TRANSPIRATION AND TEMPERATURES 



37 



signed to be both systematic and comprehensive. It embraced a 

 delineation of the principal vegetative conditions to be met with in 

 deserts, some investigations of the relations of the chief environmental 

 factors to the characteristic plants, and an examination of the more 

 important adaptations of a large number of species. One of the fea- 

 tures of this contribution deemed of great importance was the recog- 

 nition of the major problems to be encountered and an outline of fur- 

 ther researches needed upon the subject. The region included in this 

 survey consists, in large part, of mesas in which Covillea and Gaert- 

 neria are the prevailing plants. The surface layers of the soil consist 

 of gravel, sand, and boulders. An average of the data obtained by the 

 ten Weather Bureau stations nearest the region showed a rainfall of 

 about 5 inches annually, and a precipitation amounting to 1.54 inches 

 was observed in the region itself from January to June inclusive in 1S91 . 

 The extreme dryness of the atmosphere is illustrated by the fact that the 

 relative humidity at 5 P. M. taken daily during the five months men- 

 tioned was 15.6 per cent. On the 4th and 5th of August of the same 

 season a minimum of 5 per cent was recorded. A maximum temper- 

 ature of 122° was recorded five times during the summer season of 1S91 

 and a minimum of 30° was reached in January and February of the 

 same year. Vegetation in this district was seen to exhibit its great- 

 est activity during the period of maximum precipitation with medium 

 temperatures from February to May ; a quiescent condition during the 

 season of maximum temperature and dryness during June to Novem- 

 ber : and a condition of slow growth during the low temperatures of 

 December and January. 



Perhaps the most noteworthy result of this study of the flora con- 

 sisted in the discovery that the vegetation was composed almost wholly of 

 perennial shrubs and annual herbs, and but few adaptations were found 

 for storage of water. The tendency to form fleshy fruits was almost 

 lacking and even the fruits of Opuritia were comparatively dry and 

 hard. The root systems of a number of plants were examined and the 

 mesquite {Prosopis) was found to have roots more than 50 feet long. 

 Growth or increase in length and thickness was found to be extremely 

 slow in the perennials, though very rapid in the annuals which carry 

 out their entire vegetative and reproductive cycle during the period of 

 maximum precipitation. Many interesting facts are also cited as to the 

 uses of hairy coverings and resinous coating in the prevention of dam- 

 age by extreme evaporation of water and intense radiation. 



