538 History of the Doctrine of [BOOK in. 



described the rapidity with which forced plants shoot up to the 

 higher temperature, De Candolle to want of light. On the 

 other hand Ray knew perfectly well that the green colour of 

 leaves is not produced by the access of air but by the light, for, 

 as he says, plants become green under glass, and not under an 

 opaque cover ; and if they become less green under glass than 

 in the open air, this is because the glass absorbs certain rays of 

 light and reflects others. Ray however, like almost all later 

 observers till quite recent times, did not keep the elongation 

 and bleaching of etiolated plants sufficiently distinct ; his 

 account of this phenomenon is spoilt by the presence of much 

 that is obscure. 



It has been justly observed by other writers on botanical 

 subjects that no notice is usually taken of one of the most 

 remarkable of the phenomena of which we are here speaking, 

 because, being a matter of every-day occurrence, it is simply 

 accepted as something obviously in accordance with the nature 

 of things ; this is the fact, that the main stems of plants grow 

 vertically upwards and their main roots downwards. To the 

 French academician Dodart, whom we have already encoun- 

 tered in the history of the theory of nutrition, is due the great 

 merit of being the first to find this apparently simple pheno- 

 menon really very remarkable ; he convinced himself by experi- 

 ments on germinating plants, that these vertical positions are 

 caused by curvatures, and endeavoured to discover the physical 

 reason why the main roots if placed in an abnormal position 

 escape from it by curving in the downward direction, and the 

 main stems in the upward direction, till they both reach the 

 vertical line. It was a matter of minor importance that his 

 mechanical explanation, which supposed that the fibres of the 

 roots contract on the moister side and those of the stem on the 

 same side lengthen, was quite unsatisfactory ; it was much more 

 important that these remarkable phenomena were made the 

 subject of scientific enquiry, and we find that various observers 

 soon after directed their attention to them, and exercised their 



