2 5 8 



LECTURE XIII. 



begins to take place, and the results of a great number of 

 observers all tend to prove that a feeble light is less active 

 than one of greater intensity. It is probable, as Sachs points 

 out, that there is an optimum intensity above which the 

 activity of the process decreases, but it is a question whether 

 or not, in the case of any given plant, this optimum inten- 

 sity is ever reached or surpassed by the sunlight : it appears, 

 however, that, in some of his experiments, this optimum was 

 observed by Famintzin. 



In the preceding remarks upon the action of light in enabling plants 

 to decompose carbon dioxide, our attention has been confined to sun- 

 light ; a few words may be added with regard to light derived from 

 artificial sources. The experiments of Dehe'rain and Maquenne, of 

 Famintzin, and of Engelmann, made with light supplied by various forms 

 of lamps, shew that, provided the light is sufficiently intense, carbon 

 dioxide is decomposed and oxygen evolved ; this is doubtless true also of 

 the electric light, according to Siemen's observations. 



Inasmuch as light exercises so great an influence upon 

 the constructive metabolism of green plants, it may be 

 inferred that it must indirectly affect the absorption of 

 mineral food-materials by the roots. This has been shewn 

 to be the case by Rudolph Weber, and he finds further, as 

 might be expected, that rays of different degrees of refrangi- 

 bility have different effects in this respect. The greatest 

 absorption of ash-constituents took place, he found, in white 

 light, it was considerable in (impure) yellow light, very slight 

 in (impure) violet light, and least of all in green light. 



Weber experimented by growing peas in cases covered with glasses of 

 different colours, having ascertained in each case by means of the 

 spectroscope the composition of the light which passed through the glass, 

 which was never monochromatic. 



The following is a brief summary of the most important of his results: 



