296 LUTHER BURBANK 



The absorption of water by the roots of the 

 tree, and its elevation through the trunk to sup- 

 ply the deficit made by constant transpiration 

 from the pores of the leaves is a phenomenon 

 that has been perfectly familiar to botanists for a 

 long time. It was demonstrated experimentally 

 by Stephen Hales early in the eighteenth cen- 

 tury. But the forces that lie back of the phe- 

 nomenon have been very little understood. 



Very recently one of the most celebrated 

 American botanists has declared that the cause 

 of the rise of sap in trees remains perhaps the 

 most interesting of botanical puzzles. 



It is, in effect, as some one has pointed out, a 

 case of water running up hill, and many botan- 

 ists have found it mystifying that the plant tis- 

 sues are able to withstand the pressure that a 

 column of water must exert, particularly in the 

 case of tall trees. 



THE RISE OF SAP IN THE TREE 



In fact, however, it should be recalled that the 

 sap in the tree is not carried in open tubes com- 

 parable to the arteries of the animal system. 



If it were in such tubes, doubtless no plant 

 tissues could withstand the pressure that would 

 be exerted by the weight of the column of water, 

 carried, let us say, to the top of a redwood tree. 



