20 BULLETIN 31, U. S. DEPARTMENT OF AGRICULTURE. 



success in growing them. On the other hand, the short-spined cholla x 

 of southern Arizona and the South American flat- jointed species 2 

 appear to grow readily. 



The most interesting cases of proliferation are those caused by 

 enforced infertility brought on by interference with the normal 

 activities of the plant. Possibly the commonest of these is one 

 brought on by the attacks of a small fly 3 which deposits its eggs in 

 the very young flower buds, causing complete sterility of the fruit, 

 which becomes strictly vegetative and gives rise commonly to normal, 

 belated fruits of small size. These usually drop off the second season, 

 but occasionally they become incorporated as a permanent part of the 

 plant. A similar condition has been observed in the so-called Opuntia 

 puberula of European botanists by the attacks of the black opuntia 

 louse. This insect congregates on the tips of the young flower buds 

 and not only prevents them from opening but renders them com- 

 pletely sterile and proliferous (PI. VII, fig. 1), a second crop of 

 normal, fertile blossoms being induced to grow out of the ones ren- 

 dered infertile by the lice, unless they, too, should be attacked, when 

 they also become sterile. 



Proliferous fruits are of considerable importance upon the stock 

 ranges of the Southwest. In the chollas of southern Arizona and 

 southern California we have plants altogether too spiny for stock 

 to graze, but the abundant bunches of fruits are much smoother, 

 furnishing succulent and nutritious morsels which contribute not a 

 little to the sustaining capacity of the ranges in times of need. The 

 flat- jointed species of South America - would appear to possess the 

 same characteristics. The chollas of the highland of Mexico, 4 al- 

 though possessing very smooth fruits which remain attached to the 

 plant indefinitely, are too acid for stock to relish. In the United 

 States there are about half a dozen species whose fruits appear to 

 be incorporated as a permanent part of the plant. In some other 

 cases this permanency is brought about by purely artificial condi- 

 tions, all of which indicate a closer association between the stem 

 and the fruit in this group than is common. Were it possible to 

 engraft this character upon some of the spineless species, their value 

 for forage purposes would be very much increased. The fact that the 

 character occurs in both cylindrical and flat- jointed forms indicates 

 that this feat is not at all impossible of accomplishment. 



HABITS OF THE PLANTS. 



Nothing has been more clearly shown in our investigations in 

 seed germination of species of Opuntia than the fact that vegeta- 

 tively propagated plants are far from normal in general aspect. This 



1 Opuntia mamillata. * Asphondylia opuntiae. 



'Opuntia monacantha. "Opuntia imb?'icata. 



