94 LIGHT, VEGETATION AND CHLOROPHYLL 



Incidentally, it may be remarked in passing that, 

 according to some calculations, the total quantity of carbon 

 dioxide absorbed in a year by photosynthesis is ^ of that 

 contained in the atmosphere. 



The possibihty of increasing the crop by providing plants 

 with a larger quantity of carbon dioxide has practical apph- 

 cations in a glasshouse or in frames. Rotting manure gives 

 off this gas in abundance and its influence can be seen in 

 manure hotbeds. SoUd carbon dioxide, or dry ice, can also 

 be supplied; this white sohd passes to the gaseous state 

 without passing through the hquid state and sublimes to give 

 off" a very pure gas. Naturally, an increase in carbon dioxide 

 can benefit the plant only at times when the illumination is 

 sufficiently strong; otherwise the deficiency of fight will fimit 

 the photosynthesis. 



Temperature can also be a limiting factor; when it is 

 too low, it can arrest photosynthesis, even if carbon dioxide 

 and fight are present in abundance. 



Such, briefly, are the results of observations on the 

 influence of fight, combined with that of the carbon dioxide 

 content of the atmosphere and of temperature. 



Other observations show that the process of photo- 

 synthesis is subject to multiple and complex influences which 

 may often mask the simple laws just explained. 



One curious fact is that in certain plants the process is 

 checked, and may even be stopped completely, when the 

 fight is too strong. A desert plant of this kind accom- 

 pfishes afi its photosynthesis in the early hours of the 

 morning. 



With Trichomanes radicans (a shade fern), Montfort found 

 that photosynthesis steadily increased as the illumination 

 increased from up to J of the daily maximum, then 

 steadily diminished and stopped completely at half the 

 maximum. 



The cause of this inhibition is not very weU understood, 

 but it is clear that in some cases light which is too strong may 

 be more detrimental than useful. In other cases, on the con- 



