TRANSPIRATION. 4 1 



serious injury tlirough 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 ac- 

 cumulation of certain acids in the cell-sap which cannot be got rid of by 

 transpiration. Under these conditions some of tlie cells liere swell out 

 forming extensive cushions, and the cell walls become so weakened 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 \\'ill begin to exude from the margins of the leaves. 



90. It should be stated that in reality there is no difference between trans- 

 piration 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 automat- 

 ically 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. Bemonstration of stomates and intercellnlar 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 blowing 

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

 immersed in water. The air is forced out through the stomata and rises as 

 bubbles to the surface of the water. At the close of the experiment some of 

 the air bubbles will still be in contact with the leaf surface at the opening of the 

 stomata. The pressure of the water gradually forces this back into the leaf. 

 Other plants will answer for the experiment, but some are more suitable than 

 others. 



