98 



PLANT SOCIOLOGY 



Oxalis shows a shade-plant curve, which, like the curves of Melan- 

 dryum rubrum and Circaea alpina, becomes horizontal at about 3^o 

 full sunshine. Light intensities above 3^o have little or no increased 

 value for these shade plants. Nasturtium gives the assimilation curve 

 of a sun plant, in which the optimum is not shown in Fig. 52, that is, 

 the curve has not yet become horizontal. 



The universally ''logarithmoid" (suggesting a logarithmic curve) 

 course of the light curve of assimilation led Lundegardh, 1925, p. 41, 

 1930) to the formulation of his law of relative assimilation: "Increase 

 in intensity of light by a certain increment induces an increase in 

 intensity of assimilation which is the greater the nearer the light is to 



to ^ 



Light in fensify 



Fig. 52. — Ecological curve for the rate of assimilation in a light plant and a shade plant. 



{After Lundegardh.) 



the minimum." In the minimum region, that is, with very dim light 

 (mostly under Mo o^ full sunlight), the relative effect of the light factor 

 is the greatest. In the maximum region, with high intensity of light 

 (}-^i~M of total sunlight), the relative effect is greatly reduced, or zero. 

 Lundegardh (1925, 1930) has discussed the autecological signifi- 

 cance of this assimilation curve. It may be added that, according to 

 the studies of Henrici (1918) on carbon assimilation of alpine plants, 

 specific assimilation curves for species of wide distribution are not 

 especially significant. Individuals of the same species from the 

 lowlands and from the alpine heights show different curves. The 

 individuals of the plains begin to decompose carbon dioxide with light 

 of very low intensity. In the intense illumination of the full alpine 

 sunlight the assimilation curve of the alpine individuals continues to 

 rise while that of the plains individuals has long bent to the horizontal 

 or even dropped below. Perhaps these results, like those of Bonnier 

 (1920), have to do with the influence of alpine climate upon the plant 

 as a whole — with individuals of the same species to be sure, but with 

 different ecotypes. 



