32 PLANT RELATIONS. 



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 oi 

 moisture given off by a great mass of vegetation, sucli 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- 

 cumstani-es water va])or is not given off. The same is true 

 of green water plants without leaves (such as algfe). It is 

 evident that under such circumstances leaf work must be 

 carried on without transpiration. 



2'7. 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 l)()tanists 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 



