THE FOOD OF THE PLANT 



21 



be made up of a number of little parts like elongated 

 boxes in shape. 



These are the "vessels" of the wood; they do not 

 contain any cell-sap or any solution similar to the 

 sugar solution experimented with in the thistle-funnel, 

 or the acid cell-sap of the root-tip; that is, in the 

 vessels of the wood, there is no osmotically active 

 substance. 



How then can the water pass upwards through the 

 wood of the stem against the force of gravity, which 

 must always be pulling it back again ? The following 

 questions naturally present themselves: Can it be 

 pulled up from above, or forced up from below ? 



Experiment 9 



Aim. — To find out if the leaves exert any pull which 

 helps the water to rise in the stem. 



Method. — For this experiment two leafy twigs are 

 required. The twigs must be as nearly alike as possible 

 so that fairly comparable results may 

 be obtained. To this end it is well 

 to cut them at the same time and 

 from the same branch. 



Two test-tubes are filled with water 

 to the same level and the level in 

 each case is carefully marked. A 

 small quantity of some light oil such 

 as cedar- wood oil is then poured over 

 the water so that none of it may be 

 lost by evaporation. 



All the leaves are stripped from 

 one of the branches, and the branches 

 are then put into the test - tubes 

 (Fig. 7). 



Observations. — The water in the 

 test-tube which contains the leafy 

 branch diminishes at a much greater 

 rate than that in the test-tube in which the leafless 

 branch is standing. 



Fia. 7 



