154 HYDROPHYTES sect, iv 



CHAPTER XXXVH. FORMATIONS OF AQUATIC PLANTS 



The different oecological factors just dealt with yield many distinc- 

 tions in the environments of aquatic plants, according to the modes in 

 which they are combined. But another factor of equal importance to 

 the production of formations, and one that lies at the very base of this, 

 is edaphic in kind : it is the nature of the substrattmi, which brings with 

 it many differences, including those in the mode of fixation. 



The first formation to be founded is that of piaiikton, which is consti- 

 tuted of those microphytes that float free in water and are adapted to 

 this mode of life. 



Closely allied to plankton, but of a subsidiary and less important 

 nature, is the glacial community forming the cryophyte-ionnsition, which 

 is composed of microphytes that are periodically exposed to ice-cold 

 water. 



A third type is the hydrocharid-iovmation or pleuston^ which is 

 constituted of macrophytes floating on water, or, more rarely, floating 

 in it, and adapted to this mode of existence. 



Forming a sharp contrast to the preceding is benthos,^ which includes 

 aquatic plants that are fixed to the substratum, or, hke some diatoms, creep 

 over it. In opposition to free-living species, these display many types 

 of construction associated with mechanical rigidity, and various other 

 structural features. The formations under this head may be referred to 

 two great and widely different groups dependent upon the nature of the 

 substratum, according as this consists of solid rock or loose material. 



Accordingly the fourth group of formations is constituted of litho- 

 philous hydrophytes, which are water-plants hving attached to stones : 

 this group may be subdivided into several formations. 



The remaining plant-communities associated with a loose substratum 

 are to be separated into those composed solely or mainly of microphytes, 

 and those in which vascular plants or larger algae play the chief part ; 

 and after this a further subdivision may be adopted according as the 

 plants live in saline or fresh water. In addition the kind of soil is of 

 influence, according as it mainly consists of inorganic bodies or organic 

 fragments.^ Thus there result the following formations : microphyte- 

 formation ; enhalid-iovnidXion (in the sea) ; and limnaea-ioxnx^Xion (in 

 fresh water) : the last two are closely allied. 



Hydrophytic formations and their associations are often distributed 

 in a definite manner, usually m zones, according to depth of water and 

 the conditions involved in this : there are deep-water associations and 

 littoral associations. In the deepest parts of the sea and of fresh water 

 there is often mud, in which only saprophilous communities of microphytes 

 {abyssal vegetation) can exist. In less deep parts there is a miuch richer 

 littoral vegetation, which consists of more highly organized algae and 

 cormophytes, and is arranged zonally according to depth, that is, accord- 

 ing to illumination. The arrangement of these associations is greatly 

 influenced by differences of soil and movements of the water. Very 



' (C. Schroter und) Ivirchner, i8q6, p. 14; ' macroplankton ' (Chodat). 

 ^ Haeckel, 1890. * See Chapter XVI. 



