296 PRINCIPLES OF BOTANY, ETC. 
stantly exhale oxygen gas, and perhaps most of them 
do it. They seem, however, likewise to require the 
stimulus of the oxygen of the atmosphere, as in- 
closed in hydrogen and azotic gas most of them 
decay rapidly. All plants, however, do not bear 
the stimulus of light and oxygen equally well, 
Each stimulus must be in proportion to the vege- 
table fibre, and when too strong it acts in ie 
contrary way, and destroys it. All subterrancous 
plants, as was found by Scopoli and Humboldt, 
decayed in atmospheric air. And in summer all the 
species of Boletus, which grow in cellars, suffer 
from the access of atmospheric air. Daily expe- 
rience indeed proves this, as rooms and chambers 
which are damp and mouldy, are soon freed from 
this nuisance when air is freely admitted. So strong 
is the stimulus of the little oxygen of the common 
atmospheric air to those plants, that they suffer from 
it and perish. | 
Though a moderate degree of light and warmth 
favours vegetation, too great a heat is uncommonly 
noxious. .The burning rays of the sun debilitate 
plants too much, and impair their irritability by the 
relaxing power of heat. Mimosa pudica loses al- 
most entirely its irritability by a long continued heat, 
and the leaves of Hedysarum gyrans cease to move. 
Grown up leafy plants during sultry days resist the 
rays of the sun, though entirely exposed to them, 
better than young germinating plants, In the 
shade, and in milder light, plants germinate most 
successfully. Thus nature has carefully provided 
for the small delicate plants, which grow in the 
shadow 
