38 



TEXTBOOK OF PLANT PHYSIOLOGY 



cally (marking on one axis the quantity of carbon dioxide which is 

 decomposed during a time unit, and on the other the intensity of 

 light), for instance, in fractions of direct sunlight falling on the 

 surface of the earth in the noon hours of a bright summer day, it 

 will be seen that the curve of the dependence of assimilation on 

 the amount of light will show, in general, a logarithmic character 

 (Fig. 13). 



Not in all plants, however, is the dependence of assimilation on 



the amount of light expressed by a curve of this nature. In those 



peculiar to open, sunny habitats, assimilation increases until the 



intensity of light has attained the intensity of full sunlight (the 



6 



c 4 



£ 



if) 



< 2 



<NI 



o 



V60 '/ZO VlO V 6 V 4 



L i gh+ Intensi+y 



'/3 



Fig. 13. — Dependence of assimilation on the intensity of light in a light plant 

 (Nasturtium) and a shade plant (Oxalis) {after Lundegardh) . 



continuous curve in Fig. 13). In plants adapted to shaded habi- 

 tats, as for instance Oxalis (the broken curve in Fig. 13), assimila- 

 tion increases until light has attained a comparatively low inten- 

 sity, or about one-tenth of full illumination. Further increase of 

 light remains without effect or even induces a depression of the 

 assimilation process. In respect to their response to light intensity, 

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

 shade plants. Similar to other ecological groups, no sharp line 

 of demarcation can be drawn between these two. They are con- 

 nected by transition forms, or shade-tolerant plants. 



Light plants develop the better, the more they receive direct 

 sunlight. They will not tolerate the slightest shading and they 

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





