THE INFLUENCE OF LIGHT ON LIFE. 241 



it has been ascertained that the red, yellow, and orange rays are the 

 most powerful in causing growth, whereas the blue and violet rays 

 are almost entirely without effect in inducing the chlorophyll to 

 perform its function. On the 26th January I sowed some seeds 

 (canary, hemp, and rape) on pieces of flannel kept moist with 

 water. These seeds were then exposed to the light under different 

 coloured glasses, with the following result : — Under the yellow 

 glass the growth was strong and rapid ; under the red it was nearly 

 the same ; under the green the growth was, during part of the 

 time, very much delayed, but eventually it gained strength ; and 

 under the blue the growth was the slowest, and the green of the 

 plants did not appear to be of so bright a hue as in the other 

 cases. Although the green glass was the most transparent, yet the 

 growth was not nearly so rapid as under the much less transparent 

 red glass. Prof. Stokes ascribes to the greenish-yellow the most 

 active power, but my experiments, just quoted, appear to support 

 Messrs. Kerner and Oliver's conclusions that the red, yellow, and 

 orange rays are those which the chlorophyll requires to fulfil its 

 functions. It should, perhaps, be remarked that experiments with 

 coloured glasses are not so conclusive as experiments performed 

 with colour rays obtained directly from a prism. 



There seems, however, to be no end to the instances which might 

 be quoted of the influence of light upon plant life ; indeed, the 

 greater puzzle seems to be to find something which is not due to its 

 influence. Our present object, however, besides trying to show how 

 plant-life is altogether dependent on light, is to endeavour more par- 

 ticularly to show that an increase or a decrease in the normal amount 

 of light, or an alteration in its quality, has a great influence on 

 plants; that there is a certain fixed quantity which is most acceptable 

 to each species, and that when that fixed quantity is exceeded or 

 diminished to any great extent, evil consequences must follow. I 

 think that these conclusions may with safety be deduced from 

 what has just been said, and it therefore seems reasonable to con- 

 clude that, if investigations are properly conducted, we ought to 

 be able to ascertain with a great degree of nicety, the exact supply 

 that is best for each species, and so be able to measure out, in cul- 

 tivating a plant, just as much as will exert the highest and best 

 influence. 



