THE FLORAL ENVIRONMENT 55 



are only small thickets near such rivers as the Tigris 

 and Zarafshan, which flow through deserts. Nearly 

 all the larger natural oases have long ago been 

 totally ruined by man, who has destroyed the 

 characteristic and very local flora and fauna, planted 

 carrots and cabbages and dates (Figs. 10, 12), and 

 introduced along with them the common garden 

 weeds and the common garden insect pests, so that 

 to a naturaUst a " well-developed " oasis is one of 

 the most uninteresting sights in the world. The 

 water which turns a given part of the desert into an 

 oasis may come from any source, from springs, or 

 seepage from a water-bearing stratum, from a passing 

 river, or even from an artificial canal or artesian well. 

 The deserts of the Old World are great plains, 

 and the vegetation is siu*prisingly monotonous and 

 uniform over immense areas. Many genera and 

 species of plants range from the western Hmits of 

 the Sahara, or even from the barren parts of the 

 Azores, Canary and Cape Verde Islands, eastwards 

 to Sind and Turkestan. Conspicuous examples are 

 the Colocynth {CitruUus colocynthus), the Saltbushes 

 (Suceda and Salsola), and members of the genus 

 Zizyphus (Fig. 25) ; very many others might be 

 mentioned which are distributed over the greater 

 part of the vast area. The uniformity of surface 

 and of vegetation, which is so characteristic of the 

 Old World deserts, is much less observable in 

 America. MacDougal states that he found only 

 thirteen species of flowering plants during a month's 

 trip in the Libyan Desert : twelve of the thirteen 

 belonged to very widely distributed species, and he 



