GROWING PLANTS UNDER GLASSES OF VARIOUS COLOURS. 71 



it is not only the (optically) brightest rays of the spectrum 

 which are concerned in the process of assimilation, but that 

 there is a second series of rays in the more refrangible half of 

 the spectrum, as is proved by the experiment with bacteria. 

 It was remarked above that no plant bore flowers under any 

 coloured glass, and but a very few blossomed under clear glass. 

 Sachs has given reasons for inferring that the ultra-refrangible 

 rays may be concerned in producing flowers. If this be so, and 

 if these rays were cut off by the coloured glasses, it would 

 account for their absence. In the case of clear glass, however, 

 the same reason could not be held, for the only difference must 

 be a higher temperature. Flowers, however, are, of course, 

 produced through the agency of foliage. So that we may justly 

 conclude that the failure to produce them was due to the 

 abnormal conditions which engendered a weaker vegetative 

 growth. Plants being naturally attuned to full sunlight, when- 

 ever any rays are withheld, or are in excess, as of obscure heat, 

 they instantly exhibit the ill effects due to these causes. 



Moreover, the percentage of dry substance in the case of 

 plants grown in the open is lower than that of plants under 

 -clear glass, and equals that under blue. This is an unexpected 

 result, when compared with Mustard and Lettuce. But the fact 

 that the plants were in full bloom in the open will probably 

 account for it, as one maximum of respiration is always 

 coincident with the flowering process. 



Virginian Stock also illustrates specific individualities, so to 

 say, in that under all the coloured glasses the plants were 

 peculiarly incapable of development, never exceeding to any 

 marked degree the germiuating stages of growth. 



Experiment No. III. 



Lettuces grown from the Seedling to the Adult Stage 

 {during thirty-two days). 



Eighteen seedling Lettuces were weighed, and their average 

 weight calculated as 10 grammes. Three were planted in each 

 frame on May 29, 1889. The actual weight of each individual, 

 being known, was calculated as 10 grammes. 



In order to draw comparisons between the amount of dry 

 substance in the plants used for the experiments, both in their 



