358 STRUGGLE BETWEEN PLANT-COMMUNITIES sect, xvir 



this tract the more favourable conditions promote the production of new 

 species to a greater extent than in savannah .^ 



Birds all tend to carry seeds over long distances to a greater extent 

 over sea than over land, because in the former case they find no resting- 

 places on which they can alight and discharge the seeds. It is certain 

 that ocean-currents can carry seeds very far.^ Sernander^ has classified 

 Scandinavian plants according to their means of dispersal ; and of special 

 interest are his investigations concerning drift cast up by sea and fresh- 

 water, and concerning wind-dispersal ; in his work he deals with the 

 transport not only of seeds and fruits, but also of vegetative shoots. 

 He proves that various plants are adapted for dispersal at definite seasons, | 

 and that the different species can travel to greater or smaller distances.* 



CHAPTER XCVI. CHANGES IN VEGETATION INDUCED BY 

 SLOW CHANGES IN SOIL FULLY OCCUPIED BY PLANTS 



t 



The struggles discussed in the preceding chapter are to some extent 

 between different communities, of which one community prepares the 

 soil for the other, and, so to speak, persistently works out its own destruc- 

 tion : as illustrations of this we may cite the production of marsh, also 

 the change from shifting to stationary dune. When slow changes in the 

 nature of a station take place the consequent struggles are due, in the 

 vast majority of cases, to changes in the nature of the soil, and especially 

 in the water-content of this. Such struggles are exemplified in the follow- 

 ing cases : 



Many plants besides Salicornia and Zostera act as capturers of mud. 

 Aquatic mosses, algae, and other fresh-water hydrophytes, in rivers and 

 lakes detain mud among themselves. In expanses of fresh water in 

 Europe there is a developmental succession of which the following is an 

 outline : The plants are ranged in zones, which are determined partly 

 by depth of water and partly by nature of soil. In deeper water, besides 

 plankton, limnaea-covcvrn\imt\es dominate ; under water, MyriophyUum^ 

 Characeae, and others spread themselves ; while on the surface rest the 

 swimming-leaves of Potamogeton, Nuphar, and Ranunculus. Nearer to 

 the bank, in shallower water, commences marsh-vegetation ; farther 

 outwards reigns reed-vegetation, which is formed by the tallest and most 

 vigorous species, namely, by Scirpus lacustris, Phragmites communis, and 

 others.^ In the course of time the remains of all these kinds of vegetation, 

 together with inorganic particles that are brought hither by currents of 

 water and wind, accumulate at the bottom of the water so that its bed 

 rises little by little. By this means the site is prepared for other marsh- 

 plants that can grow only in more shallow water : among such are Slum 

 latifolium, species of Carex, Ranunculus Lingua, Menyanthes, Lythrum, 



^ Warming, 1892. 



^ See Warming, 1887, 1903, p. 674 ; Hemsley, 1885; Scliimper, 1891; Guppy, 

 loc. cit. 



* Sernander, 1901. 



* In regard to the contents of this chapter also, the papers by Cowles (1899^ 

 1901) and Clements (1904) should be consulted. 



* See pp. 154, 183, 186. 



