THE LEAF 127 
the bell jar, the baryta water through which it must 
pass becomes milky, showing that carbon dioxide has 
been evolved. We thus see that even the green aérial 
parts of plants respire. 
There yet remains another possible fallacy to be 
removed. One might suppose that these green parts 
respire only during darkness, relying, during the hours 
of light, entirely on the energy of the sunlight. It is 
beyond the limits of this treatise to prove that such is 
not the case, but it may be mentioned that careful 
experiment has shown that respiration goes on at all 
times in all parts, in the light as well as in the dark. 
In the sunlight the respiration of green parts is masked 
by the reverse process of photosynthesis, by which the 
carbon dioxide formed is again broken up into carbon 
and oxygen, so that the oxygen taken from the air is 
finally given back to it. But respiration nevertheless 
@oes on. 
COMPARISON OF RESPIRATION AND PHOTOSYNTHESIS. 
Photosynthesis is a building up process by which 
new substance is gained by the plant. Respiration is a 
breaking-down process by which substance is lost by 
the plant. The former is carried on only by green 
parts, the latter by all parts. Photosynthesis requires 
hight. Respiration takes place in the dark as well as in 
the light. In the former carbon dioxide is taken in 
and oxygen given off, while in the latter oxygen is 
taken in and carbon dioxide given off. 
TRANSPIRATION. 
We now come to another great function of the leaf, 
the exeretion of surplus water. Though some of the 
water brought from the soil is used to form carbon 
compounds, the bulk of it, having served its purpose 
of transporting dissolved minerals, passes out through 
the stomata into the air in the form of vapour. It has 
