THE FACTORS OF DISTRIBUTION 59- 



scattered in " open formation," which serve to arrest 

 the mud, and may give rise to little hillocks or touradons, 

 on which grow species of Limonium, Statice, Plantago, etc. 

 Here, or where wind accumulates dunes, less halophytic 

 species may establish themselves. Marram-grass (Am- 

 mophila) will fix the blown sand, a scrub of such species 

 as the Sea-buckthorn (Hippophae Rhamnoides L.) may 

 spring up on the lee of the dunes, and such a sandy flat 

 may be colonised by Pines or Heaths. 



On loose, stony, over-drained mountain screes, saxi- 

 colous Lichens and Mosses are followed by xerophytic 

 grasses, such as Sheep's Fescue (Festuca ovina L.), and 

 these by successively deeper-rooted and taller -growing 

 perennial species, such as Marjoram (Origanum), Ononis, 

 Juniper, Rowan, and Beech. 



Similarly, Algae and Lichens begin the decomposition 

 of the surface of lavas, to be followed by Mosses and 

 (such rocks being generally porous) xerophytes. Of the 

 striking colonisation of Krakatau, a volcanic island at 

 a distance from the mainland, we shall be speaking in 

 the next chapter. 



On the light soil of cleared or pared heath-land, 

 Reindeer-Moss (Cladonia rangiferina) and Polytrichum 

 may be followed by scattered plants of Radiola, before 

 Calluna and Erica cinerea L. regain their foothold, 

 with Bracken; while later Birch and Pine seedlings are 

 able to colonise. Where, however, the ground has been 

 cleared by fire, certain species are peculiarly adapted 

 to obtain the first place. Such are Funaria hygrometrica 

 Sibt., Senecio sylvaticus L., Populus tremula L., and 

 Epilobium angustifolium L. The last-named species is 

 known as " fire-weed " in America, and as " ildmarke " 

 in Denmark. 



In these successions three stages are recognised: the 

 initial, when the species are few and the plants are in 

 open formation; the intermediate, in which the number 

 of species reaches its maximum; and the ultimate, in 

 which as a result of struggle, a reduced number of species 

 become dominant in close formation. In the first stages 

 the " pioneers," or first comers, will be so owing to their 

 " mobility/' or powers of seed-production and dispersal, 

 and nearness at hand. The small -spored Algae, Lichens, 

 | and Fungi will, therefore, generally come first, followed 

 ' by pappus-bearing Composites, such as Senecio, small- 



