124 VEGETABLE PHYSIOLOGY 



All seedlings, again, are not alike in the vigour with 

 which they carry on their respiratory processes. 



We may pass on to inquire what is the relation between 

 the absorption of oxygen and the formation and elimination 

 of carbon dioxide and water. It is conceivable that the 

 oxygen may unite in the plant with carbon and with 

 hydrogen to produce at once the exhaled compounds. A 

 study of the living organism at work, howeverj soon shows 

 us that the process is not of this simple nature. We have 

 said, in the course of what has already b|een advanced, that 

 the amount of the carbon dioxide exhaled and that of the 

 oxygen absorbed are approximately equal. This, however, 

 is only true within certain limits ; if each is measured 

 accurately, they are not found to show an exact correspon- 

 dence. The ratio Co 2 :0 is usually spoken of as the 

 respiratory quotient. When the two processes are equal 

 the value of the respiratory quotient is unity ; when the 

 carbon dioxide is in excess it is greater, and when the 

 oxygen is in largest amount it is less, than unity. The 

 respiratory quotient has been found to vary to a greater or 

 less extent in different plants, and in the same plant under 

 different conditions. If its value is determined in the case 

 of germinating seeds, these differences are soon evident. 

 With starchy seeds the quotient is unity ; with oily seeds 

 it is much lower. That is, in the former case, the seeds 

 absorb a volume of oxygen equal to that of the carbon 

 dioxide they exhale ; in the latter case they take up more. 



Various observers have shown that in certain cases 

 succulent leaves, such as those of the Agave or of particular 

 plants belonging to the Saxifragacece and the Crassulacece, 

 or again the phylloclades of Opuntia, one of the Cactacece, 

 are capable of absorbing oxygen without the simultaneous 

 evolution of carbon dioxide. Nor is the oxygen absorbed 

 in these cases any more than it is in others without enter- 

 ing into some form of chemical combination/ for it cannot 

 be extracted by the air-pump. The latter also fails to 

 extract any carbon dioxide from the plants. The oxygen 



