EESPIEATION 297 



activity of respiration increases up to a certain optimum 

 point, which is usually not well denned, and which varies 

 considerably in different plants. If the temperature is 

 raised only a little higher than this, the living substance is 

 rapidly injured, and its respiration is checked. Variations 

 in temperature do not affect equally the absorption of 

 oxygen and the exhalation of carbon dioxide. At low 

 temperatures the latter is smaller than the former ; at high 

 ones the reverse is the case. 



The effect of light upon respiration is not very marked 

 and is probably indirect. Plants which grow in shady 

 spots usually manifest less respiratory activity than 

 similar ones growing in bright sunlight, but this may be 

 the result of the difference in the amount of nutritive 

 material they obtain, which is incident to the difference in 

 their situation. As we shall see in a subsequent chapter, 

 light has a very marked influence on the metabolic pro- 

 cesses, and its indirect effects may be very far-reaching. 



Kespiration is considerably affected by variations in the 

 amount of oxygen which the environment of the plant 

 contains. The protoplasts can absorb even the last traces 

 of the gas which reach them, but a certain amount is 

 necessary for them to maintain a healthy condition. Great 

 variations are not usually met with, but on the summits of 

 high mountains there is much less available for them than 

 at the sea-level. If the amount of oxygen in the atmosphere 

 from any cause falls below about 5 per cent., respiration is 

 seriously impeded. Similarly plants cannot thrive in the 

 presence of too great an amount. When the pressure of the 

 gas attains the amount of twenty to thirty atmospheres, 

 respiration becomes very difficult and after a short time 

 ceases, and death ensues. 



The process of respiration is also affected to a consider- 

 able extent by the nature of the substances which serve as 

 nutritive material for the reconstruction of the protoplasm. 

 It has already been pointed out that seeds containing oil 

 absorb more oxygen during germination than those which 



