124 PLANT WORLD. 



with highly unpleasant results. It is to these structures thiit 

 attention is chieHy directed to get an opuntia useful for forage 

 and salad. A dozen species, spineless, or nearly so, are 

 known, but the breeder must get a form which does not 

 develop the glochicis beyond a rudimentary stage if the most 

 offensive features of the armament are to be avoided. 



Some of the cylindrical forms are arborescent with 

 widely spreading branches. The joints are easily detachable 

 so that a slight contact causes the spines to pierce the clothing 

 or skin and a joint of the stem comes away bristling with a 

 score of spines and some of these also pierce the flesh. The 

 instructed use a stick or a pair of tweezers to remove the pest. 

 Dogs, horses and cattle reareci in a cactus region, learn to 

 free themselves by a shaking motion. Attempts to remove 

 the detached joints by the fingers are often disastrous. 



The ease with which the joints are detached, the facility 

 with which they become attached to animals and the rapidity 

 with which these segments root and form new plants when 

 dropped, make this one of the most important methods of 

 reproduction. This is true especially of Opuntia mamUlata 

 and O. fiilgida. 



These two forms were formerly thought to be included 

 in a single species, but on all of the expeditions from the 

 Desert Laboratory, they have been found easily distinguish- 

 able. They offer a marked example of closely allied forms 

 living in the most intimate proximity. 



The acid fruits are not set free until they are three years 

 old in some instances. After the pulp decays, the hard seeds 

 lie on the ground inactive for months or even years, mean- 

 time being subject to the action of numerous rodents which 

 gnaw through the hard outer coat to get at the embryo. Both 

 species are known as "cholla," a name loosely used, and some- 

 times applied to any cylindrical opuntia, although it strictly 

 belongs to a species native to Baja California {Opnut'ia 

 cholla). 



