244 EXPERIMENTS WITH PLANTS 



root-pressure is, as a rule, observable. We may there- 

 fore leave it out of account in this connection. 



The amount of water necessary to supply the leafy 

 crown of a lar^e tree must be very large, and we 

 might naturally suppose that all the wood of the trunk 

 is employed in conveying it. It is often noticed, how- 

 ever, that in old trees the heart- wood decays and 

 leaves the trunk hollow without affecting the health of 

 the tree. Moreover, on sawing off a tree or branch six 

 or eight inches in diameter and placing the cut end 

 in eosin solution, we find that the colored liquid does 

 not rise in the inner portion of the wood but only in 

 the outer. (In case this experiment is carried out, the 

 tree may be removed from the solution and allowed 

 to dry with its leaves on and will serve for years as 

 demonstration material.) The cells of the interior 

 portion of the wood are more or less stopped up with 

 various substances, and appear more deeply colored 

 than those of the outer portion; this inner portion is 

 called the heart-wood^ and the outer the sap-wood. 



If water is prevented from traveling in the sap-wood, 

 can it travel in the heart- wood instead ? We may 

 answer this question by cutting a ring-shaped groove 

 all around the tree or branch deep enough to go 

 through the sap-wood and prevent the water from 

 traveling in it. The wilting of the leaves will indicate 

 the extent to which the water current is interfered 

 with. No one who has seen this experiment carried 



