32 PLANT STUDIES 
a second tumbler over the blade of the leaf, which projects 
above the cardboard (see Fig. 24). It will be observed that 
moisture given off from the surface of the working leaf is 
condensed on the inner surface of the inverted tumbler. 
The cardboard is to shut off evaporation from the water 
in the lower tumbler. 
When the amount of water given off by a single leaf is 
noted, some vague idea may be formed as to the amount of 
moisture given off by a great mass of vegetation, such as a 
meadow or a forest. It is evident that green plants at 
work are contributing a very large amount of moisture to 
the air in the form of water vapor, moisture which has been 
absorbed by some region of the plant. The foliage leaf, 
therefore, may be regarded as an organ of transpiration, 
not that the leaves alone are engaged in transpiration, for 
many parts of the plant do the same thing, but because the 
foliage leaves are the chief seat of transpiration. 
The important fact in connection with transpiration is 
not that moisture is given off by active foliage leaves, 
but that this escaping moisture is the external indication 
of some work going on within the leaf. Transpiration, 
therefore, may not be regarded so much as work, as the 
result, and hence the indication of work. In case the 
leaves are submerged, as is true of many plants, it is evi- 
dent that transpiration is practically checked, for the 
leaves are already bathed with water, and under such cir- 
cumstances water vapor is not given off. The same is true 
of green water plants without leaves (such as algse). It is 
evident that under such circumstances leaf Avork must be 
carried on without transpiration. 
27. Respiration. Another kind of work also may be de- 
tected in the foliage leaf, but not so easily described. In 
fact it escaped the attention of botanists long after they 
had discovered photosynthesis and transpiration. It is work 
that goes on so long as the leaf is alive, never ceasing day 
or night. The external indication of it is the absorption 
