Chap. I] THE FORMATIONS 163 



in millions by the slightest breath of wind and greatly facilitate its 

 general distribution, whilst its creeping and deeply penetrating roots 

 increase the difficulty of eradication when once this grass, so tenacious 

 of life, has established itself. I have reasons for believing that while 

 the land was in its original condition the alang-grass was restricted to 

 sundry sterile, arid, waterless tracts of the hot zone, and was chiefly 

 confined to heavy, hard, easily dried clay soil, with an iron-pan, but that 

 at the present time, wherever we meet this grass on a fertile light soil and 

 on mountain-slopes at above 2,000 feet, this state of affairs is first brought 

 about by the hand of man. ... In Northern Sumatra, especially in the 

 Batta country that has been devastated by war, grassy wastes have conse- 

 quently come into existence which cover everything far and wide with 

 a hideous uniformity and overrun plain, mountain, and valley with their 

 whitish-green mantle V 



It is probable that, unless man should again intervene, the alang- 

 steppe will, in the course of time, again give way to the forest, for 

 climatic conditions are in every way more suited to forest than to grass- 

 land. In a decided forest climate, for example along the Brazilian coast, 

 forest quickly succeeds to devastated forest, although it is of a more 

 xerophilous character than before, being the so-called ' capoeira.' 



Desert, the third leading type of climatic formation, originates when, 

 on account of too great drought or cold, climatic conditions are hostile 

 to all vegetation ; the types of both woodland and grassland then become 

 stunted and their differences become obliterated, for the struggle between 

 them ceases. The soil is then monopolized by such woody or herbaceous 

 plants as can still contend successfully against the inclemency of the 

 climate. Transition forms between desert on the one hand, and wood- 

 land or grassland on the other, are termed semi-deserts. 



In the woodland and in the grassland such plants alone can thrive as 

 are at their oecological optimum in respect to all external factors, other- 

 wise they would perish in the struggle with stronger competitors. In 

 deserts this is no longer a necessary condition, as the struggle between 

 the plants ceases. Woodlands and grasslands are closed formations, at 

 least in an oecological sense ; more components cannot be admitted 

 into them and numerous seedlings are continually perishing in the general 

 conflict. The desert, on the contrary, is oecological ly an open formation. 

 Most seeds do not germinate in it, and seedlings frequently succumb to 

 the inclemency of the climate. Others prolong their miserable existence. 

 Many plants die and their places are not reoccupied. There are always 

 many vacant spaces to be filled in the desert 2 . 



1 Junghuhn, op. cit.. Bd. I, p. 153. 



- Very sparsely stocked stations in climatic districts suitable for woodland or grassland 

 must not be confounded with climatic deserts. See under Edaphic Formations, p. 176. 



M 1 



