57 



>upply of CO,,. Wilh a lighl-intensily oC ,\, lo .,',, Ihc culmiiialing 

 poinl is already leached by Ihe laller, while in the casc of \asliir 

 tium Ihe curve slill lends slighlly upwards al .1 liglil. In holli 

 cases Ihe assiniihilion is obviously ahnost direclly proporlional lo 

 Ihe inlensity of light with intensities under /„ ^'„. Whilc, Ihcii, 

 the curve of Ihe shade-planls — in good agreemenl wilh IJlack- 

 man's theory — passes över inlo a more or less hori/.onlal Mne, 

 indicative of the limiling elTecl of the carbon dioxide, Ihe curve of 

 the sun-plants has a gentler course, most nearly rehaled lo tlial of 

 a logarilhmic i iirve. 



The curve for a green alga reccnlly ohlained h\ O. Wamiu lu; 

 1 1919, p. 253) seems once more to point lo a bimolecuhu- reaclion. 

 AVarburg considers it reasonable to assume thal the velocitv of 

 the assimilation is proporlional to the CO., concenlralion and lo 

 Ihe concenlralion in anolher subslance wilh which Ihc carbon 

 dioxide reacls. It remains lo be seen whelher this explanalion can 

 lay claim to be of universal application. In higher planls more 

 especially, a complicaling factor is inlroduced in Ihe dilTusion of 

 the carbon dioxide from the outer air Ihrough the stomata, inler- 

 cellular syslem, and cell walls, and il is obvious thal the assimila- 

 tion can under no circumslances proceed more rapidly than is 

 permitled by Ihe resislance of dillusion. O. Warburg (1919, p. 

 254) has poinled oul the possibilily thal in regard lo curves of 

 the Blackman l}'pe (see Blackman and Smith 1911) it might be 

 the resislance of dillusion, and not, as Blackman Ihinks, Ihe small 

 CO.> concenlralion, Ihal acled as Ihe sharply limiling factor. In 

 the shade-planls — as I hope lo show in anolher place — the 

 proportion between the absorbing surface and the mäss of the 

 chlorophyll bodies is lower Ihan in the sunplants, which undoubt- 

 edly poinls to the correctness of O. Warhurgs explanalion. 



Ecologically the shade planls are suiled to the low light-intensily 

 of the förest, and the curves also show Ihat in lighls under j',, lo 

 2^y Ihey ^vork as economically as the sun-planl Xastniiium. But 

 even in the darkened förest the lighl varies very strongly. and niay 

 in the sunniesl places reach values of .\ to h And since every 

 spol of ground, excepl to Ihe norlh of the Irees, is Ht up by direcl 

 sunlighl several limes during Ihe course of the day in the passage 

 of the sunbeams Ihrough the leaves, the shade-i)lants can only 

 utilise Ihe favourable moments if the CO.^ concenlralion is higlior 

 than 0.57 per litre. As will be shown in the nexl seclion. il aclually 



