Isothecium Bridel, 1827.

Plants assigned to the genus Isothecium are so variable in growth habit that even experienced bryologists have difficulty recognizing the genus in the field. In this genus, we have strongly julaceous examples, pendent and remotely leaved examples, and plumosely branched examples. In contrast the genus Isothecium is very easily recognized in microscopic examination: the alar region of densely chlorophyllose, mostly isodiametric cells is abruptly demarcated from the more elongate cells of the remainder of the leaf. The character of leaf marginal toothing is unique among California mosses: above the leaf middle, the margin has a pattern of alternating large and small teeth such that every large tooth is flanked by two very small teeth.

Key to Isothecium

Species included are in Brachytheciaceae:
Isothecium cardotii Kindberg
Isothecium cristatum (Hampe) H. Robinson
Isothecium myosuroides Bridel
Isothecium obtusatulum Kindberg
Isothecium spiculiferum (Mitten) Renauld & Cardot
Isothecium stoloniferum Bridel

In the field, some of the species of Isothecium can be recognized by the olive-green coloration (often with a russet wash), the rather coarse serration near the leaf apex, and the rather strong differentiation of larger sheathing perichaetial leaves. Similar coloration, serration and perichaetial leaves are found in Pterogonium gracile. This latter species should not be mistaken in the field for any Isothecium because the regular branching pattern and hamate branch tips gives Pterogonium the appearance of a many-toed bird foot.

Species of Isothecium differ greatly from one another in degree of julaceous condition, in relative length/width ratio of the leaves, and in degree of plumose-dendroid branching. These differences between the species are, however, often blurred by many intermediate specimens. Allen (1983) did statistical studies on European materials that have mostly been called Isothecium myosuroides, and on North American materials under such names as Isothecium stoloniferum, Isothecium cardotii, Isothecium spiculiferum, and Isothecium myurellum. While Allen made the decision not to recognize the latter four North American names, we have observed such a strong degree of ecological fidelity of the first three of those four species that we have decided to recognize them at the species level. In so doing, we have narrowed our concept of I. myosuroides among California mosses.

A. Leaves lingulate with an obtuse apex, mostly less than 1.5 mm long .....Isothecium obtusatulum
A. Leaves ovate-lanceolate to very broadly ovate, not obtuse at apex, mostly larger .....B

B. Alar cells present in a triangular group that extends along margin nearly to mid-leaf; stems always strongly julaceous .....Isothecium cristatum
B. Alar cells present in a roundish, often concave group that seldom extends more than ten cells up the margin; stems sometimes not julaceous, often flagelliform .....C

C. Plants pendent to prostrate, hardly julaceous; alar region round in surface view, strongly excavate on larger leaves; most of the cells of distal region prorate .....Isothecium spiculiferum
C. Plants decumbent to ascending with branches strongly julaceous; alar region various; prorations on distal cells sparing or absent .....D

D. Acumen well-demarcated and occupying about 1/3 of leaf; distal portion of acumen with cells more elongate than in median region of leaf .....Isothecium myosuroides
D. Acumen only gradually defined; distal portion of leaves with cells similar to or shorter than median cells .....E

E. Plants plumose and erect to decumbent from an inconspicuous prostrate main axis; alar region round but not excavate; leaves concave, with acumen abruptly defined, mostly shorter than remainder of leaf .....Isothecium cardotii
E. Plants pinnately branched but only somewhat plumose; alar region strongly excavate; leaves concave, broadly ovate-lanceolate with acumen about as long as remainder of leaf .....Isothecium stoloniferum