223 
Inter-Relationships of the Anacrogynce. 
origin of the differentiation into stem and leaves of the presumably 
primitive thallus—a differentiation which has clearly arisen in 
several distinct lines and by several distinct methods. 
The simplest type is, apparently, the “ two-sided ” apical cell, 
that is, a cell from which two sets of lateral segments are cut off, 
alternately right and left. An apical cell of this kind occurs in 
Aneura, Metzgeria, Podomitrium, Umbraculum, Syniphyogyna , and 
Fossombronia. In each case the apical cell is triangular as seen in 
a surface view of the growing-point (or a horizontal section through 
it) and only two sets of segments are cut from it, but there are 
differences in detail as regards the relation of the apical cell to the 
tissue formed by the further divisions of the segments. In Metzgeria, 
for instance, the apical cell is nearly square as seen from the front, 
and nearly hemispherical as seen in a vertical longitudinal section 
through the growing-point. In the other genera, however, a smaller 
portion of the apical cell is exposed on the upper and lower sides, 
the dorsal and ventral tissue covering it to a varying extent. This 
is due to the more active growth of the upper and lower portions of 
each lateral segment, the midrib in these genera being more massive 
than in Metzgeria. In Fossombronia, this rapid growth of the 
segments is especially marked, and the apical cell has the form of 
a spindle, elongated vertically, when seen from the front. In 
Metzgeria, Podomitrium, Umbraculum, and Syniphyogyna, each 
segment divides first by a vertical wall into an inner (adaxial) and 
an outer (abaxial) cell; the latter divides only by walls vertical to 
the plane of the thallus and forms the single-layered wing, while 
the former divides also by walls parallel with the surface of the 
thallus and contributes to the formation of the midrib. In the 
wingless species of Aneura, all the cells formed from the segments 
divide by both horizontal and vertical walls, so that the thallus is 
several layers in thickness. In Fossombronia, each lateral segment 
divides first by two superposed horizontal walls; the upper and 
lower cells contribute to the formation of the stem, while the middle 
cell grows out to form a leaf. 
In Blyttia, Morckia, Blasia, and Pellia calycina, the apical cell 
is wedge-shaped, four sets of segments being cut from it—dorsal 
and ventral segments as well as right and left lateral segments. In 
a vertical longitudinal section through the growing-point, the apical 
cell is triangular, while in surface view or horizontal section it is 
usually four-sided. The dorsal and ventral segments contribute to 
the thick axial portion of the plant-body. In Blyttia the lateral 
