636 



MANUAL OF POISONOUS PLANTS 



animal, to which, causing friction against one another, their perfectly round form 

 is attributable." As is well known, the Opuntias produce spines and two kinds 

 of trichomes. In the Cylindropuntias, each spine is invested by a deciduous 

 sheath, "which is downwardly barbed, so that a person or animal brushing care- 

 lessly against a plant is certain to remove some of the barbed sheaths." In the 

 Platopuntias, to which the ordinary flat-stemmed prickly pears belong, the spines, 

 when present, are destitute of such a sheath. The protection to the plant is af- 

 forded simply because of their rigidity and pungenc}'. The spines have their 

 origin in pulvini, and in this particular genus of cacti are coated with delicate 

 flexible hairs, divided into partitions. These hairs are lightly attached to the 

 epidermis of the plant, so that when the pulvinus is touched they are almost 

 certain to be removed in considerable numbers. The points of the stiffer hairs 

 penetrate the skin, the barbs with which they are closely beset preventing their 

 ready withdrawal. Dr. Trelease, in summing up the injurious effect of cacti, 

 says : 



It is a frequent practice in Texas to cut the branches of cacti which are fed to stock into 

 half-inch lengths. In this way, every one of the obliquely set longer spines of Opuntia Engel- 

 manni (and of some other species which are so used) is almost certain to be cut off, so that the 

 danger from the spines is removed. This treatment, however, does not destroy the barbed 

 hairs of the pulvini, of which the bezoars under consideration are composed. It is also tfee 

 practice, in some places, to roast the fragments as a means of completely removing the spines 

 and barbed hairs, but this is objected to by some feeders, because the roasting has been as- 

 serted to add to the laxative properties of the cactus. Where some such treatment has not 



Fig. 358. Prickly Pear (Opuntia Engehnanni), 

 from the barbed trichomes of which phytobezoars 

 are sometimes formed. (U. S. Dept. Agr.) 



