126 PHOTOSYNTHESIS 



be accurately controlled. For this reason many of the older investigations, 

 in which the influence of the other factors was not recognized or in which 

 these are described in uncertain terms, can contribute but little. 



The determination of the influence of varying concentrations of car- 

 bon dioxide on the photosynthetic activity of land plants is associated 

 with many more difiiculties than is the case with aquatic plants. These 

 difficulties are largely due to the more complex structure of land plants ; 

 the opening and closing of the stomata are occasioned by water supply 

 of the leaf, temperature, light and carbon dioxide-concentration and are 

 of profound ini'uence on the gaseous exchange of the leaf, and hence, 

 of course, also on the rate of photosynthesis. It is therefore not surpris- 

 ing that some of the earlier work is difficult to interpret. 



Brown and Escombe ^^^ found a direct proportionality between car- 

 bon dioxide-concentration and rate of photosynthesis. However, the num- 

 ber of experiments was small and there was some indication of patho- 

 logical effects. They also reported an increase in the chlorophyll-con- 

 tent in plants intermittently exposed to higher concentrations of carbon 

 dioxide. This has also been found to be the case by Lundegardh. An 

 increase in chlorophyll-content would of itself tend to increase the photo- 

 synthetic activity. 



Lundegardh, ^^'* working with leaves of land plants, has arrived at very 

 similar conclusions to those of Warburg and Harder. Lundegardh's 

 curves are very much like those of Harder. In Table 21 are given the 

 results of experiments on the photosynthetic rate with varying carbon 

 dioxide-concentrations. 



TABLE 21 



Photosynthesis with Varying C02-Concentrations and Light Intensities 



ViQ, -5'L'O, AND 1/4 that OF DiRECT SuNLIGHT, IN TeRMS OF CO2 FiXED. 



(From Lundegardh.) 



Photos jTithe sis per SO Sq. cm. 

 per Hour- 



CO. 2 J_ 1 



Concentration 40 20 4 



Plant Per Cent mg. mg. mg. 



Oxalis acctosclla 0.03 0.45 LI 2.3 



0.06 0.9 2.3 2,.7 



0.09 L3 2.5 4.9 



0.12 1.6 2.7 6.3 



0.24 1.6 — — 



Stellaria memorum 0.03 0.7 LO L5 



0.06 0.9 L7 2.3 



0.09 1.0 2.4 3.0 



0.12 L3 4.0 3.9 



0.24 1.9 — 5.3 



""Brown and Escombe, Proc. Rov. Soc. London. V, 70, 397 (1902). Kreusler, 

 Lamhc. Jahrb., 16, 711 (1887); 17. 161 (1888); 19, 649 (1890). Giltay, E., Ann. 

 Jard. Buitcnzorg., 15, 43 (1898). 



"♦Lundegardh, H., Svensk. bot. Tidsskrift.. 15, 59 (1921): Biol. Ccufralbl., 42, 

 337 (1922). "Der Kreislauf dcr Kohlensaeure in dcr Natur." Jena, 1924, p. 74. 



