THE LIFE OF A PLANT 269 



oxidation. In the apple leaf there are 24,000 pores to every 

 square inch, and in the leaf of the black walnut there are 

 300,000 to the square inch. 



The important work of starch making in leaves will be 

 considered later (302). 



301. Presence of Water in Plants. If seeds are placed 

 on damp blotting paper, the root hairs of the first root will 

 turn downward to the surface of the damp paper, even if the 

 end of the root does not touch it. Several leaves may grow 

 in these conditions; this shows that water passes through the 

 root hairs into the root, and then up the root, stem, and 

 veins of the leaf, until at last it reaches the cells in the in- 

 terior of the leaf. 



Capillarity and osmosis aid in this rise of liquids through 

 the cells of the root and the stem. Capillarity makes the 

 liquids rise along the sides of the slender tube-like cells, 

 while osmosis causes them to pass from one cell to the 

 next. Root pressure is the name given to the sum 'of all 

 the agencies by which liquids are raised in opposition to 

 gravity. 



Water is constantly passing up into the leaves, where a 

 large part of it is given off or evaporated through the leaf 

 pores. We can test this fact by placing a sheet of rubber on 

 the ground around a plant, and inverting a jar over the 

 plant. On cooling the jar, we find that vapor condenses on 

 the inside. 



Experiments with sunflower plants have shown that a 

 single plant often gives off a quart of water a day, while a 

 single birch tree with its greater number of leaves may give 

 off 800 quarts in the same length of time. Grass may give 

 off as much as 13,000 quarts per acre every twenty-four 

 hours. (LABORATORY MANUAL, Exercise XXVII.) 



302. The Starch-making Process. Plants, of course, 

 do not elevate water simply for the sake of evaporating it 

 from their leaves. They put water to several important 



