LIGHT AND VEGETATION 85 



against the cold and the wind, especially when the nights are 

 cold and the days hot; the same alternations of temperature, 

 without Ught, are incapable of increasing the resistance of the 

 plant. Reserves of carbohydrate also appear to be necessary 

 to promote this action. Thus several factors are combined, 

 each being indispensable. This is a very common case. 



Chemical Composition 



The action of different radiations on the chemical com- 

 position of the plant is very imperfectly understood. Thus, 

 Dumont stated that wheat ripened under red glasses contained 

 less nitrogenous substances, and he concluded that the 

 radiations of shorter wave-length favoured the formation of 

 albuminoids. Popp, on the contrary, in the experiments 

 already mentioned, showed that the suppression of these 

 radiations caused an increase in the total quantity of nitrogen. 

 These two observations seem to be contradictory; the most 

 probable explanation is that light is not the only factor 

 involved and that other factors manifested themselves in a 

 different fashion in the two experiments. 



In the particular case of very young wheat seedhngs 

 cultivated in artificial light with incandescent lamps, it has 

 been observed that supplementary illumination by carbon 

 arcs to supply the blue radiation which otherwise would 

 have been only very sUght, favours the absorption of nitrates, 

 and more particularly of potassium nitrate; sodium nitrate 

 is not so well ultilized. 



This example shows that there is a relationship between the 

 illumination, in quality and in quantity, and the utiUzation of 

 the mineral substances that the plant draws from the soil. 

 Thus the question of fertilizers is perhaps connected very 

 closely with the action of hght. 



The synthetic production of organic substances by the 

 plant is also dependent on the nature of the illumination. 

 It has been observed, for example, that tobacco leaves grow 

 larger and more abundantly under sHghtly lower illumination 

 than they receive in the open air, but their yield in nicotine 



