38 PLANT PHYSIOLOGY 



(photochemical) reaction. As will be seen in Chapter VI, these 

 two points of view may be reconciled if it develops, as seems prob- 

 able, that photosynthesis is carried on in at least two stages, one 

 of which is physical and one of which is chemical. Brown and Heise 

 thus may have been measuring one step in the process and Mat- 

 thaei another. 



Atmospheric Pressure. — Friedl (1902) showed that variations 

 in the atmospheric pressure exert a marked influence upon 

 photosynthesis. When the pressure is high, photosynthesis is 

 increased. 



Oxygen. — Since photosynthesis is a function of green plants, 

 which require free oxygen for their normal life processes, one would 

 expect oxygen to be necessary for photosynthesis. Briggs (1920) 

 found this to be the case, but Harvey (1928) reports that marine 

 algae can carry on photosynthesis when illuminated even in the 

 complete absence of oxygen. This matter should be studied fur- 

 ther. 



Leaf Structure. — Some leaves contain more stomata than 

 others and, although in most leaves the stomata are spaced so as 

 to permit of optimum diffusion (Chap. XVIII), leaves with stomata 

 on both sides are especially favored in this respect. If the stomata 

 are plugged by smoke and dust, inward diffusion of carbon dioxide 

 will be hindered. One of the chief sources of harm of smelter 

 smoke and dust is this plugging of the stomata, which more than 

 counteracts the beneficial effect of the increase of carbon dioxide 

 in the atmosphere of the vicinity. House plants are benefited by 

 an occasional washing of the leaves, which removes all collected 

 dust particles; and one of the benefits of a rain aside from increasing 

 the water supply is this cleansing effect upon the vegetation. 



Amount of Water. — Photosynthesis depends primarily upon 

 water, since in the manufacture of food the carbon of the carbon 

 dioxide is united to the water brought up from the soil; but, in 

 accordance with the law of limiting factors, increasing the water 

 supply of the leaf will increase the photosynthesis only up to a 

 certain point. Wilting leaves not only contain too little water to 

 photosynthesize efficiently but the wilting results in closed stomata 

 thus prohibiting the entrance of carbon dioxide. In the cowslip 

 (Caltha) and Hydrangea, however, the stomata remain open even 

 in the wilted condition and the leaves are still able to make some 

 starch. 



