225 
Inter-Relationships of the Anacrogynce. 
segment divides, like the lateral segments in Fossombronia, into 
three superposed cells, of which the dorsal contributes to the midrib 
and the ventral to the wing on either side of the midrib, while the 
middle cell forms one of the lamellae on the upper surface of the 
wing. 
The differentiation of the plant-body into stem and leaves is 
associated with all the three chief types of apical cell found in the 
group—the “ two sided ” Metzgcria type, the “ four-sided” or wedge- 
shaped type, and the tetrahedral type, This fact strengthens the 
evidence, drawn from other considerations, that leaves have evolved 
independently in several distinct lines of relationship among the 
Anacrogynce. Whatever the form of the apical cell, each lateral 
segment divides at an early stage into a portion which contributes 
to the thicker axial region of the plant-body and a portion which 
grows out and gives rise either to a continuous wing or to a leaf. 
Absolutely no morphological distinction can be drawn between a 
plant-body with a midrib and lobed wings, as seen in species of 
Syinphyogyna, and one with distinct stem and leaves. We find in 
the Anacrogynae every stage in the transition from a thallus with a 
continuous entire wing to a sharply differentiated leafy shoot, and 
the differences which are seen in the mature plant-body are 
differences of degree only. For instance, in Blasia the inner or 
axis-forming portion of each lateral segment grows more actively 
than is the case in Fossombronia, hence in the former genus the stem 
is broader and the leaves less sharply marked off from it than in the 
latter. The same explanation may be applied to those species of 
Blyttia and Syinphyogyna in which the wing is either regularly and 
deeply lobed or bears regularly arranged filamentous marginal 
appendages. If the outer portions of the lateral segments grow 
uniformly, those of successive segments remaining coherent, we get 
a thallus with or without sharp distinction into midrib and wings ; 
if the outer portion of each segment grows out independently, we 
get a leafy stem ; and between the two extremes we get all stages 
of transition. 
The only Anacrogynae in which the plant-body is purely 
thalloid and shows no signs of differentiation into stem and leaf are 
PeUia, Calycularia, Makinoa, Morckia, and the four genera we have 
classed under Aneuraceae. In all these genera there is a more or 
less marked midrib with a thin wing on either side, though in some 
species of Aneura this differentiation is not shown, 
In Umbraculum and the “Dendroid” species of Blyttia and 
