Ch. XIV] THE HYDROPHYTES 561 



phytes. The inorganic habitats fall in turn into two prin- 

 cipal subdivisions, — (a) those under water, occupied by 

 forms called hydrophytes (understood as autophytic), and 

 (6) those upon land, or more properly, in the air, occupied 

 by forms which, reviving an old word, obsolete in its former 

 somewhat different sense, we may call aero phytes (also 

 understood as autophytic). We may most conveniently 

 treat our subject under three divisions, as follows : 



(1.) The Water Habitats, and the Vegetation Forms 



called Hydrophytes. 

 {2.) The Aerial Habitats, and the Vegetation Forms 



called a'erophytes. 

 (3.) The Organic Habitats, and the Vegetation Forms 



called hysterophytes. 



(1.) The Water Habitats, and the Vegetation Forms 

 called Hydrophytes. 



In the true water habitats the plants are wholly sub- 

 merged, for the floating and partly submerged kinds, which 

 we often include among hydrophytes, are really aerophytes 

 with roots in water, as will later appear. 



The physical conditions of the submerged habitats, to 

 which the structure of hydrophytes must conform, are well 

 known. The carbon dioxide, oxygen, and mineral salts 

 accessible to the plants are such as occur in solution in the 

 water, and thus are equally in contact with all parts of the 

 plant body, which, free from danger of desiccation, needs no 

 waterproof epidermis. Correspondingly, water plants ab- 

 sorb their needed gases and mineral salts anywhere over the 

 body, and do not possess the differentiation into absorbing 

 root and leafy shoot so characteristic of land plants; and 

 for the same reason they lack differentiation into conducting, 

 storage, and other special tissues. Again, gases diffuse so 

 slowly in water as compared with air, that interior tissues 

 cannot be adequately aerated through passages from the 

 surfaces, — as effected so easily in land plants. Corre- 

 2o 



