158 PLANT LIFE. 



of capillarity. It has been found that the bubbles of gas 

 here mentioned often exist under negative pressure, as shown 

 by the fact that a stem cut under mercury allows the mercury 

 to ascend for some distance within the vessels. This negative 

 pressure of the gases is due to the evaporation of water from 

 the leaves, and the most recent researches point to this as a 

 very important or even the chief factor in lifting the water. 

 That the movement is not a function of living cells is shown 

 by experiments in which stems of plants have been subjected 

 to poisonous agents, or heated for many hours to a degree 

 sufficient to kill all the living cells, yet without materially 

 affecting the supply of water. 



208. The loss of water. — Water is constantly evaporating 

 from the whole surface of the plant exposed to the air. Since 

 this loss is probably more or less modified by the vital activ- 

 ity of the plant, it has received the special name, transpira- 

 tion. 



209. Transpiration. — In the higher plants transpiration 

 from the surface is reduced by the waterproofing of the epi- 

 dermis, so that most of it takes place from the surfaces of 

 internal cells into the intercellular spaces, wherever these 

 exist. Since the intercellular spaces are connected with each 

 other and also, through the stomata, with the outside air, 

 water vapor is constantly passing off by diffusion. The leaves, 

 affording the largest exposure, are especially organs of trans- 

 piration. After they have become fully expanded no appre- 

 ciable amount of water is lost directly from their surfaces. 



210. Amount and regulation. — The amount of transpira- 

 tion, therefore, varies with the structure of the leaf rather 

 than with its area. The temperature, percentage of water 

 and movements of the air affect profoundly the rapidity of 

 transpiration. Hence arises the need of regulation by the 

 plant, to prevent excessive loss. The guard cells of the 

 stomata are irritable, so that external conditions affect their 



