ABSORPTION OF CARBON 39 



The common weed of prairie regions, Amaranthns retroflexus, may 

 be mentioned as an example of such plants. If this plant is 

 deprived of but one-quarter of the total daylight, it is considerably 

 checked in its development and loses its capacity of blooming. 

 On the other hand, shade plants, for instance, Teucrium scorodonia, 

 commonly found in the woods of western Europe, shows marked 

 signs of depression when grown in open places. According to the 

 experiments of Combes, Teucrium thrives when it receives but 

 one-third of the total daylight, or under conditions where 

 Amaranthus is hardly able to exist (Fig. 14). 



Light plants differ from shade plants by many anatomical 

 as well as physiological characteristics. The leaves of light 

 plants are thicker. This is produced by the greater development 

 of the palisade parenchyma. Frequently, in such plants, the pali- 

 sade parenchyma is formed not only at the upper side of the leaf 

 but also at its lower side. Moreover, the cells are of smaller size, 

 the number of stomata per unit surface is considerably greater, 

 carbon dioxide penetrates into the leaf more rapidly, and the net- 

 work of conductive bundles is considerably denser. It is of inter- 

 est 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 character- 

 istics (Fig. 15), depending whether they are from the north or the 

 south exposure, from the surface or from the depth of the crown. 



Shade plants differ from the 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 not been utilized and have been left 

 over by the sun-loving plants. Moreover, with the same light 

 intensity assimilation goes on in them at a higher rate than in 

 light plants, as may be seen from the broken line in Fig. 13. But 

 with high intensity of light, their thin leaves, which are insuffi- 

 ciently supplied with water by the fine network of vascular bun- 

 dles, 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 by their larger dimensions (Fig. 16). 



It has been impossible, as yet, to find an exact numerical 

 expression for the degree of light adaptation of a given plant, as 

 in natural habitats plants develop under light conditions that vary 



