24 CONTROL OF TRANSPIRATION 



Only a small portion of the water is required and the surplus 

 is forced out of the eel's and finds its way in the form of vapor 

 along the intercellular spaces in the spongy mesophyll through 

 the stomata to the air. The transpiration of these large volumes 

 of water must be of importance also in keeping down the tem- 

 perature of the plant during the burning summer heat. Trans- 

 piration is often compared to the evaporation of water from a 

 dish. While it is controlled to a limited extent in the same 

 manner as evaporation, it should be borne in mind that the giving 

 off of water is intimately associated with the vital activities of 

 the cells and that the loss of water is to a degree under the control 

 of the plant. The cuticle, which is practically impervious to 

 water and gases, extends as a thin coat over all parts of the 

 plant body, and materially assists in controlling the amount of 

 transpiration. As a result of this water-proof coat the vapor 

 can only escape from the leaf through the stomata. These minute 

 openings are one of the most interesting features of the leaf. 

 When there is an adequate supply of water the elliptical guard 

 cells are drawn apart and give free exit to vapors and gases. 

 However in cases of drought when a continued loss of water 

 would prove harmful to the plant, or when an interchange of gases 

 is no longer required then the guard cells close and so offer such 

 a barrier against further loss of water that only the severest con- 

 ditions, such as prolonged heat and drought, can overcome. It 

 has been claimed that the stomata do not regulate transpiration 

 because it has been observed in some cases that the stomata are 

 not closed when wilting begins or fully opened at the time of 

 maximum transpiration. Further observation is required to 

 settle this question and for the present we must believe that so 

 elaborate a mechanism as the guard cells is of significance. The 

 stomata in the majority of cases range in size from .0002 sq. mm. 

 to .0008 sq. mm. so that in comparison a needle prick would 

 appear as a huge hole, but they are so numerous, 40 to 300 to the 

 sq. mm., as to comprise about i per cent, to 3 per cent, of the 

 area of the leaf surface. It might be questioned if this extent 

 of opening is sufficient to permit the entrance and exit of the 

 large volumes of gases and water handled by the plant. It has 



