24 



THE CACTACEAE. 



name, however, comes from Engelmann, who first used it as a subgenus of Mammillaria 

 (Proc. Amer. Acad. 3: 264. 1856). 



The position of this group has always been puzzling to cactus students. Dr. Poselger 

 believed that it was a section of Echinocactus and transferred certain of these species which 

 had been described under Mammillaria to Echinocactus. In its vertical, nearly central 

 flowers it does approach the Echinocactanae, but otherwise it is quite distinct. 



In the origin of their large flowers, in the shape and structure of their fruit, and in the 

 color and form of their seeds the species compose a rather natural group, but they are 

 diverse in form and armament. The species are most common in central Mexico, a few 

 extending into the southern United States, and one extending into southern Canada. 



The groove on the upper side of the tubercle which is so characteristic of the genus 

 does not occur on seedlings or on very young plants, but it is always found on old flowering 

 plants and seems to be associated with the inflorescence, for the flowers appear only in the 

 axils of grooved tubercles and originate at the bottom of this groove. Plants which grow 

 in conservatories for a long time without flowering lose this groove ; * we have had one 

 plant of this kind under observation for fifteen years. 



Key to Species. 



A. Seeds brown, not angled; flowers usually large. 



B. Tubercles grooved to middle or a little below; ovary bearing scales with woolly 

 axils. Series Macromeres. 



Tubercles elongated, bright green i. C. macromeris 



Tubercles short, grayish green 2. C. runyonii 



BB. Tubercles grooved from tip to base except in young plants; ovary naked. 



C. Grooves of tubercles bearing large yellow or red glands. Series Recurvatae. 



Flowers white 3. C. ottonis 



Flowers not white. 

 Stems globular. 



Radial spines more or less recurved 4. C. recurvaia 



Radial spines spreading or ascending. 



Spines dark, sometimes black 5. C. poselgeriana 



Spines yellow or sometimes tinged with red. 



Central spines slender and flexible 6. C. miiehlenpfordtii 



Central spines stout and rigid. 



Radial spines subulate 7. C. guerheana 



Radial spines acicular '. . . 8. C. echinoidea 



Stems cylindric. 



Stems bluish green 9. C. clava 



Stems yellowish green. 

 Central spine usually one. 



Glands in groove red 10. C. octacantha 



Glands in groove yellow 11. C. exsiidans 



Central spines 2 12. C. erecta 



CC. Grooves of tubercles without large glands. Series Sulcolanatae. 

 D. Outer perianth-segments not ciliate. 



E. Flowers purplish or rose 13. C. elephaniidens 



EE. Flowers yellow or white. 



F. Tubercles very large, broader than high 14. C. humamma 



FF. Tubercles of medium size, if large, longer than broad. 



G. Plants large for this genus (often 8 cm. in diameter); seeds 



3 mm. in diameter 15. C. robustispina 



GG. Plants much smaller than in C. robustispina; seeds 2 mm. 

 in diameter or less. 

 H. Central spines usually wanting. 



Secondary cluster of spines developed in upper part of 



areoles and connivent at top 16. C. connivens 



Secondary cluster of spines not developed. 



Spines pectinate 17. C. peclinata 



Spines not pectinate. 

 Spines 14 or more. 



Spines slender with long black tips 18. C. nickelsae 



Spines rather short with light tips. 



Spines subulate 19. C. compacta 



Spines acicular 20. C. radians 



Spines fewer than 15. 



Spmes slender and weak , 21. C. sulcolanata 



Spines not slender 22. C. retusa 



* Mammillaria potosiana and M. polymorpha seem to have been based on such plants. 



