COLOUR. 109 



ded, scarcely any colour was developed at all, such 

 plants being called etiolated. 



141. Under Digestion we have seen that it is only 

 when exposed to the influence of light that the evolu- 

 tion of oxygen from the plant takes place, and that this 

 ensues from its green- coloured portions ; that a con- 

 nection, therefore, between this and the presence of co- 

 lour in a plant exists, was concluded, and from what 

 we now know we are at liberty to say, as a general 

 rule, that the presence of colour in a plant depends 

 upon the action of light calling into play those vital ac- 

 tions by which the decomposition of carbonic acid is 

 effected, and so causing the oxygen to be evolved, 

 whilst the carbon remains behind ; and that it is from 

 this carbon undergoing certain changes, and producing 

 a particular product, that colour has its origin, and in a 

 ratio with the negation of light, so will be its deficiency. 



142. But there are many circumstances well known 

 that tend to modify this theory very materially. " The 

 Fucus vitifolius," says Bonpland, " removed from a 

 depth of 190 feet in the ocean, offers a curious pheno- 

 menon in vegetable physiology. Experiments made by 

 divers render it probable that beyond a depth of ninety 

 feet the penetration of light is very little, notwithstand- 

 ing which this Fucus was of as fresh a green as the 

 young leaves of our Vine or Grasses." Though the 

 statement regarding the penetration of light is too 

 limited in the above instance, there exist facts to show 

 that a green colour may exist, and have been developed 

 in complete darkness. Humboldt found some Grasses 

 and a Wall Flower quite green in the subterranean gal- 

 leries of the mines of Freyburg ; and a Crocus that he 

 himself planted there, produced green leaves, flowers, 

 and anthers full of pollen. Besides, it must also be 

 allowed that solar light may be dispensed with in the 

 production of colour, and that artificial light is suf- 

 ficient for it, though not for the evolution of oxygen 

 from the coloured parts. Decandolle grew some plants 

 in the light of six lamps, which he observed to become 



