18 TREE-CACTUS. 



quantity of water during the annual rains, and 

 which they return hy evaporation from their 

 depths, in the drier seasons of the year 

 scantily, it is true, hut yet in sufficient quantity 

 to support vegetation ; while the succulent 

 character of the leaves, and bulbous form of the 

 roots of the greater number of the plants, tend 

 much to economise their supply of moisture. 

 To the botanist this spot alone offers a rich 

 field for useful exertion : amongst more than 

 seventy plants collected during our short stay, 

 the majority prove to be new species, and several 

 must be regarded as new genera. 



The more abundant, or conspicuous vegeta- 

 tion includes some splendid examples of the 

 Cactus family. One of these is peculiarly con- 

 spicuous on the plains, rising in an erect and 

 columnar form to the height of fifteen or twenty 

 feet ; its sides deeply fluted, (the angles armed 

 with clusters of black thorns,) and its summit 

 ramifying scantily. Some of the more aged 

 examples have a bole four feet in circumference, 

 destitute of thorns, and covered with a smooth 

 white bark the leaf in this stage of growth as- 

 suming the decided character of a caulis, or 

 trunk. We observed neither flower nor fruit in 

 this species. A vegetable column of this de- 

 scription, rising isolated in the midst of the 



