CALIFORNIA COLLEGE 



of PHARMACY . 



KESPIKATION 308 



ture rises from this minimum point, the activity of respira- 

 tion 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 processes, and its 

 indirect effects may be very far-reaching. 

 ( Eespiration is considerably affected by variations in the 

 amount of oxygen which the environment of the plant 

 contains. The protoplasts can absorb even the least 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 



