8o HISTORY OF BOTANY 



tions ; they do not assimilate the oxygen either directly 

 or indirectly ; they change it into carbonic acid gas, which 

 is found in small quantities, stored or dissolved in the 

 succulent parts as it would be in pure water, otherwise 

 they do not alter it." As to the source of carbon, De 

 Saussure claims that, whether cultivated in water or in 

 air, plants obtain all their carbon from the carbon dioxide 

 normally present in the atmosphere, and records a series 

 of experiments to show that pea plants in an atmosphere 

 containing about 8 per cent of carbon dioxide gained con- 

 siderably in weight when exposed to direct sunhght, but 

 that when the light was of feeble intensity the slightest 

 addition of carbon dioxide to the air was detrimental. 

 De Saussure was quite famihar with the importance of 

 the green pigment, but he fell into the error of thinking 

 that other pigments could take its place. Thus he 

 experimented with plants that had red or purple foUage, 

 and observed that such plants also gave off oxygen, 

 and jumped to the conclusion that the green pigment 

 was not essential to the decomposition, not noticing 

 that it was actually present though masked by coloured 

 cell sap. 



De Saussure drew attention to the fact that in day- 

 Ught a plant usually reassimilates all the carbon dioxide 

 it has formed in the process of respiration, and hence that 

 that phenomenon is not readily demonstrable while 

 carbon assimilation is going on. He expressed astonish- 

 ment that plants make no use of carbon monoxide or 

 any other gaseous compound of carbon. A further point 

 of great importance on which he lays stress is that neither 

 carbon dioxide nor water are decomposed apart from 

 each other, a discovery the significance of which was not 

 appreciated for very many years. The extent of the 

 advance that had been made in this relation you will 

 better apprehend if you recall the fact that Ingen-Housz 

 looked upon water merely as a vehicle for the transport 

 of nutriment from the roots to the leaves. It was left 



