RESPIRATION 297 
activity of respiration 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 pro- 
cesses, 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 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 
