1002 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



or almost sufficient, for light saturation during most of the day, unless the 

 sky is heavily overcast. On the other hand, the carbon dioxide supply, 

 under natural conditions, may often be less constant than it is in labora- 

 tory experiments with a circulating gas containing 0.03% carbon dioxide 

 and this may cause considerable variations in the rate of photosynthesis. 

 In quiet open air, a carbon dioxide-deficient air layer will form around the 

 plants, and cause a decline in rate, while the exhalations of the ground, 

 which contain carbon dioxide produced by the decay of organic matter and 

 by the respiration of the roots, can be caught in the foliage, and can provide 

 an increased supply of carbon dioxide (c/. page 902). A certain amount of 

 carbon dioxide, of the same origin, also may reach the leaves with the trans- 

 piration flow from the roots (c/. page 910). 



Thus, variations in the supply of carbon dioxide, under natural condi- 

 tions, may be considerable, and these variations, more than alterations in 

 the intensity of illumination, may cause the rate of photosynthesis of 

 land plants to vary from location to location, and to fluctuate with time 

 in a given location. The same appears to be true of the multicellular 

 aquatic plants. As mentioned on page 878, Gessner (1937) found that the 

 dechne in rate of photosynthesis of higher aquatic plants with time, re- 

 ported, among others, by Arnold, was largely caused by insufficient circu- 

 lation of water. Although the carbonates contained in natural waters 

 (particularly hard waters) represent a considerable reserve of carbon di- 

 oxide, the diffusion of dissolved electrolytes is so slow that a carbon dioxide- 

 deficient alkaline layer can easily be formed around a submerged plant. 



Gessner (1938) found {cf. Table 28.V) a rather low value (7.0 mg./hr. 100 cm.^) 

 for the maximum rate of carbon dioxide reduction by Potomageton crispus in hard tap 

 water; the value for Potomageton perfoliatus was even lower (4.8 mg. C02/hr. 100 cm.^). 

 These yields are less than one third of those of land leaves in ordinary air, and five to 

 ten times smaller than the maximum yields produced by land leaves provided with an 

 abundant supply of carbon dioxide. This indicates that Gessner's water plants might 

 have been in a "carbon dioxide-limited" state, despite the relatively high carbonate con- 

 tent of the medium. (Gessner attributed the low yield of aquatic plants per unit area 

 to the fact that these leaves consist of only a few layers of cells; however there is no in- 

 dication that the light absorption in the leaves of aquatics could be three or five times 

 smaller than in ordinary leaves.) 



Unicellular algae, w4th their extremely favorable ratio of surface to 

 volume, are unlikely to feel any deficiency of carbon dioxide supply, at 

 least as long as the average carbonate concentration in the medium is high 

 and stirring sufficiently intense. Since these algae form a vast majority of 

 the organisms living in the sea, we can conclude that the total photosynthe- 

 sis of the marine flora — in contrast to that of the continental flora — is not 

 much affected by limitations of carbon dioxide supply. 



