22 BULLETIN 31, IT. S. DEPAKTMEXT OF AGEICULTUKE. 



agriculturists, especially in Australia and South Africa, in recog- 

 nizing their plants from descriptions and illustrations. 



MOISTURE REQUIREMENTS. 



The adaptability of species to varying conditions of humidity has 

 already been partially covered on another page, especially in so far 

 as the effect of a large supply is concerned. The important point in 

 this connection is the minimum moisture under which the plants will 

 thrive. It can not be too positively urged that a minimum require- 

 ment is easily reached, or that our deserts are incapable of support- 

 ing crops of native pear — much less of the spineless introduced 

 varieties. 



At neither Brownsville, San Antonio, nor Chico do well-established 

 plants under cultivation ever suffer from drought. At San Antonio, 

 on the other hand, common experience teaches that the native species 

 become badly withered during prolonged periods of drought. The 

 same condition is occasionally seen in the vicinity of Brownsville, 

 but less frequently. Under cultivation, however, the supply of mois- 

 ture is sufficiently conserved to tide over any drought that ever occurs 

 without any wilting of the plants. 



There is a vast difference in the moisture requirements of different 

 species. In 1907 a collection of about 100 species was growing at 

 Chico. The plants were mostly in their second year's growth and 

 consequently well established. Owing to a misunderstanding the 

 plantation was not cultivated or irrigated until the month of August. 



Plate VIII, figure 2, shows the effect in a very graphic manner. In 

 the illustration are shown mainly spineless and Mexican highland spe- 

 cies, all of which are badly wilted. All of these species thrive without 

 the least sign of wilting when well cultivated. Scattered through the 

 plantation shown in the figure are a number of Arizona flat and cylin- 

 drical jointed species, none of which appear to have suffered in the 

 least from lack of attention. The conspicuous plants in the fore- 

 ground are from the region of Guadalajara and are in the worst 

 condition of any. The next to suffer were the spineless forms, and 

 those which suffered the least or which suffered none at all were the 

 truly desert species. 1 Since the season of 1907 all of these species, 

 together with many others, have been grown under good cultivation. 

 None of them have shown any indication of suffering from drought, 

 excepting an occasional plant whose root system had received acci- 

 dental injury. Even cuttings of the spineless species, if well estab- 

 lished in early spring and well cultivated, do not suffer from drought, 

 and they are much more likely to grow than the desert species. In- 



1 Opuntia discata, O. canada, O. fulgida, O. spinosior, O. rufida, O. laevis, O. 

 phaeacantha, O. macrocentra, etc. 



