:HAr. ml 



LIGHT 



59 



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

 o a fjreater extent from the surface of the water. The means of protection 

 i<Tainst illumination are less marked in terrestrial plants and usually 

 ;oincide with those that are intended to restrict transpiration, so that it 

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

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

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

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

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

 ;urfaces, various foldings, and so forth '. 



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

 he quality of the illumiuation. The different kinds of rays are of unequal 

 )liysiological signifi- 

 :ance, and, since they 

 u'c unequally ab- 

 sorbed by the air and 

 he aqueous vapour, 

 he question of the 

 romparativeefficiency 

 )f the several parts 

 if the spectrum is 

 lot unimportant in 

 jhyto-geography. 



The less refrangi- 

 ble half of the visible 

 ;pectrum, from the 

 ed to the commence- 

 iient of the green, 

 ;ontains the rays that 

 ire most effective in 



fHnrino- rarhnn HI J"'iG- 3S- Chylocladia reflexa. A Superficial cell with a small 



cuuv.11]^ Lrtiuuii ui jgflg^ting plate, seen from above. B Side view of a similar cell 

 )xideby means of the magnified 4.^0. After Berthold. 



hlorophyll. It is not 



■et clearly decided whether the eflficiency is greatest in the red rays, corre- 

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

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

 las no important bearing on the question before us ''-. The manufacture of 

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

 )lue and violet rays are the most effective in the assimilation of nitrates ■*. 



' VViesner, II ; Johow, op. cit. 



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

 m the basis of the researches of Pfeffer, Reinke, Engelmann, and others. 



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

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



