35^ 



ZONES AND REGIONS 



[Pt. Ill, Sect. I 



with a gradual transition to desert as the moisture decreases. To the African 

 thorn-forest in particular belongs Engler's ' dense bush ' of the lower bushland, 

 and his ' steppe-bush thicket ' of the inland. 



Engler emphasizes the systematic uniformity of the ' bush-forest ' of tropical 

 Africa, its sj'stematic resemblance to that of Cis-gangetic India, and its phj'sio- 

 gnomic resemblance to that of Central and South America (Mexico, Argentina, and 

 Chili). Systematically, the abundance of various species of Acacia is characteristic 

 of this forest, and the genera Dichrostachys and Albizzia, which also have bipinnate 

 leaves, are represented by numerous individuals. Woodj' plants with pinnate 

 leaves are seldom as plentiful (Bignoniaceae, Odina belonging to the Anacardia- 

 ceae, Harrisonia one of the Simarubiaceae, some Rutaceae, Burseraceae, 

 Connaraceae, Caesalpiniaceae). Plants with digitate leaves are common, for 

 example Commiphora, Rhus, Jasminum, Vitex. Most plants of the thorn-forest _^ 



Fig. 192. Forest tract in the coast region of German East Africa. From a photograph. 



have simple leaves, which are persistent in most of the species and have a very 

 thick cuticle; for example, species of Euphorbiaceae, Celastraceae, Rhamnaceae 

 Rubiaceae, Sterculiaceae, Verbenaceae, Compositae ; and inconspicuous whitish 

 flowers are common. In the dense woods there are very few herbs, but these 

 are plentiful in the clearings. Twining and climbing plants, possibly only 

 thin-stemmed forms, are rich in species ; Peperomia and Angraecum appear a: 

 epiphytes. 



The second type of the xerophilous low forest, the savannah-forest, is apparently! 

 far less developed in Africa than is the thorn-forest. A typical savannah-forest 

 in the sense in which I use the word, is Engler's ' steppe-forest,' which occur 

 in particular in Unyamvvezi. 'Erect trees, 7-12 meters high with stems 3-4 centi' 

 meters thick, form the prevailing type; Leguminosae with pinnate leaves,'^ whicH 



