RESPIRATION 303 
ture rises from this minimum point, the activity of respira- 
tion increases up to a certain optimum point, which is usually 
not well defined, 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. 
Respiration 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 
