CHAPTER XX. 



MESOPHYTES. 



423. I. Mesophytes show certain general relations to ex- 

 ternal conditions, many of which are also shared by other 

 forms. Except to these minor variations in the environment, 

 they show no special adaptations ; or, rather, they are looked 

 upon as the normal plants, and the ways in which others 

 differ from them are spoken of as special adaptations. In 

 reality, however, the general methods by which they adapt 

 themselves to their environment, which are now to be con- 

 sidered, are quite as much special adaptations as those shown 

 by plants living in extreme climates. These adaptations will 

 be discussed in relation to each of the main factors of the 

 environment. 



424. i. Air. — The composition of the air varies little from 

 place to place. It is only in those regions in which it is 

 rendered impure by artificial means, such as the vicinity of 

 cities and factories, and in the few isolated regions in which 

 it is vitiated by natural means, as in volcanic regions, that 

 any special adjustments may be looked for. Artificial vitia- 

 tion of the air kills off certain plants. A few plants have 

 adapted themselves to air in the neighborhood of fumaroles, 

 where they are subjected to vapors containing large amounts 

 of sulphurous acid. Whatever special adaptations are found 

 are internal, since only the very simplest plants find it pos- 

 sible to live in such conditions. 



The movements of die air, however, influence profoundly 



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