OPUNTIA. 137 



Key to Species. 



Joints elongated 132 . 0. megarhiza 



Joints obovate to orbicular. 



Fruit small, 2 cm. long or less 133. O. hallii 



Fruit large 2.5 to 6 cm. long. 



Flowers red to purple 134. 0. pottsii 



Flowers yellow. 



Areoles large, more or less elevated on old joints; joints glaucous, purplish about the 



areoles 135. 0. setispina 



Areoles small; mature joints green throughout. 



Joints usually orbicular; seeds 5 to 6 mm. broad 136. O. mackensenii 



Joints obovate; seeds 4 mm. broad or less 137. 0. tenuispina 



132. Opuntia megarhiza Rose, Contr. U. S. Nat. Herb. 10: 126. 1906. 



Roots long and thickened, sometimes 3 to 6 dm. long, 5 to 6 cm. in diameter; stems low, 2 to 



3 dm. high, much branched; lower joints elongated, 2 to 3 dm. long, cuneate below, thin, 3 cm. 

 broad; lateral joints appearing along the margins of the older joints and often, if not always, in the 

 same plane; spines 2 to 4, acicular, i to 2.5 cm. long, brown; leaves minute; flowers lemon-yellow, 

 often tinged with rose, 5 cm. broad; petals about 13, obovate, mucronately tipped; stigma-lobes 7, 

 greenish; ovary clavate, 3 cm. long; fruit and seeds unknown. 



Type locality: Alvarez, Mexico. 



Distribution: San Luis Potosi, Mexico. 



This species is not very closely related to the 

 other species of this series, but it is referred here on 

 account of its very slender spines. 



133. Opuntia ballii Rose, Contr. U. S. Nat. Herb. 13: 309. 

 1911. 



Plants low, spreading; joints obovate, 6 to 10 cm. long, 

 thickish, pale green, glaucous; spines 2 to 4, brownish, a little 

 flattened, usually ascending or erect, the larger ones 4 to 7 cm. 

 long; glochids conspicuous; fruit small, about 2 cm. long, 

 clavate, glaucous, spineless; seeds thick, 3.5 mm. broad. 



Type locality: Pecos, Reeves County, Texas. Fig. 172.— o. baiui. Part of type, xo.5- 



Distribution: Western Texas. 



Wooton and Standley in their Flora of New Mexico refer this species to Opuntia fili- 

 pendula, but 0. ballii grows in a different habitat, has smaller fruit, stouter and erect 

 spines, and different areoles; it grows on the dry mesa beyond Pecos, Texas. 



Illustrations: Contr. U. S. Nat. Herb. 13: pi. 64. 



Figure 172 is copied from the illustration above cited. 



134. Opuntia pottsii Salm-Dyck, Cact. Hort. Dyck. 1849. 236. 1850. 



Opuntia filipendula Engelmann, Proc. Amer. Acad. 3: 294. 1856. 



Low, spreading plant, 3 dm. high or less, from thickened tuberous roots 2 to 3 cm. in diameter, 

 these sometimes moniliform; joints broadly obovate, 3.5 to 12 cm. long, pale green to bluish; areoles 

 few, either small or large; spines confined to the upper and marginal areoles, i or 2, slender, 2 to 



4 cm. long, usually white Ijut sometimes purplish; glochids yellow, usually few but sometimes abun- 

 dant; flowers large, 6 to 7 cm. broad, deep purple; ovary slender, 3 to 3.5 cm. long, with only a few 

 scattered areoles; fruit spineless. 



Type locality: Near Chihuahua City, Mexico. 



Distribution: Central Chihuahua, Mexico, to Texas and New Mexico. 



This species was described by Prince Salm-Dyck in 1850 from material collected by 

 John Potts, who was manager of the mint at Chihuahua and who sent many cacti to F. 

 Scheer at Kew between 1842 and 1850. No types of his species seem to have been retained. 



