Mosses and Lichens 



but their work does not stop here, for as the older plants die and 

 crumble and mingle with the disintegrated rock, an incredible 

 amount of earth-mould is formed which is a favourable site for 

 higher forms of mosses, ferns, and other spore-bearing plants. 



That the leafy parts above arrest to a remarkable degree the 

 dust which pervades the atmosphere, not only along dusty road- 

 sides and open plains, but also in remote mountain 

 valleys, in Arctic ice fields, and in most of the 

 elevated parts of the earth's crust, will be evident 

 to one who detaches and examines a small tuft of 

 Barbula, which everywhere occurs on roadside 

 walls. He will be surprised to learn the extent to 

 which the road dust has been lodged in the older 

 dead parts of the plants, and he will be equally sur- 

 prised to learn with what tenacity the dust is held. 

 The power the older parts of the plants have of 

 holding the dust is due to certain alterations which 

 take place in the lifeless cell-tissue. To be con- 

 vinced that fine dust is also carried to the more 

 remote and elevated regions, one must examine the 

 lichens and dark Grimmias, Andr&as and other rock 

 mosses which grow in small cushion-like tufts on 

 weather-beaten mountain crags, when he will find 

 that not much less dust has been arrested by them 

 than by the Barbula living near the dusty roadside. 

 Old crumbled lichens, together with dust blown 



Andraa rupestnt ,.,.... , , , 



Plant with spore- thither by the wind, accumulate under the thallus, 

 case - or leaf-like expansion of the lichen, and soon form 



a suitable home in which moss spores may grow. The mosses 

 in turn add their share to the accumulation of humus preparatory 

 to the coming of the ferns, and the ferns in turn prepare for the 

 trees with winged seeds, the evergreens and birches, which 

 require no very great depth of soil, sturdy pioneers of mountain 

 forests. 



It is true that all green plants do a similar work, but they do 

 not work under such primitive conditions as do the mosses and 

 lichens. 



Aquatic mosses possess, perhaps to a greater degree, the power 

 of arresting and retaining mud and fine sand hurried along by a 

 violent rush of water. The plants of Hypnum rusciforme and 



14 



