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Andreaea Hedwig, 1801.

Recent work has shown spore size and morphology to be very important in Andreaea. Fortunately, the unique (for a moss) pattern of sporophyte maturation allows access to spores even when no exserted sporophytes can be found. Perichaetial leaves are usually larger or otherwise differentiated, and searching inside of the perichaetial leaf cluster will often yield a young but fully mature sporophyte whose pseudopodium has not yet elongated. The crushing of such a hidden sporophyte will yield spores appropriate for investigation. Once encountered, Andreaea will be reliably recognized by the red-brown to almost black coloration of even the younger portions of the plant. This color recognition can be confirmed by close observation of the sporophytes, dehiscent not by an operculum but by longitudinal slits, a feature reminiscent of capsule dehiscence in liverworts.

Key to Andreaea

Andreaea is a moss that grows exclusively on acidic rocks, primarily granite. In a gametophytic state it is almost non-descript and might be interpreted as a diminutive Didymodon or a muticous Grimmia. There are three basic forms of the genus in California. The commonest expression is an ecostate and papillose form; another form is costate and smooth-celled with somewhat falcate leaves; the third form is a large snow-melt species that has costate and papillose leaves. Our most common species of Andreaea may form cushions or tufts in rather moist sites; some of the other species may be far less conspicuous with the plant growing in tufts so small as to find easy camouflage among the hornblende crystals of its granite substrate.

Species included in this key are all in Andreaeaceae:
Andreaea alpestris (Thedenius) Bruch & W. P. Schimper
Andreaea blyttii W. P. Schimper
Andreaea heinemannii Hampe & C. Müller Hal.
Andreaea nivalis W. J. Hooker
Andreaea rothii Weber & D. Mohr
Andreaea rupestris Hedwig
Andreaea schofieldiana B. M. Murray

A. Leaves ecostate; perichaetial leaves convolute-sheathing, much larger than adjacent vegetative leaves .....B
A. Costa present, at least above leaf base; perichaetial leaves not strongly convolute-sheathing .....C

B. Leaves somewhat secund; papillar salients on dorsal portion of mid-leaf mostly higher than leaf thickness .....Andreaea rupestris
B. Leaves nearly straight and symmetrical; papillar salients low to inconspicuous throughout leaf .....Andreaea alpestris, in CA the splitting off of A. alpestris from the prior A. rupestris strains credibility: healthy clones proported to be A. alpestris have shoots with all the characters of A. rupestris

C. Median laminal cells sharply papillose; leaf margins crenulate to minutely serrate .....Andreaea nivalis
C. Median laminal cells smooth; leaf margins entire or nearly so .....D

D. Costa in extreme base of leaf only 1-3 cells thick, or not visible; plant dioicous; cells of lamina and abaxial cells of costa smooth; leaf limb moderately or not at all differentiated from the leaf base .....E
D. Costa present and not much flattened toward leaf base; plant autoicous; leaf at least somewhat constricted above an oblong-ovate base to a subulate or lanceolate limb .....F

E. At least some of the leaves bluntly rounded at apex; costa missing in extreme leaf base; median basal cells of leaf quadrate; spores mostly less than 18 µm .....Andreaea heinemannii
E. Leaves subulate, not bluntly rounded; costa at least two cells thick to base; median basal cells of leaf rectangular with unpitted, straight lateral walls; spores mostly 25-30 µm in diameter .....Andreaea blyttii

F. Leaf limb more than twice as long as the leaf base; spores 35-50 µm in diameter .....Andreaea rothii
F. Leaf limb less than twice as long as the leaf base; spores 20-30 µm in diameter .....Andreaea schofieldiana

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