226 
F. Cavers . 
Symphyogvnn, however, the plant-body is differentiated into a 
creeping wingless sympodial rhizome and an erect fan-like assimi¬ 
lating portion formed by repeated “ dichotomy ” of a winged axis. In 
these three genera, as well as in Podomitrium, the axis is traversed by 
a strand of pitted cells; in Podomitrium and in the creeping species 
of Blyttia and Symphyogyna, the strand is less strongly developed 
than in Umbrnculum and the “ Dendroid ” species of Blyttia and 
Symphyogyna ; in the “ Dendroid” forms, again, the strand is more 
bulky in the erect than in the creeping portion of the plant. The 
observations of Tansley and Chick on Morckia hibernica and M. 
Flotowiana (syn. M. hibernica , var. Wilsoniana) have already been 
referred to. In the latter species the writer found (9) that in the 
new branches produced by plants cultivated in glass-covered dishes 
in moist conditions (the plants came from sandy flats on the 
Yorkshire coast), the midrib showed no trace whatever of the two 
conducting strands which are normally present. It may be noted 
that several species of Anenra show a close approach to the 
“ Dendroid ” habit found in the genera Blyttia, Symphyogyna, and 
Umbraculum, though in Aneura the assimilating portion shows 
pinnate instead of fan-like “ dichotomous ” branching. 
It is clear that both the “ Dendroid” habit and the conducting 
strands have arisen independently in the different genera in which 
these developments occur. The most striking form of parallelism 
met with in the Anacrogynse, however, is that which has led to the 
differentiation of the plant-body into stem and leaf. This is seen 
in its simplest form in those species of Blyttia and Symphyogyna in 
which the wing of the creeping thallus bears on its margin regularly 
arranged filamentous outgrowths or teeth, one such outgrowth being 
formed from each lateral segment of the apical cell. As Goebel 
(24, 26) has suggested, the primary function of these outgrowths is 
the protection of the growing-point, and it would seem reasonable 
to extend this view to account for the evolution of leaves in general. 
Where no leaf-like outgrowths arise from the lateral segments of 
the apical cell, the growing-point is provided with gum-secreting 
papillae which arise from the cells above or below the apical cell, or 
from the segments at either side of it, or in all these positions. In 
many cases, the leaves arise in exactly the same way, the young 
leaf forming at first a unicellular club-shaped gum-papilla, which by 
active growth and divisions in the cell that bears it, becomes terminal 
on a cell-row or a cell-plate. Apart from Blyttia and Symphyogyna, 
in which the evolution of leaves only occurs in certain species, and 
