300 Mr Philip Sewell on the [sess. lit. 



there resuked a remarkable predominance of leaves, and a 

 scarcity of ilowers ; whilst in white light the vegetation was 

 less vigorous, and flowers very abundant. He was led to 

 believe that the production of flowers was therefore favoured 

 by the ultra-violet or invisible rays, which excited some 

 unorganised ferment to take effect at the growing points. 

 AVe may regard this conception of Professor Sachs as 

 showing that when assimilation or the vegetative power is 

 (practically) nil, then, as in the ultra-violet rays, elabora- 

 tion of Ilowers or of colouring matters may take place. 

 Whether or not the destruction of the protoplasm neces- 

 sary to colour is aided l)y a " princi})le acting like a 

 ferment," in however small quantities, remains to be seen — 

 it would not alter the conditions, which would be indeed 

 the more characteristically katabolic. 



The gradations in form and colour from the assimilating 

 leaves of the stem to the coloured non-assimilating and, at 

 the same time, smaller bracts or leaves of the flower, is 

 remarkably well seen in widely different plants — very clearly 

 in the brilliant intiorescence of Canna and in Epilohium. 

 Although all will agree that the small size of coloured 

 bracts of the inflorescence is evidence of a checking of 

 vegetation, and of the absence of assimilation in such parts, 

 yet it is perhaps necessary to refer to the comparatively large 

 size of most petals. It is found, however, that though petals 

 arc often large, they are formed of the most transitory sub- 

 stance, they are, in fact, so produced that they make a 

 display at as little expense as jwssible. Undoubtedly if a 

 flower normally 1^ inch become li inch in diameter, we 

 might suppose that such an exuberant growth would be 

 liable to be influenced l)y the selective power of an insect's 

 discerning eye. But we may call to mind the very large 

 flower of the niglit-flowering Cactus, which is ])roduced when 

 assimilation, and what van Tieghem calls chloro-vaporisa- 

 tion, have ceased, and when the plant is therefore in a con- 

 dition to pour its surplus elaborated material into its flower- 

 bud. This growth, too, of the most elementary tissue is par- 

 ticularly evanescent, being suited only to last the night. 



But not to confine our attention to these thiee chief con- 

 ditions of colouring, nor to tlie correlation l)etween flower- 

 production and colour-production, we may notice tliat all the 



