TRANSPIRA TION. 



39 



membrane tlien wliicli is only i-lowl}- permeable to the water of the cell-sap 

 is here a very important factor in checking tlie too rapid losi of water from 

 the leaves. 



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

 are all connected, and that the slomata, where they occur, open also inlcj 

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

 the intercellular spaces to escape when the slomala are open. 



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

 vapor wlien 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 w.iter supplied the plant by ro(jt 

 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 st^lution 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 wiU. The stem may tlien 

 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 wa)-: Place the cut end of a leafy shoot in 



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



rn 1 ■ r 1 TT n • i i n i Experiment to show 



fill this arm of the U tube with water, and add mercury lifting power of trans- 



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, 



