THE LEAF 175 
ranged in layers alternating with damp cotton bat- 
ting or blotting paper (Fig. 228); close the jar 
tightly and leave from 12 to 24 hours in the dark 
to prevent the action of photosynthesis. Then 
insert a thermometer and note the rise in tem- 
perature. Ifa lighted taper is plunged in, it will 
quickly be extinguished, showing that respiration 
has been going on. 
191. Respiration in leaves. — We see 
from experiments like the foregoing that 
the leaf, besides carrying on the functions 
. i ; Fic. 228. — Arrange- 
of digestion, photosynthesis, and trans- ment of apparatus to 
‘ : : . 4 show that heat and car- 
piration, is also an active agent in the handiestdoane cecie® 
work of respiration. In this function by leaf buds. 
oxygen is used up and carbon dioxide 
given off, just as in the respiration of animals;.but the 
process is so slow in plants that it is much more difficult 
to detect than the contrary action in photosynthesis, and is, 
in fact, not perceptible at all while the latter is going on, 
though it does not cease even then. 
But while the leaf is the principal organ of respiration, the 
process is carried on in other parts of the plant as well, 
else it could not survive during the leafless months of 
winter. It appears to be most active at night, but this is 
only because it is not obscured then, as during the day, by 
the more active function of photosynthesis. Indeed, it was 
for a long time supposed that plants ‘‘ breathed ”’ only at. 
night, and it was thought to be unwholesome to keep them 
ina bedroom. It is now known, however, that respiration 
goes on at all times and in all living parts of the plant, but 
the quantity of oxygen taken in is so small from a hygienic 
point of view that it may be disregarded. 
192. Distinctions between respiration and photosynthesis. 
— While these two functions are contrasting and antipodal, 
so to speak, in their action, they are mutually complemen- 
tary and interdependent, the one manufacturing food and the 
other using it up, or rather marking the activity of those 
