452 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



underground bulbous or tuberous storage organs, send up aerial 

 shoots Avhich flower and fruit but die down with the resumption of 

 drought. Hence the ' flowery carpets ' of delicate mesophytes that 

 some of the more seasonally-varying deserts often exhibit between 

 their scattered bushes after adequate rainfall. In contrast to these 

 ephemerals and ' deciduous perennials ', the shrubs commonly have 

 very small evergreen xerophilous leaves, or sometimes larger 

 deciduous ones which are lost after the rainy period ; others may 

 have the leaves reduced to scales, photosynthesis being carried on 

 instead by green twigs or leaf-like or succulent stems. 



Most of the desert plants having perennial aerial parts are extremely 

 xeromorphic, exhibiting such features as excessive development of 

 fibrous tissues, thickened or otherwise covered epidermis, sunken 

 and protected stomata, and reduction or ' waxing ' of the transpiring 

 surface. They also commonly exhibit the xerophytic feature of high 

 osmotic value of the cell-sap. Frequently several of these char- 

 acteristics are shown by a single plant, sometimes to an extraordinary 

 degree. Dispersal of seeds from whole plants detached by the wind 

 and acting as tumble-weeds is fairly common, and often seeds will 

 remain dormant for years on end before germinating when sufficient 

 water becomes available. Even if many xeromorphic desert plants 

 may transpire fairly rapidly when water is plentiful, they are able 

 when necessary, by such features as those already mentioned and 

 by keeping the stomata closed all day, to reduce their water-loss to 

 a minimum and so often survive prolonged drought. They also 

 exhibit considerable resistance to wilting and to injury as a result 

 of water-loss. Thorny shrubs or broom-like switch-plants and 

 other drought-endurers are particularly characteristic of deserts, as 

 are, of course, extreme succulents and manv ephemerals, but it is 

 too ' con^enient ' to classify desert plants into any such stereotyped 

 categories. A few root-parasites also occur rather widely in deserts. 

 Contrarv to popular supposition, the larger succulents are unable 

 to withstand the conditions of the drier deserts. 



Where there is a lasting supply of water, as along the banks of 

 rivers whether permanent or seasonal, or where the ground-water 

 rises to near or sometimes above the surface, as in oases, the vegeta- 

 tion is able to demonstrate at once the natural fertility of the soil 



other ephemerals in his desert quadrats to grow up, flower, ripen seed, and die 

 down — all within a period of about five weeks, though this is only in the warm- 

 temperate belt. It is, however, possible that germination had taken place before 

 the main rains came, and so closer observations must be made in future. Fig. 156 

 shows the ' before and after ' effect of heavy rain in a desert of central Iraq. 



