36o STRUGGLE BETWEEN PLANT COMMUNITIES sect, xvii 



Hypnum) began to extend into the water. Gradually development advanced 

 from the margin of the depression to the centre, in the form of a 

 supernatant Sphagnum-moor on which Eriophorum, species of Carex, and 

 other plants grew. As the chmate became milder the higher surrounding 

 ground acquired vegetations of trees in the following order : Populus 

 tremula and Betula, Finns sylvestris and Quercus.^ Trunks of these trees 

 were overturned by wind and buried in the moor together with their leaves, 

 fruits, and so forth : thus arose the forest-moors which were rich in trees 

 and are so common in the north of Seeland. Above them often lies low- 

 moor or Sphagnum-vegetation ; but many of them are covered with 

 meadow or, thanks to cultivation, with pastures and cornfields. 



There are many other means by which water-filled depressions are 

 occupied by upward growth : some of these have not been sufficiently 

 investigated, others cannot be discussed here. In peat-hollows one may 

 sometimes see rhizomes or even prostrate horizontal assimilatory shoots 

 of Equisetum limosum growing from the margins or sides towards the 

 centre and gradually preparing the way for other plants. 



As a whole the development of vegetation in Denmark and in many 

 other countries for centuries or even millennia past has proceeded in the 

 direction of gradual drying, and is still so moving. Aquatic vegetation is 

 being suppressed, lakes and ditches are vanishing, streams are being 

 narrowed : in favour of these statements there is much historical, archaeo- 

 logical, and geological evidence. The choking up of lakes in Denmark and 

 on the Baltic coast is generally influenced by the direction of the wind, 

 as Forchhammer^ pointed out some decades ago. Klinge^ has noted 

 that the same is true in Russian provinces bounding the Baltic Sea. The 

 western banks of the lakes are mostly shallow, flat, and marshy, while 

 the eastern banks have steep, stony sides. The reason for this is that the 

 western banks are more protected and sheltered from the prevailing 

 south-west and west winds than are the eastern banks, where the violence 

 of the waves prevents the process of filling up by plants. On the western 

 banks the marsh-vegetation can consequently advance and the banks 

 recede, whereas the eastern banks rather move in a landward direction. 



Another interesting example of the production of land by the activity 

 of vegetation and of the coincident displacement of one type of vegetation 

 by another is provided by the behaviour of mangrove-vegeta.tion.'^ The 

 zone farthest out to sea is formed by species of Rhizophora. Thousands 

 of their aerial roots weaken the force of the waves ; organic and other 

 transported particles collect here and are deposited. In this way the rhizo- 

 phoras prepare the soil for other mangrove-plants that are not capable 

 of advancing so far out to sea. On the landward side the final stage is 

 the development of xerophytic littoral forest, such as Barringtonia-forest, 

 on dry soil.^ Thus in favoured spots mangrove-vegetation constantly 

 advances farther in the seaward direction. 



A peculiar developmental succession evoked by increasing dryness is 

 exhibited in Lapland,^ where Sphagnum-moor undergoes the following 

 changes. Bog-mosses gradually disappear, as their tufts become over- 

 grown by other mosses that are content with less moisture, and by lichens. 



' Steenstrup, 1841. See p. 203. 



' Forchhammer, in lectures at the University of Copenhagen, i860. 



' Klinge, 1890. * Seep. 234. ' See p. 228. * Kihlman, 1890. 



