04 



JOURNAL OF THE ROYAL HORTICULTURAL SOCIETY. 



germination more than others, the question to ask is, Are there 

 more heat rays associated with those coloured ones or not ? 



Such is the case with the least refrangible, or red and yellow 

 rays ; so that we seem to trace a clue to at least the higher rates 

 of germination under these colours. 



Green glass absorbs heat rays as well as many others, while 

 chlorophyll does not absorb green light to any important extent. 

 Consequently, we think we can see why this glass alone may nofc 

 only be merely inefficient but injurious. 



The curve of heat is now regarded as having one maximum 

 in the red, falling steadily towards the most refractive, or violet, 

 end of the spectrum ; and although the temperature is conse- 

 quently considerably lower under blue than red and orange, 

 yet we shall see that the assimilative powers under the blue 

 are approximately equal to those under yellow.* 



If the heat is reduced under blue glass, we may ask the 

 question, Why are the effects so nearly equal to what takes place 

 under yellow ? 



Perhaps the following facts may lead us towards an answer. 

 "We know that the chlorophyll absorption-bands are broader 

 under the most refractive end of the spectrum, and therefore 

 more of the rays are absorbed at that end, so that it would seem 

 that there is a compensating power here; while fewer and "more 

 powerful" rays, if we may so imagine them to be, are absorbed 

 under red and yellow, i.e. by the bands known as Nos. I and II. 

 Hence the light absorbed represents the capability of the same 

 amount of " work " at both ends of the spectrum. 



That light can supply the place of heat is well known, as in 

 the case of plants growing in arctic regions and high altitudes, 

 which germinate when the heat is very deficient. 



Again, it is now known that the light absorbed by chloro- 

 phyll is the chief agent in producing transpiration, by its being 

 converted into heat within the plant ; so that it would seem 

 reasonable to infer that the rays absorbed in the blue and violet 

 end may compensate for the absence of obscure heat-rays. 



* Many experiments have been made by surrounding plants with 

 coloured solutions, as of the yellow-orange bichromate of potash, and the 

 blue one of sulphate of copper ; but in these cases the water absorbs a 

 great quantity of heat, and so must necessarily complicate the problem. 

 In fact, unless the experiments are carried out in the light of the pure, 

 spectrum itself, without the aid of solutions or coloured glasses, the results 

 can never be more than approximately accurate. 



