CHAPTER XIV 



TRANSPIRATION 



Transpiration.— (We have already learned that water is taken 

 in through the roots of the plants and that there is a movement 

 of the materials dissolved in it throughout the greater part of 

 the plant. Some of this water is used in the formation of car- 

 bohydrates (see page 195), but most of it is given off through 

 the foliage and other green parts of the plant. Transpiration 

 refers to the giving off of vs^ater by the plant. The water is 

 given off from the leaves and other green parts of the plant and 

 to some extent from other parts of the plant as a vapor. In 

 some plants this process is very rapid, while in others it is rela- 

 tively slow. The amount of water in the plant at any time is 

 not an index of the amount which passes through the plant. 

 Transpiration must not be confused with respiration, which will 

 be referred to later (page 221). The transpiration of water by 

 the plant was originally supposed to be the same or similar to 

 the exhalation of vapor from the lungs of animals. But later 

 studies have shown that these processes are not the same, and it 

 is better to consider the transpiration of water from the plant 

 as a process in which the plant is passive. Of course, we do not 

 know what conclusions further study may lead to, but for the 

 present we will describe it as a mechanical or physical process 

 very similar to evaporation but differing from it in that it is 

 modified or controlled to some extent by the structure of the plant 

 and the living protoplasm within the cellsi 



Transpiration varies in different species of plants, in differ- 

 ent parts of the same plant and during different periods of the 

 170 



