Mamillaria micromeris Engelm. Syn. Cact. 260 (1856).
With depressed top and very rarely branching, 1 to 3.5 cm. in
diameter: tubercles very small (about 1 mm. long) and wart-like,
crowded, shedding the spines with age and giving the base of the
plant a tuberculated appearance: spines from white to ashy-gray,
1 to 3 mm. long; in young plants and on lower tubercles of adult
plants about 20, equal and radiant; on flower-bearing tubercles
30 to 40, stellate-porrect in every direction, the 6 to 8 upper
ones two to four times longer than the rest (4 to 8 mm.), clavate
toward the apex and acute (the clavate top at length deciduous),
intermixed with loose wool of about the same length and forming a
small tuft on the top of the plant which includes and partly
hides flowers and fruit: flowers whitish to light pink, almost
central, very small (6 mm. in diameter), much reduced (3 to 5
sepals, 5 petals, 10 to 15 stamens, 3 stigmas): fruit 8 to 12 mm.
long: seeds 1.5 mm. long, black and shining. (Ill. Cact. Mex.
Bound. t. 1 and 2. figs. 1-4) Type, the specimens of Wright in
Herb. Mo. Bot. Gard.
On naked mountain tops and sides, extreme southwestern Texas (Val
Verde County to El Paso) and southward into Coahuila and
Chihuahua.
Specimens examined: Texas (Wright 227 of 1849, also of 1852;
Nealley of 1892): Coahuila (Bigelow of 1853): Chihuahua (Pringle
212): also growing in Mo. Bot. Gard. 1893.
The plants densely covered above with delicate ashy-gray spines
and with naked tuberculate base are readily recognized. It still
remains an open question whether the flowers are developed from
the axils of tubercles of the same season or the last ones of the
preceding season. Dr. Engelmann inclined to the latter view, as
all the other characters of the plant associate it with the
"lateral-flowered" species; and in the absence of definite
observation we have retained it there. If the nearly central
flowers indicate that they are produced from growth of the same
season the species would seem to be allied to Coryphantha, in
which group its small flowers and small tubercles would be
anomalous.