ABSORPTION AND MOVEMENT OF WATER 151 



Kain and dew collecting over the stomata or passing into 

 the intercellular spaces would prevent the diffusion of gases 

 except as they are dissolved in the water. This disadvant- 

 age is avoided in the first place by placing the stomata ordi- 

 narily on the lower side of the leaf, the one least likelj to 

 be wet by rain. It is further avoided by the fact that when 

 the leaf is wet, transpiration virtually ceases while absorp- 

 tion may continue for a time, thus producing such a degree 

 of turgor in the guard and other epidermal cells that their 

 expansion closes the stomata. 



Certain plants living in extremelj^ moist places, where the 

 danger of excessive transpiration at any season is reduced 

 to the minimum, are unable to close their stomata. Herbs, 

 shrubs, and even trees (notably the willows) possess this 

 character. It is for this reason that these plants cannot 

 bear transplanting to dryer places for cultivation, but are 

 distinctly swamp plants. 



GASES AND THE MOVEMENT OP GASES 



If the plant or any part of it dries — that is, loses water 

 faster than it absorbs it — air takes the place of the evapo- 

 rated water. If the ceU-walls and intercellular spaces are 

 equally permeable to gases and to water-vapor, air will pass 

 in as rapidly as evaporation removes the water, but if 

 water-vapor makes its way through cellulose, cuticula and 

 cork faster than air, drying wiU tend to produce in the 

 plant a gas pressure lower than that of the surrounding air. 

 If again the component gases of the air diffuse and diosmose 

 at different rates, the evaporating water will be replaced by 

 nitrogen, oxygen, and carbon-dioxide in proportions different 

 from those in which they occur in the air. But if the living 

 cells of the plant use any of these gases, the composition of 

 the air in the plant will be influenced by this means also. 

 In all living cells of higher plants under normal conditions 

 oxygen is consumed in respiration and carbon-dioxide is 

 liberated, usually in equal volumes. In aU chlorophyll-con- 

 taining cells carbon-dioxide is consumed and oxygen lib- 

 erated, under the influence of light. Carbon-dioxide diffuses 

 and diosmoses more rapidly than oxygen, and the latter 



