140 THE PHYSIOLOGY OF STOMATA. 



of starch by the same plastids, and the ratio between the two functions may 

 and probably does vary with the amount of chlorophyll present. On this 

 view it may be seen that the movements of the stoma are not wholly and 

 directly dependent upon the process of photosynthesis within the guard-cell 

 plastid, and there are thus brought into harmony the dissenting explanations 

 of various workers in this field; and the activity of stomata which have 

 no chlorophyll, or very little, is thus explained. The disappearance of starch 

 from the guard-cell in the early part of the morning, and its oft-observed per- 

 sistence in darkness, especially in plants in which a supply of food materials 

 is available, compels the assumption of a mechanism of digestion of a different 

 kind from the ordinary diastase of the leaf. The presence of such an agent 

 has not been demonstrated, though its existence had been previously 

 suspected (Kohl). The difference lies in its absence or inactive condition 

 during the night, and its marked activity in the early part of the day, in 

 sharp contrast to the inverse order of activity shown by leaf diastase. It 

 is believed that a clear understanding of this assumed ferment will enable 

 us to account for the movements of stomata which have remained unex- 

 plained by the generally prevalent photosynthetic theory. 



The relation of stomatal activity to normal, though high, temperatures 

 and to the very low and presumably trying relative humidities of the desert 

 appears to be the same as to these conditions anywhere, and there is nothing 

 which may be seized upon as affording an illustration of adaptive behavior 

 particularly suited to the supposedly unkind climate to which the plants 

 studied are subject. Ewart (1897) in his studies of the effect of tropical 

 insolation, came to a somewhat analogous result with reference to other 

 matters. As long as a water-supply is available, and so long as the plant is 

 kept cool by transpiration, it suffers no more in the tropics than in temperate 

 regions. The same may be said of desert plants. There is at present, at 

 any rate, no evidence that the behavior of the stomata tends to mollify the 

 rigor of the environment. Many plants of the desert have, to be sure, many 

 peculiar anatomical features connected with the stoma, to which, however, no 

 regulatory behavior may be attributed. By virtue of such characters, the 

 plants of the desert may claim their lease of existence where many others 

 would succumb. That the activities of such apparently very important 

 organs as the stomata, though subject to the exigencies of the desert, have 

 not changed adaptively in the course of uncounted centuries makes it appear 

 improbable that plants have become gradually fitted to this environment, 

 but that, being fitted, they have survived. 



