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 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 foli- 

 age 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. 



In case the leaves are submerged, as is true of many 

 plants, it is evident that transpiration is practically checked, 

 for the leaves are already bathed with water, and under such 

 circumstances water vapor is not given off. It is evident 

 that under such circumstances leaf work must be carried 

 on without transpiration. In some cases, as in certain 

 grasses, fuchsias, etc., drops of water are extruded at the 

 apex of the leaf, or at the tips of the teeth. This process 

 is called guttation, and by means of it a good deal of 

 water passes from the leaf. It is specially used by shade 

 plants, which live in conditions which do not favor tran- 

 spiration. 



27. Respiration. — Another kind of work also may be 

 detected in the foliage leaf, but not so easily described. 

 In fact it escaped the general attention of botanists much 

 longer than did 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 



