Fig. 38, 62, 63.
Type: Polytrichum undulatum [(Linnaeus, Sp. PI. 1117. 1753. Bryi) Hedwig, Descr. 1: 43. 1787.] Hedwig, Sp. Muse. 98. 1801. typ. cons.
Catharinea [Ehrhart, Hannov. Mag. 1780: 933. 1780.] Weber f. & Mohr, Index Mus. PI. Crypt. (2). 1803. nom. rejic.
Brotherus (1925) lists 41 species of Atrichum, many of which are based largely
on the sexual condition: whether the antheridia and archegonia are associated
within the same involucre, or occur in separate involucres on the same shoot, or
are borne on separate shoots. The vegetative gametophytes are extremely uniform.
The monoicous taxa seem to be polyploid, with chromosome numbers of n = 14
and 21 known in nature. Some morphological features of the gametophyte, such
as the size of the cells of the lamina, can be correlated with each level of polyploidy
(Noguchi and Osada, 1960; Ireland, 1969).
Atrichum has long been considered closely allied to Oligotrichum, because of
the 32-tooth peristome and the restriction of lamellae to the leaf nerve in both
genera. The two were traditionally included in the genus Catharinea, together
with other Polytrichaceae having a sparsely hairy, or glabrous calyptra. Apart
from the calyptra, these same features are found in many Pogonatum species (e.g.,
P gymnophyllum Mitt.), whose gametophytes more closely resemble Atrichum
than do those of most species of Oligotrichum.
Both Atrichum and Pogonatum lack stomata. The peristome of Atrichum is
composed of simple, linear, rather closely approximated teeth, without strong
coloration. It is thus quite different from the peristome of Pogonatum, with its
compound, usually intensely reddish teeth. The Atrichum peristome reminds one
of the peristome of Psilopilum sens. str. (cf Fig. 7N).
Fleischer's notion (1922) of Atrichum as the most primitive genus of the
family is inconsistent with its derived characters in both gametophyte and
sporophyte: bordered leaves and the absence of stomata. Atrichum has clearly
not given rise to any other g(>nus of Polytrichaceae. The genus is essentially a
temperate type, with its main distribution in the Northern Hemisphere (Fig. 80).
Atrichum is the only genus of Polytrichaceae wliose species have leaves that
are bordered by linear, thick-walled cells, commonly in more than one layer. The
weakly bordered i)erigonial bracts of many Atrichum sjiecies indicate how this border could have originated from marginal series of slightly elongated cells forming denticulations.
The leaves of Atrichum subulirostrum Schimp. ex Besch. are bordered by lax,
papillose cells in a single layer (Fig. 81). This border is often interrupted, and
may be absent in some leaves. The almost cucullate leaves and the diminutive
stature of this species are quite unlike any other species of Atrichum, and are
more reminiscent of Psilopilum sens, str., or of Oligotrichum. Moreover, a close
examination of the base of the capsule discloses the presence of rudimentary
stomata (Fig. 82).