as we have used. We have 

 at least found out that a powerful force is at work. 



Is the water raised by a force acting from above 

 (i. e., pulled up), or by a force acting from below (i. e., 

 pushed up), or by both methods? As we have already 

 learned, there is in the leafy branch a powerful force 

 at work lifting the water, and this too when the branch 

 is completely separated from the root. In this case 

 the force must reside in the stem or in the leaves. 

 Determine the amount and rapidity of sap -flow in a 

 leafy branch, as compared with a similar branch 

 deprived of its leaves (and with the wounds caused by 

 their removal sealed with sealing-wax) . Does the result 

 indicate that the force resides principally in the leaves? 



It would appear probable from this and other experi- 

 ments which have been made that the water is pulled 

 up by a force acting from above. If this were the case 

 it would create a partial vacuum in the ducts whenever 

 the supply of water from the roots ran low. Do we 

 find any evidence of such a state of things ? Allow a 

 vigorous plant (Squash, Sunflower, etc.) to wilt 

 slightly, thus showing that the supply of water from 

 the root is running low. Now bring a part of the 



p 



