80 ELEMENTS OF PLANT ANATOMY. 
found occasional forms resembling cormophytes, and some 
authorities claim that the two organs, stem and leaf, may be 
clearly distinguished in these plants. It is not within the 
province of this work to discuss purely morphological questions, 
yet they cannot be entirely ignored, as some method of classi- 
fication is necessary if the anatomical characters of plants are 
to be made plain. For our purpose, it is unimportant whether 
we class certain of the higher forms of algae as thallophytes or 
cormophytes, for the evident transition from thallus to leafy 
stem takes place in a group of plants above the algae, namely 
the Hepaticae. That is, this group is the lowest in which the 
distinction between stem and leaf is so evident that there is no 
difference of opinion regarding it. Even here the stem does 
not in all respects correspond to the perfectly radial organ of 
that name in higher plants. In other words, the stem of the 
leafy hepatic is dorsiventral, though in other respects it is a 
true stem. Therefore simply for the sake of convenience, and 
with no intention of entering into the controversy concerning 
what constitutes leaf and stem, we class all plants below the 
foliaceous Hepaticae as thallophytes. 
In respect to the possession of stem and leaf, the group of 
plants known as Hepaticae contains three forms, a simple 
thallus, a thallus with rudimentary leaves, and a leafy stem. 
In other respects their morphological characteristics are such as 
to cause them to be classed together. 
The lowest of these three forms consists of a very simple 
thallus in which it is difficult to distinguish more than a single 
tissue. Above this in the course of development come others 
increasing gradually in complexity, but in such a way that two 
distinct series are formed ; the one retaining the thallus form, 
but with increasing complexity of anatomical structure; the 
other rising in the scale by means of outward differentiation, 
such as to lead to pronounced morphological differences. The 
former series culminates in the family Marchantiaceae and in- 
