200 GROWTH 



The plants mentioned showed a regular increase in length 

 from I to 5 inclusive and a decrease from 6 to 8 inclusive. 

 The behaviour of all plants, however, was not the same ; the 

 beetroot and the potato developed better in the blue-violet 

 than in the green, and better in the green than in the yellow. 

 In general terms, the more the short rays, especially the ultra- 

 violet, were removed, the greater the growth up to a certain 

 limit. It also was observed that earlier flowering was pro- 

 moted by culture under 3 and 4 of the series. 



Similarly Popp * grew plants in precisely the same con- 

 ditions except the quality of the illumination. In light from 

 which all rays shorter than 529 /m/x had been abstracted, the 

 plants, as compared with those grown in normal conditions, 

 showed thinner stems, fewer branches, curled leaves, but with 

 a good development of chlorophyll, a reduction in the amount 

 of anthocyan, less differentiation in the tissue of the leaf and 

 stem, less compact and thinner walled cells, and a reduction 

 in mechanical tissue. There was also a delay in flowering ; 

 the flowers were fewer, the fruit weak and the seed poor. 

 There was also a decrease in the fresh and dry weight, a de- 

 ficiency in total carbohydrate and an increase in total nitrogen. 

 It would, therefore, appear that the blue-violet part of the 

 spectrum is requisite for normal development but not the 

 ultra-violet, for when these rays were excluded the plant 

 growth was normal. 



In addition to the above-mentioned observations, the 

 effect of ultra-violet radiation on the growth of the plant has 

 been the subject of many investigations. In general, the 

 effect of these rays is to promote a dwarf habit which is 

 accompanied by various structural changes. Since plants 

 flourish in the absence of ultra-violet light, the matter is a 

 subject of special rather than of general interest, for which 

 reason it is not necessary to pursue it further on the present 

 occasion.f 



Of energy other than light, field experiments with wheat 



* Popp : " Amer. Journ. Bot.," 1926, 13, 706. 



t See Delf, Ritson and Westbrook : '* Brit. Journ. Exp. Biol.," 1927, 

 5, 138, and the literature there quoted. Also Nadson and Philippov : 

 " Compt. rend. Soc. Biol.," 1928, 78, 366, 



