Chap. Ill] 



LIGHT 



59 



m 



WH 



already less intense, by reason of the oblique incidence of the rays, is reflected 

 to a greater extent from the surface of the water. The means of protection 

 against illumination are less marked in terrestrial plants and usually 

 coincide with those that are intended to restrict transpiration, so that it 

 seems hardly possible at present to decide against which of the two dangers 

 a definite protective device originated. Among such, for instance, may be 

 placed the manifold movements and the fixed lie of the leaves in relation 

 to light, by means of which they escape the direct incidence of the sun's 

 rays ; in addition there are coatings of hair, smooth strongly reflecting 

 surfaces, various foldings, and so forth 1 . 



The effects of light on plants depend not only on the amount, but also on 

 the quality of the illumination. The different kinds of rays are of unequal 

 physiological signifi- 

 cance, and, since they 

 are unequally ab- 

 sorbed by the air and 

 the aqueous vapour, 

 the question of the 

 comparative efficiency 

 of the several parts 

 of the spectrum is 

 not unimportant in 

 phyto-geography. 



The less refrangi- 

 ble half of the visible 

 spectrum, from the 

 red to the commence- 

 ment of the green, 

 contains the rays that 

 are most effective in 

 reducing carbon di- 

 oxide by means of the 

 chlorophyll. It is not 



yet clearly decided whether the efficiency is greatest in the red rays, corre- 

 sponding to the broadest absorption bands in the chlorophyll, or in the 

 yellow rays, as many investigations render probable, but this uncertainty 

 has no important bearing on the question before us 2 . The manufacture of 

 chlorophyll depends on the presence of yellow or orange-yellow light. The 

 blue and violet rays are the most effective in the assimilation of nitrates 3 . 



1 Wiesner, II ; Johow, op. cit. 



2 These questions are thoroughly discussed in all manuals of physiological botany 

 on the basis of the researches of Pfeffer, Reinke, Engehnann, and others. 



3 Regarding the assimilation of nitrates and the formation of organic calcium-com- 

 pounds depending on this, see Schimper, I and II. 



Fig. 38. Chylocladia reflexa. A Superficial cell with a small 

 reflecting plate, seen from above. B Side view of a similar cell 

 magnified 450. After Berthold. 



