134 THE PHYSIOLOGY OF STOMATA. 



If the theory above outlined is, in its chief conception, true, the tempera- 

 ture and moisture relations are not difficult of understanding, although the 

 behavior of stomata in prolonged darkness still offers some difficulty. The 

 gradual disappearance of starch when the plant is starved is possible only 

 if an agent of starch dissolution is present, and the appearance of this material 

 may be the result of this starvation. The probable slight movement of sto- 

 mata in darkness demands also the presence of the ferment in small quantities, 

 though it is possible that the osmotic adjustments between the cells of the 

 stomata and epidermis on the one hand and the cells of the epidermis and 

 chlorenchyma on the other during darkness may account for the movement. 



The behavior of the stomata which do not close at night, belonging, accord- 

 ing to Darwin, to perhaps 13 per cent of land plants, remains to be explained, 

 but it seems clear that these difficulties are less troublesome on the theory 

 that photosynthesis is of relatively less importance than has hitherto been 

 thought. And there is no real objection to supposing that the ferment which 

 is normally rendered active in the early part of the day is present in small 

 quantities and is proportionally active during the night, and more so in some 

 plants than in others. These points, while they must ultimately be explained, 

 are of subsidiary importance to the main question. 



The higher normal temperatures, according to Kohl and Francis Darwin, 

 favor the opening of the stomata. I have not, for my own part, examined 

 by experimentation ad hoc into this relation, but my observations of the 

 rates of stomatal opening and starch dissolution at different times of the year 

 seem to leave little doubt that the lower normal temperatures of spring delay 

 these processes from 2 to 3 hours. This may be understood in the light of the 

 conditions under which enzymatic processes take place. In the spring, when 

 the minimum temperatures range near 45 F. and the daily maxima are 40 

 to 50 lower than in the summer, the whole action of the plant is sluggish, 

 as is evidenced by the slower growth. The difference in the quality of the 

 insolation may, however, be in part responsible for these differences, and it 

 will require further research to separate these factors. 



As regards the effect of relative humidity, the case seems to be compara- 

 tively simple with respect to the principle involved. So long as the tissues 

 of the plant are supplied with a sufficient water content, the stomata may be 

 expected to behave normally. If it happens that the vapor-tension is so 

 low as to make it difficult for the plant to keep up the supply, or, what amounts 

 to the same thing physiologically, if the water-supply at the roots is reduced 

 too far, wilting ensues, and the stomata close concomitantly. The stomata 

 do not prevent wilting ; they only manage to conserve the water after wilting 

 has occurred. Although Francis Darwin's results and mine are not wholly 

 in accord on certain other points, especially matters of fact, I may express 

 my own views in this regard in his words "they [the stomata] close or 



