242 EXPERIMENTS WITH PLANTS 



stem under eosin solution in a shallow pan, and cut 

 the stem underneath the liquid by a quick, clean stroke 

 of a sharp knife or scissors. After two or three min- 

 utes, remove the stem and determine how far the eosin 

 has penetrated. 



In what manner do the leaves exert this force ? We 

 have already learned (page 122) that the root-hairs 

 draw up water by reason of the water -attracting sub- 

 stances which they contain. We determined the water- 

 attracting power of the root -hairs by placing them in 

 weak solution of salt or sugar and increasing the 

 strength of the solution until it attracted water from 

 them and they became flabby; we may do the same 

 with the leaves, and we then find that it takes a much 

 stronger solution to attract water from them than from 

 the root- hairs. If, then, the leaves are so much richer 

 in water-attracting substances 1 than the root-hairs, the 

 most probable explanation seems to be that they 

 attract water from the root -hairs through the interven- 

 ing cells and ducts, and so supply what they lose by 

 evaporation. It is at present doubtful whether this 

 explanation can account for the rise of sap for more 

 than 150 to 200 feet. 2 



1 The youngest leaves, which are richer in water-attracting substances than 

 the old ones, are also the first to be colored when a short piece of stem is 

 placed in eosin; this indicates that they attract water more powerfully than 

 the older leaves. 



2 Two factors which are little understood as yet must be reckoned with; 

 the tensile strength of the water-column and the frictional resistance of the 

 ducts, tracheids, etc. 



