26 THE STOMATA 



proportion to the ratio of transpiration to absorption. So we 

 find that it becomes thicker as the volume of water given off 

 comes nearer and nearer to the volume absorbed. In this way it 

 serves as a very effective check in preventing a fatal loss of water. 

 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 conditions, 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 transpira- 

 tion. 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 been shown that a mem- 

 brane pierced by sufficiently small openings which are separated 

 from one another by a distance of at least ten times their diam- 

 eter, permits diffusion of gases as readily as though there were 

 no membrane at all. The structure of the epidermis is an 

 admirable example of such a -perforated membrane and it is so 

 perfectly adjusted to the work in hand that it could accomplish 

 much more than the necessities of the plant demand. 



14. Noteworthy Features of Leaves. — Now that some idea has 

 been gained of the nature and extent of the work performed by 

 the .leaves we are prepared to comprehend the meaning of the 



