146 THE BIOLOGY OF FLOWERING PLANTS 



have appeared which, if they receive confirmation, may 

 necessitate some modification of the hmiting factor hypo- 

 thesis. In the first place, both Lundegardh (1921, 1922) and 

 Harder (1921) claim to have shown that assimilation rate is 

 not directly proportional to light intensity ; the graph 

 representing these relations is not a straight line but a 

 curve. Blackman's figures, which are supported by results 

 of Willstatter, are definitely at variance with this. It seems 

 as if the experimental methods of Lundegardh and Harder 

 may be at fault. The fundamental temperature relation 

 does not seem to have been established. Lundegardh used 

 a closed air space, and Harder, working with submerged 

 plants, used a solution of sodium bicarbonate ; in both 

 cases the carbon dioxide concentration must fall off during 

 the experiment, though it need not reach a limiting value, 

 and in the latter there must be other changes, e.g. in hydrogen 

 ion concentration. Lundegardh 's published figures indicate 

 very large experimental errors. It may be noted, however, 

 that for " aurea " plants Willstatter found the graph for light 

 to be a curve, and that Warburg (19 19) found the Hght- 

 assimilation graph to be a curve for certain algae. The other 

 result obtained by Lundegardh and Harder is that two 

 factors may influence the rate of assimilation at the same time. 

 Lundegardh found, for instance, that a carbon dioxide 

 concentration which was limiting for a fight intensity 

 of one-fortieth sunlight, yet gave a higher assimilation 

 rate with light of one-twentieth, and a still higher rate 

 with light of one-fourth sunlight. The much more exact 

 results of Matthaei and of Blackman and Smith do not 

 agree with this, though perhaps the number of experiments 

 bearing on this point is too small and the results not 

 sufficiently uniform to give definite refutation. Smith 

 (191 9) writes : " It is conceivable — and indeed probable — 

 that when, so to speak, two factors are close to the limiting 

 value a change in the one not limiting may have some 

 appreciable effect on assimilation. This will show itself 

 about the inflexion of the curve when the limiting factor 

 is changing. For example, when carbon dioxide is limiting. 



