Chapter VIII —131— Natural Factors 



Stocking of Unoccupied Territories: — Territories unoccupied by 

 vegetation, if we exclude those places where plant life is impossible, no 

 longer exist on the globe. The entire surface of the earth, every spot 

 where conditions are at all fit for the existence of organisms, is in- 

 habited. But unoccupied areas may arise as a result of chance, catas- 

 trophic circumstances — alluvial deposits destroying the vegetation 

 buried beneath them, avalanches, volcanic eruptions covering huge 

 areas with ash and lava, upheaval of land from beneath the surface of 

 the sea — or as a result of the destruction of vegetation by man. 



According to Comes, the first plants that made their appearance on 

 the Vesuvian lava fields after they had become cooled were unicellular 

 Algae, next came lichens, then mosses and ferns, and, lastly, the flower- 

 ing plants. Hence, it appears that for the higher, flowering plants to 

 become established it is necessary that sterile soils should first be sub- 

 jected to certain changes brought about by the activities of lower 

 organisms (Chevalier and Guenot, 1925). Ulbrich (1928) watched 

 for a number of years the invasion by plants of a former gravel pit 

 near Berlin. This pit was abandoned in 1894 and during the subse- 

 quent years was gradually invaded by plants. In 1900 there were 

 found on its territory 109 species, including 50 cryptogams; by 1903 

 there were already 268 flowering plants and 113 cryptogams; by 1922 — 

 a total of 959 species, of which 429 were flowering plants. Most of the 

 latter were plants whose seeds and fruits are wind-dispersed. A large 

 number of the plants belonged to species that grew in the surrounding 

 territory, but there were also many that grew only at a distance of 

 150 km. from the place of observation. 



Very often, when a territory is deprived of its plant covering by 

 some catastrophe, there at first appears on it a very homogeneous 

 vegetation, sometimes arising as a result of the rapid spread of a single 

 species, which subsequently begins to die out as a result of competition 

 between its own individual plants and then, little by little, is crowded 

 out by the later comers that gradually put in their appearance. An 

 interesting case of such a rapid spread of a single species is reported by 

 MoLiscH (1927), who observed it immediately after the 1923 earth- 

 quake in Japan. In Tokyo the trunks of the trees along the sides of 

 the street and in the parks, due to the widespread fires throughout the 

 city, were badly charred, and, consequently, aU the epiphytic vegeta- 

 tion on them was destroyed. This territory, free of competitors, was 

 very rapidly inhabited by the fungus, Monilia sitophila, which soon 

 covered all the charred tree trunks. Ordinarily this fungus is rarely 

 encountered in Japan, Molisch having found it only infrequently on 

 discarded ears of corn lying on damp ground. 



An interesting example of the stocking of unoccupied territory, not 

 far distant from occupied land, is provided by the Aland Islands. The 

 entire process of the colonization of these islands by plants was sub- 

 jected to detailed study by Palmgren (1922, 1925, 1929), who found 

 that such a process depends on the size of the territory being colonized 

 and the biological peculiarities of the immigrant species. The upheaval 

 of these islands began in the post-glacial period and is still in progress, 

 in the course of each century the surface of the islands rising higher by 

 about 0.5-0.6 m. Consequently, we have here, as a result of this 



