202 



LIGHT, VEGETATION AND CHLOROPHYLL 



682 mg. of glucose, 648 mg. of saccharose or 613 mg. of starch. 

 The plant never succeeds in using more than 77 per cent of 

 this quantity and seems powerless to fix it when its proportion 

 in the atmosphere falls below 0-01 per cent. 



Lack of carbon dioxide seems often to be a limiting factor, 

 especially in calm weather when the assimilation is rapid. 

 The leaf soon deprives the surrounding air of the greater part 

 of its carbon dioxide so that the wind or the absence of wind 



700 m/x 



Hlillll 



600 



500 



4.00 





•;V>--r.'-*>'.-.'i:f;'*/.".":''.r<v- ■ ••. . ./';*r:*'>v/i-'«-.'--\'V.'-.-".-v;''';.-, •'•!.•■. 



.^•-^•:v^^i^•^^ ;^'•^ ^^!:^-'^■^^^^"^v^^••-^^^'«^^ l•^ •-•.•■■•.■::■■•■.••■■■...;■ ■.■ a^.t- 



^.v-.'>"'~.'"-'-''"''''''l iv;VV ■•:;••■ -v. ■■y''.Vv-V3tc-;.V\^V\A->^^^^^^^ l.-r^-.v.Vi' 



•:r:^--;~'uv:-:V;;.--:-; 



•:>:.••• "^ 









Fig. II, 5. Engelmann's experiment. Above: the absorption 



spectrum of chlorophyll. Below: filament of green algae 



surrounded by bacteria avid of oxygen and grouped in the 



regions where most is liberated 



is seen to have an influence. The leaf takes carbon dioxide 

 much more easily from the air when its content is in the 

 neighbourhood of 0-03 per cent and this content scarcely 

 varies when the air is frequently renewed. 



If the carbon dioxide content of the air is increased, the 

 rate of assimilation increases proportionately until it becomes 

 ten times greater, but this content cannot be increased 

 indefinitely. The optimum, although it varies with the species 

 of plant, is nevertheless rather high — on an average between 



