TRANSPIRA TION. 



39 



membrane then which is only slowly permeable to the water of the cell-sap 

 is here a very important factor in checking the too rapid loss of water from 

 the leaves. 



By an examination of our leaf section we see that the intercellular spaces 

 are all connected, and that the stomata, where they occur, open also into 

 intercellular spaces. There is here an opportunity for the water vapor in 

 the intercellular spaces to escape when the stomata are open. 



84. Action of the stomata. — Besides permitting the escape of the water 

 vapor when the stomata are open they serve a very important office in regu- 

 lating the amount of transpiration. During normal transpiration the stomata 

 remain open, that is, when the amount of transpiration from the leaf is not 

 in excess of the supply of water to the leaves. But when the transpiration 

 from the leaves is in excess, as often happens, and the air becomes very dry, 

 the stomata close and thus the rapid transpiration is checked. 



85. Transpiration may be in excess of root pressure. — If the supply of 

 water from the roots was always equal to that transpired from the leaves 

 during hot, dry days the leaves would not become flaccid and droop. But 

 during the hot and dry part of the day it often happens that the trans- 

 piration is in excess of the amount of water supplied the plant by root 

 pressure. 



86. Negative pressure. — This is not only indicated by the drooping of 

 the leaves, but may be determined in another way. If the shoot of such a 

 plant be cut underneath mercury, or underneath a strong solution of eosin, it 

 will be found that some of the mercury or eosin, as the 

 case may be, will be forcibly drawn up into the stem 

 toward the roots. This is seen on quickly splitting the 

 cut end of the stem. When plants in the open cannot 

 be obtained in this condition, one may take a plant 

 like a balsam plant from the greenhouse, or some other 

 potted plant, knock it out of the pot, free the roots from 

 the soil and allow to partly wilt. The stem may then 

 be held under the eosin solution and cut. 



87. lifting power of transpiration. — Not only does 

 transpiration go on quite independently of root pressure, 

 as we have discovered from other experiments, but 

 transpiration is capable of exerting a lifting power on 

 the water in the plant. This may be demonstrated in 

 the following way: Place the cut end of a leafy shoot in 

 one end of a U tube and fit it water-tight. Partly 

 fill this arm of the U tube with water, and add mercury 

 to the other arm until it stands at a level in the two 

 arms as in fig. 37. In a short time we note that the mercury is rising in 

 the tube, 



Fig- 37- 

 Experiment to show 

 lifting power of trans- 

 piration. 



