224 
F. Cavers. 
segments divide in much the same way as in Metzgeria ; in PcIIia 
calycina their division resembles what is seen in the wingless species 
of Amur a \ and in Morckia we have a transition between the two 
types, the thick median portion of the thallus usually thinning out 
into the wing on either side more abruptly than in Pellia caly cina. 
In Blasia, the lateral segments divide in practically the same way 
as in Fossombronia, horizontal divisions giving three superposed 
cells of which the dorsal and ventral add to the stem tissue while 
the middle cell forms a leaf. 
In Pellia epiphylla the apical cell is semi-circular in a vertical 
longitudinal section through the growing-point, and there are cut 
from it three sets of segments, two lateral and one posterior, the 
latter extending the whole depth of the thallus and only later dividing 
into dorsal and ventral cells. As Campbell (6) has pointed out, this 
type has probably been derived from the wedge-shaped type (with 
four sets of segments) “ by a gradual increase in the angle formed 
by the dorsal and ventral walls of the apical cell, which finally 
became so great as to form practically one plane.” The midrib is 
usually decidedly thicker, vertically, in Pellia calycina than in 
P. epiphylla, and the differences in the form of the apical cell and 
its mode of segmentation may perhaps be connected with this 
difference in the depth of the midrib in the two species. 
Finally, in Noteroclada, Petalophyllum, Treubia, and the 
Calobryacete, we have a tetrahedral cell, similar to that found in 
most of the Acrogynae and Mosses, from which three sets of 
segments are cut off in spiral succession. In the Calobryaceae, 
which grow erect and are more or less distinctly radial in symmetry, 
each segment produces a leaf, but the leaves of one row are frequently, 
if not always, either larger or smaller than those of the other two 
rows and further differ from them in being inserted transversely 
instead of obliquely on the stem, so that even in this group we get 
the bilateral symmetry and anisophylly which are so characteristic 
of the Acrogynae. In the other three genera of Anacrogynae with 
a tetrahedral apical cell, the plant grows horizontally and is 
bilaterally symmetrical, and leaves are only formed from the lateral 
(strictly, dorso-lateral) segments, though in Noteroclada and 
Petalophyllum each ventral segment produces hairs which represent 
the ventral leaves or “ amphigastria ” of the typical Acrogynae. In 
Noteroclada, the upper portion of each dorso-lateral segment 
contributes to the formation of the stem, while the lower portion 
grows out to form a leaf; in Petalophyllum, each dorso-lateral 
