THE LEAF 161 



you can of the number of leaves on each tree, and calculate the number 

 of kilograms of water it would give off at that rate in a day. 



EXPERIMENT 64. THROUGH WHAT PART OF THE LEAF DOES THE WATER 

 GET OUT ? Take some healthy leaves of tulip tree, grape, tropseolum, 

 or any large, soft kind attainable. Cover with vaseline the leafstalk and 

 upper surface of one ; the stalk and under surface of a second ; the stalk 

 and both surfaces of a third, and leave a fourth one untreated. Suspend 

 all four in a dry place by means of a thread attached to the petioles so 

 that both surfaces may be equally exposed. The leaves must be all of 

 the same species, and as nearly as possible of the same age, size, and vigor, 

 and care must be taken that none of the vaseline is rubbed off in handling. 

 Examine at intervals of a few hours. Which of the leaves withers soonest ? 

 Which keeps fresh longest? From what part would you conclude, judg- 

 ing by this experiment, that the water escapes most rapidly ? 



179. Transpiration, nutrition, and growth. We learn 

 from the foregoing, and from Exps. 58 and 59, that plants 

 give off moisture very much as animals do by perspiration. 

 The two processes must not be classed together, however, 

 for they are physiologically different. The action, in plants, 

 is called transpiration. It is usually assumed that a large 

 amount of water must pass through the plant in order to 

 bring to it the necessary supply of food material ; but since 

 the entrance of mineral salts is brought about by osmosis, 

 conditioned by the living cells of the root; and since osmosis 

 of salts may take place in a direction opposite to that of the 

 greater movement of water, it follows that the entrance of 

 salts is independent of transpiration. 



Inasmuch, however, as a certain amount of water is 

 necessary to bring the living cells into a condition of turgor 

 (7) so that they may grow, it follows that there is a relation 

 between transpiration and growth. If transpiration exceeds 

 absorption for any length of time, the tissues will be de- 

 pleted of their moisture, as is shown by the wilting of crops 

 in dry, hot weather; and if the unequal movement continues 

 long enough, the plant will die. Hence, a knowledge of the 

 laws governing this important function is necessary to all 

 who are interested in cultivating agricultural products. 



