LOSS OF WATER 



45 



sap, the physiological process of secretion is also of very 

 great importance. Moreover, on this basis we are able 

 to account for the ascent of sap in submerged aquatic 

 plants, like eel-grass, pond-weed, and others, where tran- 

 spiration is not possible, or in land plants in very humid 

 tropical regions where the nearly or quite saturated air 

 greatly reduces transpiration or even wholly prevents it 

 for extended periods of time. In fact it may be shown, 

 experimentally, that a leafy branch can raise water through 



Fig. 35. — Experiment to show that secretory action in the cells of a 

 leaf are able to cause the rise of liquid in a branch, when evaporation from 

 the leaf-surfaces is impossible, i, Beaker containing solution of eosin; 

 2, cork; 3, inverted glass bell-jar containing water; 4, iron support. In 

 this experiment the eosin rose rapidly in the branch. (Modified from 

 H. H. Dixon.) 



the fibro-vascular bundles, even when submerged. The 

 apparatus is set up as shown in Fig. 35, where the leafy 

 branch, immersed in water in an inverted glass bell-jar, 

 has the cut end of the stem in a solution of eosin or red 



