246 LITHOPHYTES SECT, vm 



tion. The vertically cleft basalt rocks are well-nigh devoid of plant-life, 

 because they are quite incapable of retaining water. The irregularly 

 cleft basalt rocks are inhabited by a few strongly xerophytic under- 

 shrubs and some herbs. Where strata of basalt and tuff alternate, the 

 difference in their powers of conducting water becomes obvious. The 

 tuff, which is a good conductor of water, is clothed with Adiantum Capillus 

 Veneris and other plants, or, near cultivated soil, with Parietaria judaica, 

 and stands out in the form of horizontal bands on the walls of rock. Yet 

 during summer these mesophytes are in a partially faded condition. 

 Rocks consisting entirely of tuff display vegetation quite as scanty as that 

 of basalt rocks, because there is no impervious stratum lying below. 



In tropical places where the air is moist or where the site is shady, 

 for instance in forests, or in humid mountain-valleys where mist often 

 hangs over the ground, we may encounter a vegetation of chasmophytes, 

 including dense, green cushions of moss and many mesophytes just as 

 in moist temperate situations. 



CHAPTER LXIV. FORMATIONS ON SHINGLE AND RUBBLE 



CLOSELY allied to formations on rock are those on rubble, which 

 develop on a soil that has arisen through the disintegration of rock by 

 atmospheric agencies (heat and frost). This soil produced by weathering 

 is composed of detached, angular pieces of rock, and smaller or larger 

 stones which have fallen from the faces of rocks ; at the base of rocks 

 in mountainous countries there are often found accumulations of this 

 kind talus. In some cases the fragments of rock are so large as to 

 remain stationary, but in others they are smaller and produce an unstable 

 rubble that slips down. The formations produced show corresponding 

 differences. 



In the former case, in the course of time earth accumulates between 

 the fragments of rock in greater or smaller quantities, and thus provides 

 a soil capable of sustaining highly organized plants and even trees. And 

 when this is not the case many herbs may send their roots down between 

 the stones into the subjacent soil and grow luxuriantly. Among such 

 masses of stones one often finds delicate mesophytes, including ferns, 

 that with this exception are confined to forest : the reason for this is 

 that the roots under and between the stones find sufficient water to replace 

 that lost in transpiration. There occur not only non-migratory plants, 

 but also migratory plants, because beneath and beside the stones there 

 is adequate space for rhizomes and runners. 



On talus of this kind two formations occur intimately mingled 

 together : 



1. Lithophytes on the stones. 



2. The remaining vegetation between the stones. As time passes, 

 the accumulation of earth and humus between the stones may become 

 so considerable that the stones are covered and the lithophytes vanish. 

 The vegetation thus has a developmental history, and presents an appear- 

 ance that changes with its age. 



On talus consisting of small-stoned, slipping rubble, the species 



