108 LECTURE VII. 



first instance upon the turgidity of the guard-cells. When 

 they are absorbing water, the only way in which they can 

 yield to the hydrostatic pressure set up within them is, inas- 

 much as they are firmly connected at their ends, by becoming 

 curved, and this necessarily produces a space between them, 

 corresponding to that part of their adjacent surfaces along 

 which they are not coherent. When the guard-cells are not 

 turgid they are straight, their adjacent walls are in contact, 

 and the opening of the stoma is obliterated. He concludes 

 that the influence of light upon the guard-cells is of such a 

 nature as to increase their power of absorbing water, pro- 

 bably by causing the formation within them of osmotically 

 active substances : in connexion with this it should be men- 

 tioned that the guard-cells always contain chlorophyll-cor- 

 puscles. Possibly, however, the effect is to be attributed 

 rather to an increased resistance to evaporation than to an 

 increased power of absorption. However this may be, we 

 can account, to some extent at least, for the great difference 

 between the diurnal and the nocturnal transpiration of a 

 plant, by the fact that its stomata are open during the day 

 and are closed during the night : their function is not to 

 check but to promote transpiration. 



These considerations naturally lead us on to enquire into 

 the relations existing between transpiration and the external 

 conditions of the plant generally. The hygroscopic condition 

 and the temperature of the air may be first considered. A 

 plant in an atmosphere saturated with moisture will not 

 exhale any watery vapour, provided that the temperature of 

 the plant is not higher than that of the air. It may happen 

 that, in consequence of great metabolic activity, the tempera- 

 ture of the plant is raised above that of the air, and then 

 transpiration will take place, although the air is at its point 

 of saturation. Under ordinary circumstances, when the tempe- 

 rature of the plant and that of the air do not perceptibly 

 differ, the activity of transpiration diminishes with increasing 

 moisture of the air ; but when the temperature of the air is 

 high and the proportion of moisture in it small, transpiration is 

 promoted. Further, transpiration is affected not only by the 



