CHAP. XLIII ADAPTATIONS. FORMATIONS 187 



to lacustral conditions. It therefore exhibits a zonal arrangement of 

 its occupants according to the degree of adaptation of these to the lacus- 

 tral conditions. . . . The geographically denned " boundary zone " 

 must be classified biologically into three main subdivisions : (a) Meadow- 

 swamp, lying nearest the dry land and inundated only for a short time ; 

 (b) zone that is being converted into land . . . : (c) strips of gravel or 

 sand poor in vegetation.' Only the last two concern us here. In these there 

 grow land-forms of plants derived from the lake-flora, also typical occu- 

 pants of the ' boundary zone ', as well as plants that have advanced to 

 this point though belonging to swamp meadows and ditches. The above- 

 named botanists distinguish, in connexion with Lake Constance, two 

 associations : a heleocharetum (with Heleocharis acicularis, Littorella, 

 Ranunculus reptans, Myosotis palustris var. caespititia, Agrostis alba, 

 and others) and a polygonetum. They furthermore distinguish a fourth 

 formation, namely that of alluvial plants, including a tamaricetum- 

 association (with Tamarix germanica and Hippophae and others), which 

 occupies the ' boundary zone ' where this takes the form of a gravelly or 

 sandy shore, and including also plants from the riparian alluvia as well 

 as alpine plants. 



Swamp vegetation is also allied to that which is described by Drude 

 under the heading of Formations near springs and brooks, and includes 

 tall herbs like Ulmaria, Geranium palustre, Impatiens noli-me-tangere, 

 Equisetum Telmateia, and small herbs, mosses, and algae. 



As these last formations have been oecologically investigated only 

 to a slight extent, we shall here consider in detail only 



1. Reed-swamp Formation. 



2. Bush-swamp Formation. 



CHAPTER XLIV. REED-SWAMP OR REED-FORMATION 



REED-SWAMPS occur either in fresh flowing water, or in stagnant, 

 more or less acid, water. Many species, such as Phragmites communis, 

 seem to be not in the least exacting as regards choice of soil. The swamps 

 of Arundinaria macrosperma are on acid peat soil, and the vegetation 

 thus provides a transition to oxylophytes. 



The vegetation is mainly composed of tall monocotylous perennial 

 herbs, grows in more or less deep, usually standing, water, and seems 

 to be most nearly allied to the communities of fresh-water plants ; 

 between the individual shoots and leaves one sees everywhere clear water, 

 which entertains representatives of the plankton-, hydrocharid-, and even 

 limnaea-formations. 



ASSOCIATIONS IN TEMPERATE ZONES 



Among the various genera and species to be found are Phragmites 

 communis, Scirpus lacustris, Typha, Butomus umbellatus, Glyceria 

 spectabilis, and other species, Phalaris arundinacea, Iris Pseudacorus, 

 Cladium Mariscus, Carex paniculata, C. rostrata, C. gracilis, C. filiformis, 

 C. acutiformis, C. stricta, C. riparia, C. vesicaria, and other species, 



