178 FORMATIONS AND GUILDS [Part II 



iii. OPEN EDAPHIC FORMATIONS. 



In many places the physical texture of the soil is such that it does 

 not permit the existence of closed formations. A feature that in deserts 

 is due to climate, in this case is due to the nature of the soil. The 

 soil is occupied by those plants that are able to establish themselves 

 on it in spite of the unfavourable conditions. There are but few of 

 such plants, however, and the formation remains open throughout, so 

 that there is still space left for many plants, and accordingly there is no 

 struggle between competitors. Whatever the climate may be, such 

 places possess the character neither of woodland nor of grassland, but 

 produce a confused mixture of woody and herbaceous plants that are 

 quite independent of one another. 



To the open formations of the kind just described belong, in the first 

 place, those of rock-plants. Naked rock, after cooling down from a 

 molten condition, or after separation from a larger mass of rock, 

 remains bare of vegetation for a longer or shorter period. Sooner or 

 later, sooner in a damp climate than in a dry one, plants appear on its 

 surface, at first small Algae and lichens later on, and after these most 

 accommodating plants have produced a little humus, mosses and higher 

 plants. The vegetation on the surface of rocks or stones may be termed 

 that of lithophytes. Crevices in rocks, in which more finely grained 

 components and more water accumulate than on the surface, produce 

 a somewhat more copious vegetation, that of the cJiasmopJiylcs. A 

 formation of plants on rock consists either of lithophytes only, especially 

 if the rock is free from cracks, or of lithophytes and chasmophytes. 



Lithophytes are low, fiat, spreading plants, the superficial development 

 of which is sometimes determined chiefly by the roots, sometimes by the 

 shoots, which by the help of small roots — or in thallophytes by rhizoids 

 — become attached to the hard substratum. Mosses and phanerogams 

 frequently assume the form of cushions. Chasmophytes, as opposed to 

 lithophytes, are long straggling plants, since their substratum often lies 

 at the bottom of a crevice at a great distance from its mouth and 

 therefore from the light. Hence, many chasmophytes possess extremely 

 long rhizomes and roots, yet such extreme forms are less frequent in 

 rocky crevices than among gravels, which owe their origin to the dis- 

 integration of rocks under the influence of atmospheric agencies, and which 

 usually form large heaps at the foot of the masses of rock from which 

 they have fallen, or create the moraines along the course of glaciers. 

 On these gravels lithophytes are much less frequent than chasmophytes, 

 and the chasmophytes exhibit the frequently extraordinary growth in 

 length to which reference has just been made. 



Some of the fragments of rock come down to the water-courses, 



