CHAPTER VI.— ANATOMY OF CORMOPHYTES. 
THE transition from thallus to stem is said to take place in the ° 
Hepaticae of the second series, or the one formerly referred to as 
culminating in the Jungermanniaceae. Exactly what is meant 
by this expression, “transition from thallus to stem,” is seldom 
clearly explained. It may be safe to state here that some mor- 
phologists claim that the present hving forms show how such a 
transition may actually have taken place. The process may be 
described something as follows. The ordinary broad-winged 
thallus produces descendants with slightly narrower wings or 
blades; this is repeated through successive generations until 
finally a form is reached where the wings have entirely disap- 
peared, the midrib alone remaining as a stem, while the organs 
which in the earlier generations were rudimentary are devel- 
oped into real leaves. 
To show this, two examples are usually taken, Blasia pusilla 
and Fossombronia. The former consists of a ribbon- or band- 
shaped thallus, bearing on its under side two rows of toothed 
scales, the so-called amphigastria. Leaves parallel to the long 
axis of the stem are also inserted along the edge of the flat 
thallus or stem. These leaves were formerly supposed to be 
portions of the thallus, but are now known to be independent 
organs originating from the apical cell in the same manner as 
the leaves of Fossombronia. In the latter plant the thallus is 
reduced to a stem, strongly flattened on the upper side, and 
bearing two rows of leaves inserted obliquely along its two 
dorsal flanks ; while underneath, where the scales of Blasia 
occur, is found a row of club-shaped glandular hairs. These 
hairs are often widened at the base by subsequent cell division, 
so as strongly to resemble the scales or amphigastria of Blasia. 
