Key to Varieties and Subspecies of Agave americana
Howard Scott Gentry, Agaves of Continental North America, 1982, p. 281
Key to Varieties and Subspecies of Agave americana
1a. Spines relatively broad and short, 2-4 cm long; leaves straight, guttered, sometimes valleculate, the margins straight or crenate; rosettes commonly on trunks 3-5 dm long 2
1b. Spines subulate, 3-6 cm long; leaves frequently reflexed and otherwise not as above 3
[2 - LEAVES STRAIGHT]
2a. Rosettes with stems to 6 dm long; leaves glaucous gray, the margin crenate with teeth along the mid-blade on sharply angled mammae; flowers 80-85 mm long var. expansa
2b. Rosettes on shorter stems; leaves glaucous white, the margins nearly straight with close-set non-mammillate teeth; flowers 100 mm long var. oaxacensis
[3 - LEAVES RECURVED or REFLEXED]
3a. Leaves relatively short, 80-135 cm long, 4 to 6 times longer than wide, plane or guttered, straight to curving, green to glaucous gray; panicles shorter with 15-20 branches; wild ssp. protamericana
3b. Leaves longer than 100-200cm, 6-10 times longer than wide, frequently reflexed, glaucous gray or green variegated; panicles longer with 25-35 branches; cult. 4
4a. Leaves glaucous gray to light green, narrowed above base, some reflexed above the mid-blade var. americana
In the Preface to his great work, Gentry wrote, "Many boundaries between groups are not sharp, as also obtains among many species, because variation in Agave is mostly of a gradual or clinal type; one form or character changes to another by degrees."
So, we must keep an open mind as regards the cut-off points in the key (true of any key for any genus), and use experience to guide us when the key does not exactly work.
