MECHANICAL CHANGES EFFECTED IN THE GROUND BY PLANTS. 267 



forming ultimately a bed of flowering plants, including in especial abundance 

 representatives of the orders of grasses, pinks, and composites. 



Many water-plantsin particular, aquatic mosses and algae possess, in an 

 almost greater degree than lithophytes or land plants, the power of laying hold of 

 inorganic particles, and thus exercise a far-reaching influence as mud-collectors on 

 the conformation of the ground. It is wonderful how plants are able to arrest large 

 quantities of the fine sand hurried along by a flood, although they are exposed 

 to the violent rush of the water. The tufts of the dark green alga Lemanea 

 fluviatilis and of the aquatic moss Ginclidotus riparius, which cling to rocks in 

 the cascades of clear and rapid mountain torrents, are so conglomerated by mud and 

 sand that they cannot be freed therefrom until the tissue has become dry and 

 shrivelled. Limnobium molle, which grows in the turbid waters from glaciers, has 

 such an abundance of earthy particles adhering to it that only the green tips of the 

 leaf-bearing stems are visible above the grey-coloured cushions imbedded in the 

 mud. The felted masses of Vaucheria clavata, filling the channels of apparently 

 clear, gently-flowing streams, are so mixed with mud that if a lump of this alga is 

 fished out, the weight of mud clinging to it exceeds that of the alga itself a 

 hundredfold. In these cases of submerged plants, it is, again, not the living but the 

 dead parts which serve to arrest the mud. On lifting up a lump one sees clearly 

 that only the uppermost and youngest prolongations of the filaments those situated 

 at the periphery of the algal cushion as a whole are living and filled with chloro- 

 phyll; the fundamental mass has become colourless and lifeless. But these dead 

 parts, which form a thick felt of interwoven filaments, alone retain in their meshes 

 the finely-divided mud and sand in such surprising quantities; these particles slip 

 off the green living parts without adhering to them. An important consideration 

 in this connection is the fact that the dead cell-membranes swell up and become 

 slightly mucilaginous, so that fine particles of mud lodge more easily in the soft 

 swollen substratum thus formed. Wooden stakes stripped of bark and fixed in a 

 strong current show this very clearly, as do also the trunks of trees that are thrown 

 up by floods and lie stranded on the shore with their bared boughs projecting into 

 the stream. However strong the current to which wood in that condition is ex- 

 posed, it covers itself in a short time with a grey coat consisting of earthy particles 

 brought down by the water. If a piece is cut off and exposed to the air, the earthy 

 deposit does not become detached until the wood-cells have dried up and shrunk. 

 As long as they are moist the particles of mud continue to adhere to them. 



This mechanical retention and storage of dust by rock-plants and of mud by 

 aquatic plants is of the greatest importance in determining the development of the 

 earth's covering of vegetation. The first settlers on the bare ground are crustaceous 

 lichens, minute mosses and algae. On the substratum prepared by them, larger 

 lichens, mosses, and algse are able to gain a footing. The dead filaments, stems, 

 and leaves pertaining to this second generation arrest dust in the air and mud in 

 the water, and thus prepare a soft bed for the germs of a third generation, which 

 on rocks consists of grasses, composites, pinks, and other small herbs, and in water 



