How to Know the Lichens and Mosses 



Atnblystegiwn riparium, which cling to rocks in streams, are so 

 conglomerated by mud and sand that they cannot be freed from 

 it until the plants have become dried and shrivelled. Limnobium 

 molle, which grows in the turbid waters from glaciers, has such 

 an abundance of earthly particles adhering to it that only the 

 green tips of the leaf-bearing stems are visible above the gray- 

 coloured cushions imbedded in the mud. It is the dead parts 

 alone which retain in their thick felt of interwoven filaments, the 

 firmly divided mud and sand. That they are able to do this is 

 due to the fact that the cell-membranes swell up and become 

 slightly mucilaginous. 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 are crustaceous lichens, 

 minute mosses, and algae. Larger lichens and mosses are able 

 to gain a footing on the substratum prepared by them. 



" Tis spring-time on the eastern hills! 

 Like torrents gush the summer rills, 

 Through winter's moss and dry dead leaves 

 The bladed grass revives and lives, 

 Pushes the mouldering waste away, 

 And glimpses to the April day." 



Whittier Mogg Megone, Pt. HI. 



The dead filaments, stems, and leaves of this second genera- 

 tion 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 the water of pond- weeds, water-crowfoots, hornwort, and 

 related plants. The second generation is produced in greater 

 abundance than the first, and the third develops more luxuriantly 

 than the second. The third may be followed by a fourth, fifth, 

 and sixth, each successive generation crushing out and supplant- 

 ing the one preceding it. 



Another marked and important change results from these 

 small beginnings. Streams on rather flat lands are turned from 

 their courses by the accumulation of debris made possible by the 

 arrested sand and mud, ponds have their outlets choked so that 

 often new outlets must be cut, and small lakes are often cut in two 

 by a natural divide which is due to the accumulation of sand and silt 

 bound together, first by water plants and later by shrubs and trees. 



IS 



