Chapter Thirteen 

 TRANSPIRATION 



Transpiration is the loss of water by plants through the 

 process of evaporation and simple diffusion. Under most 

 conditions more than 90 per cent of the loss is from the leaves 

 and the remainder from the stem, chiefly from the younger 

 portions. Transpiration used to be considered valuable to 

 the plant to carry salts into the plant and through it. This 

 would be true if plants had large openings through which the 

 soil solution flowed; however, since the water enters, not as 

 a stream, but as individual molecules and the salts enter as 

 molecules by diffusion independently of the rate of water 

 intake, it seems impossible that transpiration is necessary 

 for salt absorption. Although numerous experiments have 

 shown that the rate of transpiration and salt absorption are 

 not dependent on each other, it is possible that the move- 

 ment of water inside the plant speeds the movement of salts 

 after they have entered the plant. 



Many experimenters have attempted to show the influ- 

 ence of transpiration on these problems, but so many factors 

 are interwoven that the results are variable. To illustrate: 

 Dr. Hasselbring grew tobacco plants in two groups, one of 

 which he grew under a cheesecloth shelter which reduced the 

 transpiration by 25 per cent. The two sets attained an equal 

 dry weight but those with the lower rate of transpiration 

 absorbed about 15 per cent more salts than the other set. 

 Other plants under slightly different conditions have ab- 

 sorbed a little less salts with retarded transpiration. In view 

 of such conflicting results, it is generally believed that trans- 

 piration itself is a useless phenomenon which the plant can- 

 not avoid because of its need for the diffusion of other gases 

 to and from the leaves. 



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