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



these partly diseased branches, which will grow vigorously for 

 about two years, then deteriorate, as stated above, repeating the 

 history of the parent plant. From all appearances this species 

 is in nature long lived. No one knows just how old the plants be- 

 come, but certainly thej^ do not die from age in nature in less than 

 15 years, and in no case have phenomena been observed similar to 

 those which occur in the ones under cultivation. 



Just what these phenomena mean or why they occur in the par- 

 ticular way in which they do is a matter well worthy of investi- 

 gation. Although not occurring at present in any of the strictly 

 economic species, the behavior is still an important one to study, 

 for if it be a communicable disease it immediately becomes very 

 important. The evidence, however, seems to be rather against 

 this view for several reasons. In the first place, the phenomena 

 have not yet been observed except at San Antonio, and in the sec- 

 ond place cuttings from the nearly dead plants reestablish them- 

 selves readily and grow vigorously for two years, even when set in 

 the identical spot in which the parent plant stood. Diseases are 

 common upon species of Opuntia in this locality, but if this be a 

 disease it behaves very differently from any of the others. It seems 

 to be always present on one species, from whatever source obtained, 

 and does not appear to do any injury in any other locality. 



EFFECT OF LOW TEMPERATURES. 



A discussion of low temperatures has necessarily occurred under 

 other topics ; consequently, only a partial analysis of it will be made 

 here. In previous publications of this Bureau it has been stated 

 that a temperature of 20° F. was about the minimum that the conven- 

 tional spineless species of the so-called Indian-fig group could 

 withstand. This is a general statement of the case. No one can 

 set a definite temperature at which they will be injured, any more 

 than one can for other plants. So much depends upon conditions 

 of temperature before and after the freeze that an exact statement 

 is not possible. The various species of the genus are killed with 

 cold at varying temperatures between the freezing point and 40° 

 below zero for the small, extreme northern forms. Many tropical 

 species, more especially these belonging to the genus Nopalea, will 

 be killed at much higher temperatures than 20° F. Even these seem 

 able to withstand lower temperatures when they are dry than when 

 gorged with water or when the ground is wet. 



The first visible effect of low temperatures on the plants is a 

 drooping condition. The tissues weaken at the articulation espe- 

 cially and become flaccid and partially disorganized. When poorly 

 supported there is a break, usually at the articulation in the flat- 

 jointed species, and the heavy limbs drop off. The power of recovery 



