TROPICS AND SUBTROPICS. 277 



Casuarina. The level ground about the base of the cone is still grass-steppe, 

 dominated by grasses, sedges, and composites, the ferns now playing a sub- 

 ordinate part. The author further concludes that 72 per cent of the phanero- 

 gams have migrated by means of sea-currents, while wind has been effective in 

 the case of 30 per cent. 



Busse (1908) has studied the effect of fires in grassland in Africa: 

 Such fires are set by the natives for a number of different purposes and con- 

 stitute an important part of their culture. Grazing in the high grass-steppes 

 is quite impossible without annual burning. The effect of this is to develop 

 steppe over wide stretches. While natural steppe occurs, the greater part 

 of the steppes was formerly woodland, which has been destroyed by fire, as 

 well as by the ax and by grazing. Abandoned cultivated lands become 

 covered with scrub with an open forest or with other xerophytic forms. The 

 grassland withstands fixes best while the mature forest suffers less than the 

 aftergrowth. 



Chevalier (1909) has described the r61e of the cyperaceous Eriospora pilosa 

 in the high mountains of western Africa: 



This plant grows upon the most arid granite rocks, where it develops a mass 

 of roots. Between the tufts of Eriospora, the roots and rootstocks form a 

 layer of turf 0.05 to 0.3 m. thick, upon which are found some mosses, as well as 

 a species of Sphagnum. This plant has an important r61e in the forestation 

 of bare, rocky plateaus; in fact, the virgin forest has been able to extend itself 

 from the plains to the summits of tropical moimtains by virtue of the soil 

 prepared by Eriospora. At the present time, this development is only occa^ 

 sionally foimd, because the forest is everywhere in regression, in consequence 

 of the influence of the natives. 



Deuerling (1909) has considered silting up processes in general with especial 

 reference to those of standing water, swamp, moor, strand, floating islands, 

 and flowing water. He gives also a survey of the barriers formed by plants 

 in Europe, Asia, North and South America, and then considers especially such 

 barriers in the rivers of Africa: 



The "sadds" of the Upper Nile are considered in particular. These are 

 produced by plant societies which are developed in the swamps of the flood- 

 plain. Such communities are those of Phragmites communis, Typha auMralis, 

 Cyperus papyrus, Vossia procera, and Herminiera. With these are associated 

 a large number of smaller plants which serve to bind the whole together into 

 a floating mass. Grass islands are formed which increase to thick masses and 

 are then driven by the wind into the current. The origin of a barrier out of 

 these swimming complexes results from flood-waters and favorable winds, 

 together with a weaker current associated with windings and narrowings of 

 the bed of the stream itself, which form them into a dam-like mass. The 

 filling up of the river-bed then proceeds not only in width and breadth, but 

 also in depth. The masses of vegetation in the sadd die only at the under- 

 most part, but elsewhere they grow luxuriantly. When the formation of the 

 sadd has proceeded far enough, the plant remains and the deposited silt make 

 an increasingly compact mass which develops into a taU-grass swamp, and is 

 finally scarcely to be distinguished from the land as a consequence of drainage 

 and drying out. 



Whilford (1911) has studied the origin of grasslands in the Philippines in 

 consequence of cultivation: 



