294 PSILOPHYTES SECT, xm 



Orchard-scrub may sometimes abound in grasses and herbs during the 

 rainy season. 



Thorn-bushland and Thorn-forest. 



Where the shrubs stand closer together orchard-scrub gives way to 

 thorn-bushland. Thorn-bushland is widespread over the tropics where 

 the rainfall is not considerable. According to Urban, in East Africa 

 two types may be distinguished : deciduous thorn-bushland, and aphyllous 

 bushland composed of succulent euphorbias. The two types frequently 

 merge into one another. East African thorn-bushland is sometimes 

 composed of many species, but at other times there are certain prepon- 

 derant species, namely acacias. Despite prolonged drought epiphytes are 

 not entirely wanting. Here and there a few herbs occur. 



Characteristic of the shrubs in thorn-bushland are a thick cuticle 

 or a coating of long persistent hairs, and the small development of the 

 leaf-surface, in addition to the precocious change of leaves and stems into 

 spines ; all of these being protective devices occurring in members of 

 the most diverse families. Species of Acacia and other Leguminosae, 

 on the contrary, guard against complete desiccation by the closing together 

 of their glabrous leaflets. In Ugogo where thorn-bushland prevails 

 there are no permanent streams, but only greenish, glittering, often 

 soda-containing pools in the valleys, and carefully guarded water-holes 

 in which rain-water must be stored often for eight months. In Ugogo 

 springtime commences about the middle of November. The large bushes 

 become green and conceal the red soil as well as their numerous thorns. 



Thorn-bushland and thorn-forest are very widespread in the drier 

 parts of Africa and Asia. 1 



Caa-Tinga. 



In the more central and northern parts of Brazil, especially on cal- 

 careous soil, one encounters Caa-Tingas, forests which have been so 

 well known ever since the travels of Martius. The majority of the trees 

 protect themselves from the prolonged drought and heat by shedding 

 their leaves, and for this reason Caa-Tinga forest is extremely hot during 

 the dry season. Among the remarkable trees present the best known 

 is the bombaceous Chorisia crispiflora, which has a barrel-like swollen 

 trunk whose loose soft wood acts as a gigantic water-reservoir 2 : Spondias 

 tuberosa apparently possesses in its swollen roots subterranean water- 

 reservoirs. Smaller trees and bushes are evergreen, and in this case 

 find in their coriaceous, thick, and stiff or white-haired leaves, protection 

 against excessive transpiration. Caa-Tinga forest abounds in thorny 

 and stinging (e.g. Jatropha) plants, in columnar Cactaceae, and in other 

 succulent plants. It is forest green in the rainy season. Scarcely has 

 the dry season given way to the first spring rains before the foliage 

 hurriedly sprouts forth ; and in one or two days all is green. Similar 

 behaviour characterizes the allied West Indian dry bushlands or bush- 

 forests. The important phyto-geographical part played by water reveals 

 itself in many ways ; if the permanent ground-water lies so near the 



1 Passarge, 1895 ; Warburg, 1893. * Martius, 1840-7, Tab. 30; Ule, 1908. 



