Depressed or flattened, simple, unarmed plants, covered with
peculiar imbricated tubercles above and their scale-like remains
below: tubercle with lower and upper parts very different; lower
part comparatively thin and flat; upper exposed part triangular
in outline and divergent, very thick and hard, the lower surface
smooth and keeled, the upper surface plane or convex, smooth or
tuberculate or variously fissured, with a broad wool-bearing
groove or simply a more or less evident tomentulose apical
areola: spine-bearing areola obsolete: flower-bearing areola at
the summit of the lower peduncle-like portion of the very young
tubercle (thus appearing axillary with reference to the exposed
part of the tubercle) and bearing a dense penicellate tuft of
long soft hairs which conceals the lower part of the flower and
the entire fruit and persists about the apical region of the
plant as matted and apparently axillary wool: ovary naked: seeds
large, black, and tuberculate: embryo obovate, straight.
According to the present views concerning generic limitations in
Cactaceae, Anhalonium must certainly be kept distinct from
Mamillaria, and to such a view Dr. Engelmann had finally come.
The generic distinction is based upon such characters as (1) the
complete suppression of the spine-bearing areolae; (2) the strong
differentiation of the tubercles into two very distinct regions;
(3) the production of the flower at the apex of the basal or
penduncle-like portion (which becomes flattened and expanded at
maturity) of a very young tubercle; and (4) the large tuberculate
seeds.
In the case of engelmanni the broad woolly groove of the upper
portion of the tubercle expands below into the flower-bearing
areola, but terminates blindly above just behind the sharp apex.
In prismaticum and furfuraceum the groove is obliterated, but
there usually remains a small (more or less tufted) areola and
depression just behind the apex to mark its upper extremity.
This apical areola therefore, does not represent a spine-bearing
areola, but the closed upper extremity of a tubercle groove.
It seems evident that Anhalonium is a much modified Cactus, and
that its affinity is with the coryphanths, through such a species
as C. macromeris, in which the flower becomes extra-axillary. If
in macromeris, with the flower standing well up on the tubercle,
the portions of the tubercle above and below the flower should
become very different from each other, the upper portion being so
much modified as to cause the spine-bearing areola to be
obliterated, the condition of things in Anhalonium would be
obtained.
* Upper surface of tubercle with a broad and deep wool-bearing
longitudinal groove which widens below.