Fig. 13, 125.
Holotype: Racelopodopsis camusii Theriot, Monde PI. II. 9: 22. 1907.
Pseudoracelopus, Racelopus, and Racelopodopsis form a natural group, united
by the almost universal lack of lamellae, a trend toward reduction in the size
and complexity of the leafy plant, the presence of collenchymatous leaf cells,
and the scabrous seta. The sporophytes of all three genera are like those of
Pogonatum, except that the scabrous condition is not confined to the exothecium.
The distributions of these genera are depicted in Fig. 127.
The most complex gametophytes occur in Pseudoracelopus. Pseudoracelopus misimensis E. Bartr., because of its radial habit and retention of some vestiges of lamellae, is perhaps the most primitive extant member of the group. The type species of Pseudoracelopus, P. philippinensis Broth., is similar to P. misimensis, but lacks lamellae. Pseudoracelopus armatus E. Bartr., and P. marginatus (Mitt.) G. L. Smith (1969b) resemble some Pogonatum species (e.g., P macrophyllum Dozy & Molk.) in their dorsiventral shoots, but are also anisophyllous. In this respect they are unique among the Polytrichaceae. The thickened leaf margin of P. marginatus is also reminiscent of Pogonatum macrophyllum and its allies, and therefore the sporadic development of lamellae in P macrophyllum takes on added significance. Pseudoracelopus latifolius Ther. & Henry, P petelotii Ther. & Henry, and P. armatus have greatly reduced leaf nerves. The habit and leaf shape of P. petelotii are similar to Racelopodopsis, exemplifying the sort of transitional form between these genera that one might expect.
The two species of the genus Racelopus are rather dissimilar in the shape
of their leaves (Fig. 123, 124), but the cross-sections of the leaves of both species
show a broad, uniform stereid strand, which is seemingly median in position
(Fig. 12). The occasional presence of a larger diameter cell on the adaxial side
of the stereid strand in Racelopus acaulis Mitt, indicates that the characteristic
Racelopus leaf structure probably arose through the obliteration of the remaining
leaf tissue by the abaxial stereid strand. The leaf of Racelopus pilifer Dozy &
Molk., to judge by its shape and areolation, is essentially a sheath, with no blade
developed, whereas R. acaulis leaves have the more usual broadened basal portion,
and the upper part of the leaf has a more blade-like areolation.
The species of Racelopodopsis have gone about as far as they can in the
reduction of the leaf structure short of the elimination of all indication of a
nei-ve (Fig. 13). The neotenous character of this transformation is underlined by
the resemblance of the uniform bistratose structure of the leaf nerve in this genus
to the earliest stages of leaf differentiation at the shoot apex in other Polytrichaceae
(cf Lorentz, 1864). Most Racelopodopsis species have leaves with an
essentially sheath-like areolation.
In Brotherus' treatment of these genera (1925), each was credited with only one species. Racelopus acaulis Mitt, was overlooked, and Pseudoracelopus marginatus was included in Pogonatum. Neither the dorsiventral, anisophyllous shoot nor the scabrous seta of P. marginatus seems to have been noticed before in the bryological literature. I recognize six species of Pseudoracelopus, two of Racelopus, and three of Racelopodopsis, the difference in numbers being largely due to species published since 1925.
In summary, the progressive reduction in the gametophytes of Pseudoracelopus, Racelopus, and Racelopodopsis mirrors similar reduction series within the genus Pogonatum, yet to a greater extent: Racelopodopsis has attained the greatest structural simplicity in the family. I do not consider Pseudoracelopus, Racelopus, and Racelopodopsis to be closely related to their Pogonatum counterparts, yet the parallel reduction in the size and complexity of the leafy gametophyte and the persistence of the protonemata further substantiate the relationship of these genera to Pogonatum.
Thus, the hypothetical primitive stock that gave rise to the Polytrichaceae may be described as follows:
The leafy gametophytes were dioicous, of medium size, bearing leaves with a
sheathing base and a firm, broadly lamellate blade. The marginal cells of the
lamellae were like the others; the margins of the blade were serrate, without
tooth cells. The capsule was inclined, somewhat dorsiventral, obtusely angled (or
terete), with abundant stomata near its base. The exothecium was smooth, and
its cells lacked thin spots. The peristome teeth were simple, and neither crested
nor spurred at the back. The calyptra produced a thick felt of hairs.
Of the living Polytrichaceae, the closest approximate to this archetypal condition
occurs in species of Polytrichastrum [e.g., P. longisetum (Brid.) G. L. Smith,
P. formosum (Hedw.) G. L. Smith]. Other taxa with a decidedly primitive overall
aspect are Notoligotrichum, and the austral species of Polytrichadelphus.
My concept of the phylogeny of the Polytrichaceae is portrayed in Figure 128.
The modern genera lie in a horizontal plane, which represents the present. Their
arrangement in that plane is intended to reflect their phenetic similarity to one
another. The base of the diagram, representing the generalized, ancestral stock,
is truncated. No group of bryophytes approaches the Polytrichaceae in kind or
degree of structural complexity. A recently discovered ancient moss flora from the
Permian of the Soviet Union (Neuberg, 1960) may hold the key to the origin
of the genus Sphagnum (cf Jovet-Ast, 1967). One can only hope that some other
fossil assemblage will provide the link between other mosses and the Polytrichaceae,
assuming that such a common ancestor ever existed.