290 Mr Philip Sewell on the [sess. lii. 



those of light, heat, moisture, and soils, and accordingly 

 we have to notice differences of colour brought about by 

 these influences in different latitudes and altitudes, or in 

 different habitats of one country or another. 



]\Iany papers have been written especially regarding the 

 colour of Arctic plants. Among the more recent of these 

 the researches of Bonnier, agreeing with those of Flahault 

 already mentioned, support the generalisation made by De 

 Candolle, Fries, Schubeler, and others, that as flowers are met 

 with sfrowing nearer the north, the colour of leaves and of 

 the petals may be seen to be more intense. M. Pellas has 

 recorded, from careful observation, that the colour of the 

 same species, grown in the same locality, but at different 

 altitudes, is also increased as higher regions are reached. The 

 colour-change is not, however, nearly so evident as in 

 different latitudes. He mentions that certain plants, as 

 Silene ritpestris, Silene infiata, BcUidiastrum, have more rose 

 colour in their petals in higher regions than in lower regions; 

 at the same time, several plants are especially referred to 

 as showing most markedly the colour-change. His facts 

 are all in accordance with the experience of growers of 

 alpine plants, who bewail the lessened brightness of their 

 especial favourites when transplanted to their gardens. 



3. Lastly, among the transitions that we may partly 

 account for, must l)e mentioned those which wo may associate 

 characteristically with definite conditions of growth, dependent 

 on the possibilities of nutrition. We have seen, in all the 

 instances of indirectly produced colour-change which have 

 been already mentioned, that the amount of nutrition or the 

 amount of assimilation have been the direct cause of such 

 variation. 



Plants, when grown iiealthily, if exposed to light and heat, 

 take on either green or other colour, and they are not able to 

 produce their normal tint if tlie assimilation is not normal. 



A vigorously vegetating condition is invariably associated 

 with the production of chlorophyll; on the other hand, when 

 plants are grown under unfavouralfle circumstances, their 

 leaves may show variation in the amount of chlorophyll in 

 proportion to the amount of etiolin, xanthophyll, and other 

 coloured products ; whilst, provided there is reserve material 



