796 THE POPULAR SCIENCE MONTHLY. 



nearest approach being missing links between other neighboring 

 orders and the Cactacece, otherwise almost utterly isolated. One 

 is the genus Pereskia, in which, especially in Pereskia grandi- 

 folia, there are developed true leaves, succulent and veiny and with 

 spines in their axils. Most species of the genus are shrubs or 

 trees, and, still further remarkable, the flowers are borne in 

 nearly panicled clusters. The thirteen species belong mostly to 

 the West India region, and one produces the so-called "Barbados 

 gooseberry." A decided analogy may be recognized by close 

 comparison between these Pereskias and the Ribeseacece, the 

 group of the saxifrage family which includes the currants and 

 spiny-stemmed gooseberries ; and this probably points to a dis- 

 tant connection between the progenitors of the Cactaceoe and 

 those of the modern genus Ribes. The other possible " missing 

 link " is the genus Rhipsalis, a curious group, mostly epiphytic, 

 and growing in long, pendent masses from the branches of trees, 

 in some instances resembling mistletoe. In these plants we see a 

 possible approach to the group of so-called " ice-plants," the or- 

 der Mesembryanthemacece. But most peculiar is the fact that 

 one species of Rhipsalis is indigenous to South Africa, Mada- 

 gascar, and Ceylon — the only instance of an Old World cactus. 

 This probably has its significance. 



The other suborder, the Tubuloscz, are undoubtedly the more 

 highly specialized cacti, and further, significant fact, they are for 

 the most part Mexican, while the OpunticB are widely scattered 

 northward. Besides several minor genera of Tubulosce- there are 

 three great and distinctive ones, which, as it is interesting to note, 

 mark successive steps in structural specialization — they are Ma- 

 millaria, JEchinocactus , and Cereus. In mamillaria there is a 

 great departure, in the character of the vegetative body, from the 

 Opuntice,. The plants are more or less globular or subcylindrical, 

 and the original joints of the stem are indicated only by the 

 conical spine-tipped tubercles which make up the surface of the 

 fleshy mass. Echinocactus, the "hedgehog cacti," has the gen- 

 eral appearance of mamillaria, but the tubercled surface is modi- 

 fied into a mere series of parallel vertical ribs, bearing clusters of 

 spines along their ridges at points corresponding to the tubercles 

 of mamillaria. Of the two genera, echinocactus is much the 

 more strictly Mexican, while mamillaria has a few representa- 

 tives spreading northeastward into Kansas and South Dakota. 

 The large genus Cereus is the crowning glory of the cacti. It 

 retains the ribbed structure of echinocactus, but its stems are 

 nearly always columnar and in many instances arborescent. With 

 echinocactus, this genus reaches its greatest development in 

 Mexico, or near the boundary line, where flourishes the monarch 

 of the Cactacece, the " giant cactus," Cereus giganteus. In cereus, 



