ASSIMILATION OF CARBON BY PLANTS 



195 



sion of photosynthesis. In respect to their response to hght 

 intensity, all plants may be divided into two large groups, light 

 plants and shade plants, but no sharp line of demarcation can 

 be drawn between these two. They are connected by transition 

 forms, or shade-tolerant plants. 



Light plants develop the better the more they receive direct 

 sunlight. They will not tolerate much shading, and they will 

 grow, under natural conditions, exclusively in open places. The 

 common weed of prairie regions, Amaranthus 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 



-Leaves of the hazelnut: a and h, leaves exposed to the sun; c, d, e, 

 shaded leaves {after Hesselman from Lundegardh) . 



in its development and loses its capacity to bloom. On the 

 other hand, shade plants, e.g., Teucrium scorodonia, commonly 

 found in the woods of western Europe, shows marked signs of 

 depression when grown in open places. According to the experi- 

 ments of Combes, Teucrium thrives when it receives but one-third 

 of the total dayhght, or under conditions where Amaranthus is 

 hardly able to exist (Fig. 57). 



Light plants differ from shade plants in many anatomical as 

 well as physiological characteristics. The leaves of hght plants 

 are thicker. This thickness is produced by the greater develop- 

 ment of the palisade parenchyma often arranged in two to three 

 layers. Frequently, in such plants, the palisade 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 



