196 PLANT PHYSIOLOGY 



penetrates into the leaf more rapidly, and the network of con- 

 ductive bvuidles is considerably denser. It is of interest to note 

 that plants of the same species, when developing under different 

 light conditions, acquire a different structure. Even leaves of 

 the same tree will show marked shade or light characteristics 

 (Fig. 58), depending on whether they are from the north or the 

 south exposure, from the periphery or from the interior of the 

 crown. 



Shade plants differ from light plants also in regard to the 

 amount of chlorophyll, showing a considerably higher content 

 in the latter. Owing to this fact, they are able to utilize the 

 small quantities of light that have filtered through sun-loving 

 plants. Moreover, with the same light intensity, assimilation 

 goes on in them at a higher rate than in light plants, as may 





Fig. 59. — ^Comparative size of plastids in sun and in shade plants: a, Taxus 

 (shade); b, larch (sun); c, locust (sun); d, beech (shade) {after Luhimenko) . 



be seen from the broken line in Fig. 56. But with high intensity 

 of light, their thin leaves, which are insufficiently supplied w^th 

 water by the fine network of vascular bundles, cease to increase 

 their assimilative activity at an earlier moment, and the curve 

 proceeds parallel to the horizontal axis of the coordinates. The 

 chloroplasts of shade plants usually differ from those of light 

 plants in their larger dimensions (Fig. 59). 



It has been impossible as yet to find an exact numerical expres- 

 sion for the degree of light adaptation of a given plant, as in 

 natural habitats they develop under light conditions that vary 

 continuously in accord with the diurnal and annual fluctuation 

 of solar radiation. One of the first attempts to ascertain the 

 relative minimal amount of light in which plants are able to 

 develop was made by Wiesner. He determined the intensity of 

 light during the noon hours in the most shaded habitats, where 

 certain plants occur, and compared it with the intensity of light 

 in an open place. The fraction obtained shows with what part 



