208 



The Living Plant 



tion, keeping it down to safe limits when water is scanty, but 

 allowing full play when water is plenty. The turgescence of the 

 guard cells, however, is influenced also in another way; for 



they (and they only of epidermal 

 cells), contain chlorophyll, which 

 has to make sugar in light and 

 thus increase their turgescence 

 and cause them to open the sto- 

 mata. This arrangement would 

 explain to perfection why light 

 increases transpiration so greatly 

 quite apart from any accompany- 

 ing heat, while a definite ecologi- 

 cal advantage seems equally 

 clear, viz., it should ensure open- 

 ing of the stomata at those times 

 when the demand for carbon di- 

 oxide is the greatest, and allow 

 them to close with the lessening 

 of this need. From the structure 

 of the guard cells, therefore, we 

 should expect them to serve 

 as automatic valves, regulating 

 FIG. 7i. Typical guard cells, with a transpiration adaptively to the 



stoma between them, highly magnified, ov * orT , Q 1 /.rmrlifirmc anrl fVmo 



in surface view and cross section. The external Conditions, and 



lower figure shows diagrammatically in {,heV have USUallv been regarded 



cross section the method by which the J 



turgescent rounding of their cavities by botanists. But this COnCCp- 



opens the stoma, the dotted walls . , , , , 



showing the closed, and the unshaded tlOn nas not been Sustained by 



!* studies, which have shown 



L. Kny, and the lower from a much- so muc h irregularity, and 

 copied diagram by Schwendener.) 



anomaly, in their action that we 



have to remain in doubt until further researches shall give us 

 the truth. Meantime we can only consider that any regulatory 

 action they may have is clumsy at the best. 



