THE AMERICAN BOTANIST 33 



acrid substances cannot be perceived for some hours after- 

 wards. 



Transpiration. — In all ordinary plants, there is a pretty 

 constant inflow of water at the roots throughout the growing 

 season and an equally constant loss of water from the leaves. 

 In the case of some plants, the loss of water from the leaves 

 may amount to many gallons annually. In the common mus- 

 tard, each pound of dry matter made by the plant requires 

 900 pounds of water and alfalfa requires more than a thous- 

 and. The use to the plant of this "transpiration stream" as 

 it is called, has been explained in various ways. It was once 

 thought that it served to carry food materials into the plant, 

 but it is now known that the food materials move indepen- 

 dently of the water. Again, the evaporation from the leaves 

 was supposed to act as a sort of boiling down process by 

 means of which the plant was able to concentrate the matter 

 dissolved in the water. From some calculations made by 

 Chas. A. Shull and published in School Science and Mathe- 

 matics, it appears that transpiration from the plant serves the 

 same purpose that it does in the case of our own bodies, name- 

 ly, to dissipate the heat energy and thus keep us cool. In 

 bright sunlight, a leaf may receive enough energy to raise its 

 temperature 30° a minute. It would require only a few min- 

 utes, therefore, for the leaf to reach the boiling point. That 

 it does not reach this temperature is due to the moisture evap- 

 orated. It is well known that to evaporate a gram of water 

 (about half a thimble full) 536 calories of heat are necessary. 

 So long as the water continues to circulate through the plant, 

 and evaporate from its tissues it is uninjured by the heat. 



