266 MECHANICAL CHANGES EFFECTED IN THE GROUND BY PLANTS. 



increased looseness of the soil in the locality, a result of the greatest importance, 

 inasmuch as it enables air and water to permeate to a depth much more easily and 

 quickly by the ways that the roots have previously opened up. Dead roots rotting 

 underground constitute, moreover, the source of the carbonic and nitric acids which 

 help to render available the mineral constituents, and so serve the turn of subsequent 

 generations of plant-settlers on the same spots, whilst they accomplish fresh disin- 

 tegration of the substance of the soil. 



If, however, the subterranean parts of plants are continually engaged in minino-, 

 and so change in various ways the position of the component particles of soil, the 

 organs above ground exert an influence in some measure opposed thereto, in that 

 they retain and bring to rest particles of earth which are set in motion by currents 

 of air or water. In the section that treats of the absorption of nutrient salts by 

 lithophytes, attention was directed to the fact that the dust pervading the atmos- 

 phere, and blown from place to place by the wind, is arrested to a remarkable extent 

 by mosses and lichens. One need only detach a small tuft of the common Barbula 

 muralis, which everywhere occurs on walls by roadsides, to convince oneself of the 

 extent to which dust from the road is lodged amongst the leaves and stemlets, and 

 of the tenacity of its adhesion. Moreover, not only such dust as rises from roads, 

 but also that variety which, though not easily observed, yet fills the air of remote 

 mountain-valleys, of arctic ice-fields, and of the most elevated parts of the earth's 

 crust, is arrested in those localities by mosses and liverworts, and by many Phanero- 

 gams besides, the growth of which is similar to that of mosses. There is not much 

 less dust clinging amongst the stemlets of the dark Grimmias, Andreseas, and other 

 rock-mosses, which grow in small cushion-like tufts on weather-beaten mountain 

 crags, than is attached to the Barbula living by the dusty roadside. If one of the 

 tufts in question is detached from its substratum, a fine powder composed of mica- 

 scales, granules of quartz, chips of felspar, and a number of minute organic frag- 

 ments pours out from between the moss-stems, whilst another portion of this finely 

 powdered earth is left clinging to the leaves and stemlets, and is found to be regu- 

 larly adnate to them. 



It is never, however, the still fresh and living upper parts of these leafy moss- 

 stems that arrest and cai-ry dust, but always the older dead parts below. The lower 

 dead half of the moss, whether still in a state of preservation or already rotting, is 

 alone capable (in consequence of characteristic alterations in the lifeless cell-tissue) 

 of holding fast the atmospheric dust. The under part of moderate-sized cushions of 

 moss constitutes a compact mass composed half of imprisoned dust and half of 

 brown lifeless moss-stems. These little cushions, clothing rocky crags, become a 

 favourable site for the germination of a whole host of seeds, which are conveyed 

 thither by the wind and detained in the same manner as the dust. The seedlings 

 arising from these seeds send their rootlets into the subjacent portion of the bed 

 of moss, where the interstices are full of dust or finaly-divided earth. Here they 

 find all the conditions prevailing necessary for their nourishment, and they expand, 

 and, little by little, crowd out the mosses which received them so hospitably, 



