AN INTRODUCTION TO PLANT BIOLOGY 97 



the opening and closing of these pores may control the 

 amount of evaporation. 



The process of evaporation of water from the leaves of 

 plants is in botany commonly called transpiration, and we 

 say that leaves transpire, meaning that they give off water 

 by evaporation. For all practical purposes the words 

 evaporation and transpiration are equivalent as applied 

 to the work of leaves. 



94. Soil- Water and Sap. Throughout this lesson we 

 have referred to water as ascending in the plant, but it is 

 never pure water. Water absorbed from the soil always 

 contains some mineral materials in solution. Also, as 

 the water passes through the plant it absorbs or dissolves 

 other substances and becomes sap. However, a large part 

 of the water which goes directly up the stem in the wood- 

 tubes is not very different from the water of the soil ; and 

 travelers in tropical countries often cut off certain vines 

 and drink the water which runs from the stems. 



One other point should be emphasized; namely, that in 

 the evaporation of water there is left behind in the plant, 

 especially in the leaves, all the substances brought in solu- 

 tion from the soil. Leaves which fall in the autumn give by 

 burning much more ashes than do leaves which fall in the 

 early summer; and the explanation is that evaporation 

 during the long summer season has left behind in the leaf 

 various materials carried up in solution in water from the 

 soil, and not needed by the plant. 



95. Use of Water in the Plant. So far in our study 

 of water and its movement through root, stem, and leaves, 

 and its final loss through evaporation, we have considered 

 the purely mechanical processes and without reference to 

 the use of the water while passing through the plant. Now, 

 plants are not elevating water simply for the sake of evaporat- 

 ing it from the leaves ; on the contrary, evaporation is neces- 

 sary to make place for more fresh water and so keep up the 



