Monographs Details: Atrichum
Authority: Smith, Gary L. 1971. Conspectus of the genera of Polytrichaceae. Mem. New York Bot. Gard. 21: 1-83.
Family:Polytrichaceae
Scientific Name:Atrichum
Discussion:

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).