454 INTRODUCTION TO PLANT GEOGRAPHY [CHAP, 



— provided, of course, that salts are not present to excess. Date 

 Palms [Phoenix dactylifera) and attractive gardens can thus be 

 cultivated in otherwise desert areas, and in large oases a fine variety of 

 tropical and subtropical agricultural crops are produced. Wherever 

 there is feed for Mammals, desert forms such as Gazelles are apt 

 to pasture, while for miles around inhabited oases little save poisonous 

 or distasteful plant material is normally left undisturbed. Along 

 dried-up watercourses (zvadis) the trees may attain large dimensions, 

 though usually remaining small-leafed and thorny, while perennial 

 Grasses, which are otherwise rare, often inhabit the sandy or gravelly 

 beds. Where desert conditions extend into temperate regions, as 

 in the Gobi, the oases are characterized (as mentioned in Chapter 

 XII) by tall Poplars and Willows. Here other vegetation is in 

 keeping with the temperate situation, the crops being such temperate 

 ones as Barley, Wheat, and Plums. On their poleward side such 

 cooler deserts are usually bordered by wide steppes, whereas hot 

 deserts are typically bordered by semi-desert scrub, at least on the 

 equatorial side. 



Mangrove and Other Sea-Shore Vegetation 



By far the most characteristic and important vegetation-type of 

 tropical and subtropical sea-shores is the ' mangrove ' or ' mangrove- 

 swamp forest ' developed on mud-flats which are exposed at low 

 tide but otherwise normally covered by salt or brackish water, at 

 least being reached occasionally during the highest tides. Par- 

 ticularly favourable conditions for the development of mangroves 

 are found in creeks and quiet bays ending river estuaries, where 

 tidewaters cause the deposit of river sediment. On the resulting 

 flats and deltas, the water-borne seeds or seedlings of the colonizing 

 plants grow, soon forming the characteristic, rather low and dense 

 forest (Fig. 157). In other cases the mangrove forest in its interior 

 may consist of sizeable trees and be quite lofty (Fig. 158). In 

 Malaya and to a lesser extent in Borneo there are large stretches of 

 uniformly tall, mature mangroves (I. V. Polunin voce). Thus in 

 Malaya the ' climax ' mangrove forest consists of relatively few species 

 which tend to be gregarious, producing stands of uniform height, 

 whereas in Borneo the stands are not so pure or uniform (J. A. R. 

 Anderson and I. V. Polunin in Hit.). Actually, real climax man- 

 grove now scarcely exists in Malaya, owing to felling on a rotation 

 of about 30 years, which leads to a retention of this height-uniformity 



