THE LEAVES 81 



the whole cell and not set apart in separate bodies, 

 as in the higher forms. Masses of pond-scum, with 

 bubbles of oxygen attached, are easily found in 

 almost any pond on a summer day. 



99. Tlie leaf and loater. — All of the food taken 

 up by the roots is dissolved in one to ten thousand 

 times its weight of water, and is carried in this form 

 up through the stem and out into the leaves, where 

 it is used with the sugars to build up protoplasm or 

 make reserve foods. Some of this great amount of 

 water may be used also, but the greater proportion 

 is not needed further, and since there is no method 

 for its return to the roots it must be thrown off. 

 This must be done principally in the form of vapor. 

 The excretion of watery vapor constitutes the sec- 

 ond important function of the leaf. The process is 

 termed transpiration. 



100. Course of the ivater in the leaf — The water 

 supply comes through the petiole, of course, and 

 when it reaches the lamina it divides into numerous 

 streams, each one of which follows a nerve or rib 

 and goes out into all of its branches. This may 

 be demonstrated if a leaf of the maple or poplar is 

 cut from the stem of a living tree, and the base of 



