4 6 



PHYSIOLOGY. 



serious injury through lesions of the tissues. This is brought about by the 

 cells at certain parts becoming charged so full with water through the 

 activity of root pressure and lessened transpiration, assisted also probably 

 by an accumulation of certain acids in the cell-sap which cannot be got 

 rid of by transpiration. Under these conditions some of the cells here 

 swell out, forming extensive cushions, and the cell walls become so weak- 

 ened that they burst. It is possible to imitate the excess of root pressure 

 in the case of some plants by connecting the stems with a system of 



water pressure, when very quickly 

 the drops of water will begin to 

 exude from the margins of the 

 leaves. 



90. It should be stated that in 

 reality there is no difference between 

 transpiration and evaporation, if we 

 bear in mind that evaporation takes 

 place more slowly from living plants 

 than from dead ones, or from an 

 equal surface of water. 



91. The escape of water vapor is 

 not the only function of the stomata. 

 The exchange of gases takes place 

 through them as we shall later see. 

 A large number of experiments show 

 that normally the stomata are open 

 when the leaves are turgid. But 

 when plants lose excessive quantities 

 of water on dry and hot days, so 

 that the leaves become flaccid, the 

 guard cells automatically close the 

 stomata to check the escape of water 

 vapor. Some water escapes through 

 the epidermis of many plants, 

 though the cuticularized mem- 

 brane of the epidermis largely prevents evaporation. In arid regions 

 plants are usually provided with an epidermis of several layers of cells to 

 more securely prevent evaporation there. In such cases the guard cells 

 are often protected by being sunk deeply in the epidermal layer. 



92. Demonstration of stomates and intercellular air spaces. A good 

 demonstration of the presence of stomates in leaves, as well as the presence 

 and intercommunication of the intercellular spaces, can be made by blow- 

 ing into the cut end of the petiole of the leaf of a calla lily, the lamina being 



Fig. 56. 



The roots are lifting more water into 

 the plant than can be giyen off in the form 

 of water vapor, so it is pressed out in 

 drops. From " First Studies Plant Life." 



