2o8 



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 

 transpiration adaptively to the 

 external conditions; and thus 



lower figure shows diagrammatically in they haVe USUally bcCU regarded 

 cross section the method by which the 



turgescent rounding of their cavities by botauists. But this COUCCp- 



opens the stoma, — the dotted walls .. ■, , ■, j. • j u 



showing the closed, and the unshaded tiou has uot been sustamed by 



walls the open position. (The upper j^^^^. g^udieS, wMch haVe shoWU 

 figures reduced from a wall-chart by ' 



L. Kny, and the lower from a much- go much irregularity, and cveu 



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. 



Fig. 71. — Typical guard cells, with a 

 stoma between them, highly magnified, 

 in surface view and cross section. The 



