442 INTRODUCTION TO PLANT GEOGRAPHY [CHAP, 



lacking. On the other hand in favourable situations where the 

 trees grow close together, there may be a fair array of xerophilous 

 epiphytic Bromeliads, Orchids, and Ferns, that seem more properly 

 to belong to the rain forest. 



Savanna-woodland of one sort or another is found very widely 

 in tropical and subtropical regions including much of Cuba and 

 elsewhere in the Caribbean, Brazil and northern Argentina, East and 

 central Africa both north and south of the Equator, and occupying 

 much of India and China as well as of northern and eastern Australia, 



3. Thorn-woodlands J with similar or allied types called tropical 

 thorn-forests, thornwoods, thornbush, caatinga, etc., are usually still 

 more xerophilous, being found in areas of still lower rainfall and 

 more prolonged drought-period than the often poorly-differentiated 

 but usually much more grassy savanna-woodlands. Indeed Grasses 

 are often lacking in the drier thorn-woodlands, or tend to be 

 segregated into clumps separated by areas of bare soil. The present 

 group of vegetation-types are found chiefly where the annual rainfall 

 is between 40 and 90 cm. but variable, with the temperature high 

 all the year round, ranging from 15 to 35° C. The terms quoted 

 are not synonymous in that they are apt to be applied to communities 

 of different physiognomy growing under different circumstances in 

 different regions. And just as the major types dealt with previously 

 in this section grade into one another, as indeed do many vegetation- 

 types elsewhere, so do the present variants intergrade and inter- 

 digitate with the savannas and grasslands dealt with below. More- 

 over, various and even quite different types and sequences may be 

 involved where local water or other conditions change markedly. 

 Thus succulents are often most in evidence in areas of particularly 

 coarse, over-drained soil, and may form distinctive communities 

 alternating with one or another type of thorn-woodland. 



The foliage of the dominants of tropical thorn-woodlands is 

 deciduous or markedly xerophilous, or often reduced to mere scales, 

 thorns or prickles being a common feature as the above names 

 imply. Switch-plants with woody photosynthetic stems are also 

 characteristic of the community. The roots of the main plants are 

 much branched, and competition among them for water may be 

 severe. Often they penetrate very deeply, shallow-rooting types 

 such as Grasses having little chance of success. Many of the woody 

 plants store water for the dry season in swollen trunks or roots, as 

 in the case of the Brazilian Bottletree {CavaniJlesia arhorea) whose 



