380 ZONES AND REGIONS [Pt. Ill, Sect. I 



ii. LIME. 



In warm climates. lime appears to exercise an action on plant-life quite 

 different from that which it has in temperate and cold climates. Weathered 

 soil of pure limestone affords less favourable conditions for the growth of 

 plants, and the number of plants whose development is favoured by liming 

 the soil is smaller in low than in high latitudes ^. 



Nothing quite certain is known regarding the influence of the chemical 

 properties of lime on the constitution of the vegetation in the tropics, 

 although several species appear to be limited to a calcareous soil. The 

 effects of a calcareous soil that have hitherto been demonstrated refer 

 solely to stony situations, poor in humus, in periodically dry regions, and 

 possibly may be traced back to the low water-absorbing power of lime, 

 therefore to a purely physical property. 



In the climate of the monsoon-forest, calcareous soil having the low 

 water-absorbing power just mentioned causes the appearance of the most 

 xerophilous of tropical forest-types, namely the thorn-forest, or it may be 

 of thorn-bush and thorn-scrub, which denote a still greater dryness. The 

 occurrence of thorn-forest on calcareous soil in Central Brazil has been 

 already mentioned-. In the periodically dry districts in Pegu, Kurz's 

 'dry forest,' a type corresponding exactly to our thorn-forest, is characteristic 

 of dry, stony, calcareous soil. It is a bush-like forest, green in the rainy 

 season, 'not very inviting on account of the prevalence of thorny trees and 

 shrubs.' The trees are there of moderate height (50-70 feet, exceptionally 

 up to 100 feet); Acacia Catechu (sha) is in such forests frequently the 

 predominant species of tree, hence the name sha-forest. Finally, forests 

 and bush of similar oecological character have been described by Warburg 

 on calcareous soil at Ceram-Laut. 



Warburg observed a diversified primary forest vegetation, where there was 

 almost no humus, on calcareous rocks, if the latter were sufficiently fissured ; this 

 vegetation consisted chiefly of bushes, some of which were armed with thorns. 

 One or two endemic species were found there alone. The following species pre- 

 dominated : Trema virgata, Bl., Dalbergia densa, Benth., Eugenia Reinwardtiana, DC, 

 Zanthoxylum diversifolium, Warb., Atalantia paniculata, Warb., Breynia cernua, 

 Miill.-Arg., Acalypha grandis, Benth., Flagellaria indica, Linn., Citrus Hj^strix, DC. 

 In places, collections of a small bamboo, Schizostachyum Zollinger!, occurred. 



iii. HUMUS\ 



Soils rich in humus cover smaller areas in the tropics than in temperate! 



zones, and pure deep humus-soils are very rare. The poverty in humus is, 



a consequence of the acceleration in the development of micro-organisms j 



occasioned by the tropical heat, which is at least for a part of the )-ear 



' Wohltmann, op. cit., pp. 134-5. ' See p. 360. = Wohltmann, op. cit., p. 173. t 



