448 



INTRODUCTION TO PLANT GEOGRAPHY 



[chap. 



dense and exceed the height of a Man, they are rarely close enough 

 together in these semi-desert bush-lands to obscure the entire horizon. 

 Fig. 154 shows a relatively well-vegetated area in Australia. 



In some subtropical regions such as occur in southwestern North 

 America, a shrubby climax may be developed where the annual 

 rainfall averages as little as about 8 cm., provided it is reasonably 

 distributed and reliable. The dominants tend to be many-stemmed 

 but sparsely open, with widely-spreading roots. Although the 



Fig. 154. — Semi-desert 'bush-land' in Australia. 



formation is commonly called ' desert-scrub ' it seems best men- 

 tioned here, even as its more temperate counterpart, characterized 

 by the Creosote-bush, was described under semi-deserts in Chapter 

 XII. Indeed the southern, subtropical extensions towards Central 

 America have little of a regular nature to distinguish them from the 

 northern type, and much the same is true in other regions. Thus 

 while the dominants tend to be lower, the Creosote-bush being in 

 the South only about a metre high and others (such as Bur-sages, 

 Franseria spp.) commonly lower, the transition is normally so 

 gradual that no further type emerges or account need be given 

 here. 



