EVANS—THE NORTH AMERICAN SPECIES OF ASTERELLA. 251 
often appear on the dorsal surface and a broad purple margin is 
sometimes a distinctive feature. The receptacles, both male and fe- 
male, may also be subject to pigmentation. 
Many of the structural features of Asterella were clearly de- 
scribed by Leitgeb? in 1881, in connection with his work on the more 
complex Marchantiales. He assigned the genus to his group Opercu- 
latae, partly on account of the method of dehiscence of the capsule, 
but partly also on account of the morphology of the female recep- 
tacle, as he conceived it. Campbell,? in 1895, showed that Leitgeb’s 
interpretation of the receptacle would not apply to the Californian 
Asterella californica (Hampe) Underw., in which the female recep- 
tacle is of the type associated with the group Compositae, and it has 
since been shown that there are other species of Asteredla and other 
genera of the Operculatae to which Leitgeb’s interpretation will not 
apply. Although one of the latter author’s most important distince- 
tions between the Operculatae and the Compositae has thus been 
proved inconstant, the groups are still to be regarded as natural as- 
semblages of genera, a fact which Cavers® has recently emphasized. 
Two types of branching are regularly found in Asterella, terminal 
branching by forking and intercalary branching by means of ventral 
outgrowths arising from the sides of the thickened median portion 
of the thallus. The terminal branches are broad from the beginning; 
the intercalary branches broaden out abruptly from a narrow stalk- 
like base. A supplementary type of branch, also intercalary in na- 
ture, is the apical innovation. This arises usually when the growth 
of a thallus is limited by the formation of an inflorescence, and the 
power of forming such branches does not appear to be at all general. 
As pointed out by Leitgeb, an abundant production of one type of 
branch is associated with a limited production of the other. Dichot- 
omous branching, in fact, is characteristic of certain species, while 
ventral branching is characteristic of others. It is doubtful, how- 
ever, if one type ever replaces the other altogether. In certain 
species the receptacles seem to be confined to ventral branches, which 
are usually limited in growth and are sometimes greatly abbreviated ; 
in other species the receptacles are much less definite in position. 
The thallus shows the usual differentiation into epidermis, green 
tissue with air spaces, and compact ventral tissue, the lower surface 
bearing scales and rhizoids of the two characteristic types. The 
epidermal cells exhibit considerable variety with regard to size and 
thickness of wall, but seem to be arranged invariably in a single 
layer. The variation in size is sometimes marked on an individual 
Unters. Leberm, 6: 84-87, pl. 4. 1881. 
? Mosses and Ferns 57. f. 19. 1895. 
* New Phytol. Repr. 4: 34. 1911. 
