PLANT PHYSIOLOQT, 101 



phenomenon, though not a simple one. It must not be 

 forgotten that the water in plant-cells contains many sub- 

 stances in solution, and consequently evaporates less rapidly 

 than pure water, in accordance with well-known physical 

 laws. Moreover, the attraction of the substance of the 

 cell-walls for the water counteracts, to some extent, the 

 tendency to evaporation ; and in the same manner, even to 

 a greater extent, the water is prevented from passing off by 

 the "imbibition power" of protoplasm. It is, in fact, 

 impossible to deprive cellulose and protoplasm of all their 

 water in dry air at ordinary temperatures. 



177. In submerged aquatics there is of course no loss of 

 water by evaporation ; it is only in aerial plants or parts of 

 plants that such a water-loss occurs. In the latter the 

 exposed parts are protected against the dry air by the epi- 

 dermal layer of cells, nearly impervious to water. More- 

 over, those plants which are exposed to drier air have a 

 thicker epidermis, while in those living in moist air the epi- 

 dermis is always thinner. These facts show that evapora- 

 tion of water is not necessary to the life of the plant, and 

 that, on the contrary, the loss of water is carefully guarded 

 against. 



178. The breathing-pores of the green and succulent 

 parts of higher plants, when open for the ingress and egress 

 of gases, permit the escape of some moisture. They are 

 placed over intercellular spaces, and these are in communi- 

 cation with the intercellular passages of the plant, which 

 are filled with moist air and gases. Now, when the breath- 

 ing-pores are open, these gases expand and contract with 

 every change of temperature or atmospheric pressure, thus 

 permitting the escape of considerable amounts of water; 

 when, on the other hand, the breathing-pores are closed, 



