LIGHT AND VEGETATION 95 



trary, photosynthesis has been known to continue in an 

 illumination forty times more intense than full sunlight. 



The Influence of Temperature 



The injfluence of temperature, considered in more detail, 

 seems to be extremely complex. According to the plants 

 used for experiments, there may be an increase of photo- 

 synthesis over a certain range of temperature rise ; above and 

 below this range, variations of temperature have no effect. 

 If the Hght is poor, photosynthesis may be more active at low 

 than at high temperatures, but with increased illumination a 

 rise of temperature will greatly favour the assimilation. 



Lundegard, followed by other investigators, found, for 

 the potato and the haricot bean for example, several suc- 

 cessive peaks in the curve of photosynthesis as a function of 

 temperature. The position of these peaks changes with the 

 illumination; as it becomes weaker they are displaced towards 

 the lower temperatures. 



Many other factors also have an influence. Nitrates, 

 phosphates and potassium are necessary and if they are 

 deficient the process is slowed down. This is one reason why 

 mineral fertilizers are useful. 



It has recently been shown that minute traces of a number 

 of chemical elements are indispensable; their action does not 

 necessarily bear directly on photosynthesis but on the ultimate 

 chemical elaborations of its immediate products, which are 

 conveyed by the sap away from the leaf where they have been 

 created. 



The Photosynthetic Effect of the Different Radiations of the 

 Spectrum 



Only visible radiations and radiations of the very near 

 ultra-violet seem to be usable by the plant for photosynthesis. 

 The limit on the side of the long wave-lengths is situated in the 

 neighbourhood of 7,500 A or 7,600 A; several observations 

 agree on this point. The limit on the side of the short wave- 

 lengths is not so well known because these ultra-violet 



