THE NATURE OE PHOTOSYNTHESIS 63 



of the elliptical cross-section of a perfectly open sunflower stomatum to 

 be 0.0000908 sq. mm. which is equal to the area of a circle 0.0107 mm. 

 in diameter. Their number varies widely, 40 to 300 stomata to the 

 square millimeter according to the species with as high as 700 in some 

 cases. The number of openings in a single leaf thus runs to enormous 

 numbers, a medium sunflower leaf containing about 13 million stomata. 



The morphology and details of the functioning of the stomata can- 

 not be entered upon here.^ Suffice it to state that the cells bordering the 

 openings, the guard-cells, are capable of controlling the size of the open- 

 ing in response to certain external and internal conditions. It must be 

 borne in mind that the stomata do not serve only for the ingress of 

 carbon dioxide into the leaf, but as well for the egress thereof when 

 the plant is in the dark as well as for the corresponding egress and 

 ingress of oxygen and the escape of water vapor. Thus, for instance, 

 when the stomata close under conditions of extreme heat the rapid loss 

 of water is prevented, but at the same time the conditions are more 

 unfavorable for the absorption of carbon dioxide. In experimental work 

 on photosynthesis it is essential that changes in the size of the stomatal 

 openings of this sort be constantly borne in mind in order to avoid 

 epurious results. 



The factors effecting the opening and closing of the stomata are of 

 an extremely complex nature. A very important role in these movements 

 of the stomata is played by the starch grains in the guard-cells. The 

 fluctuations in the starch-content of these cells do not run entirely parallel 

 to that of the rest of the leaf, and the behavior of the chloroplasts of 

 the guard-cells is in many respects quite different from that of the chloro- 

 plasts of the other cells. The movements of the guard-cells appear to 

 be intimately connected with variations in their osmotic pressure. This 

 in turn is brought about by the hydrolysis and formation of the starch 

 through enzyme action. The influence of various factors on the starch 

 economy of guard-cells has been studied by Iljin * who ascribes great 

 importance to certain kations and the hydrogen ion concentration in 

 this phenomenon. Many external factors are of influence on the move- 

 ments of the stomata. In general, light results in the opening of the 

 stomata and darkness in their closing. This is, however, not a universal 

 rule, for after prolonged darkness the stomata may be wide open. Lloyd ^ 

 found that in blue light stoma^^a open but not so much as normally or as 

 in red light. 



Considerable difference of opinion still exists regarding the effect of 

 air enriched in CO2 on the condition of the stomata. Linsbauer ^ concludes 

 that increasing the COo-content of air results in a closing of the stomata, 



' Lloyd, F. E., The Physiologv of Stomata. Pub. No. 82. Carnegie Institution 

 of Washington (1908). 



* Iljin, W. S.. Biochcm. Zcit., 132, 494, 511, 543 (1922). Wiggans, R. G., Am. 

 J. Botany. 8, 30-40 (1921). 



''Lloyd, I.e., 114. 



"Linsbauer, K., Flora, 109, 100 (1916). 



