566 A TEXTBOOK OF BOTANY [Ch. XIV 



phyllous leaves, supporting stems, and absorbing roots; 

 the inclosure of their shoots in waterproof epidermis and 

 cork, pierced by regulable stomata and lenticels; their 

 alteration in form, size, and aspect, with dryness of habitat ; 

 their penetration by intercellular passages; and their 

 attainment to massive size and firm structure. 



The aerial habitats fall into subdivisions determined by the 

 relative proportions of the six primal necessities ; and these, 

 with the distinctive vegetation forms, are the following : — 



A. The Optimum Habitats, and the Vegetation Forms 



CALLED MESOPHYTES. 



B. The Alternating Humid-Dry Habitats, and the Vege- 

 tation Forms called Tropophytes. 



C. The Dry Habitats, and the Vegetation Forms called 

 Xerophytes. 



D. The Undrained Habitats, and the Vegetation Forms 



CALLED HELOPHYTES. 



E. The Saline Habitats, and the Vegetation Forms 



CALLED HALOPHYTES. 



F. The Acid Soil Habitats, and the Vegetation Forms 



CALLED OXALOPHYTES. 



A. The Optimum Habitats, and the Vegetation Forms 



CALLED MESOPHYTES. 



These are the habitats characterized by constant warmth, 

 ample rainfall, and a well-aerated soil rich in mineral salts ; 

 and in all such places, light, carbon dioxide, and oxygen 

 are sufficient for plant needs. These conditions, which are 

 the very best for plant life, are realized only within the tropics 

 in places of ample rainfall ; and here, in the so-called tropical 

 rain forests, is attained the greatest luxuriance and diversity 

 of plant life. The vegetation forms, which, of course, are all 

 perennially green, are called mesophytes (i.e. intermediate 

 plants, or normal plants), and the principal are the following. 

 For simplicity, the word mesophytes is not repeated, though 

 understood in the names of the forms, e.g. the first would 

 read in full "deliquescent tree mesophytes," or "deliques- 

 cent arborescent mesophytes." 



