LIGHT AND VEGETATION 83 



in its initial phase, in a photochemical excitation of certain 

 molecules. 



Thus the physicist alone cannot be expected to analyse 

 these phenomena completely; he will need the results of 

 modern chemical research in which further progress has still 

 to be made. 



The rapid advance of knowledge in organic chemistry 

 and the introduction of exact methods of measuring and 

 defining the radiation are the two essential requirements in 

 research on the effects of hght on vegetation. 



Equilibrium of Development 



It has long been known that the radiations of the shortest 

 wave-lengths in the visible spectrum, the blue and the violet, 

 have a particularly marked influence on the regulation of the 

 growth and development of plants. If these radiations are 

 absent and the illumination consists of only yellow and red 

 light, photosynthesis appears to proceed normally. The plant 

 remains green, but it takes on the characteristic appearance 

 of plants which have developed in obscurity or in insufficient 

 illumination and are suff"ering from what is called etiolation. 



The most obvious eff'ects of the lack of blue Hght are: 

 long, thin and weak stems, imperfectly developed leaves, 

 intern odes which are too long and tissues which are only slightly 

 diff'erentiated. 



The greater height, or length of stem, has often been 

 interpreted as a favourable indication and a desirable eff'ect. 

 Nevertheless, the total weight, and especially the weight of 

 dry matter, is always lower in the absence of blue light; the 

 plant shows unmistakable pathological signs. 



The best experiments have been made with coloured 

 glasses filtering the sunUght. They are chosen so that they 

 abstract from the solar spectrum larger and larger portions, 

 starting with the short wave-lengths. Under the glasses which 

 transmit the largest part of the spectrum, muslin is fitted to 

 reduce the illumination to the same value as it is under the 

 glasses which transmit only a part of the spectrum. The 



