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lobed or cleft and never show very marked differences in the various parts 
of the plant, even when the stem and its branches are prostrate and closely 
adherent to the substratum. 
With the Hepaticae, on the other hand, the task would be much more 
difficult. The gametophyte here exhibits the greatest variety in different 
families. It is sometimes a flat thallus without any indication of leaves, 
sometimes a thallus-like stem with rudimentary leaves, sometimes a more or 
less cylindrical stem with distinct leaves ; and these various types are con- 
nected by intermediate conditions. In the thallose forms the cell-structure 
is sometimes uniform throughout or nearly so, and sometimes shows a high 
degree of differentiation; here again there are intermediate conditions. 
Both thallose and leafy species are almost always prostrate and show 
marked differences between the upper and lower portions. In other words 
they are “ dorsi-ventral.” In the thallose forms the dorsiventrality mani- 
fests itself in differences in cell-structure: in the leafy forms in differences 
between the leaves. 
In the eastern United States nearly three fourths of our Hepaticae are 
leafy and belong to the family Jungermanniaceae, sometimes spoken of as 
the “ scale-mosses,” and we will confine our attention to these. The scale- 
mosses may usually be distinguished at a glance from the true mosses by 
the prostrate habit to which allusion has just been made and by the fact that 
this habit usually brings about a distinctly flattened appearance for the 
whole plant, the leaves themselves as well as the stem being more or less 
appressed to the substratum. When we examine a plant carefully we find 
that the leaves are more regularly arranged than in most of the mosses ; 
looking at a stem from above {Fig. /) we see two distinct longitudinal ranks 
of leaves spreading out on either side; looking at the same stem from below 
{Fig. 2 ) we can usually see a third rank of leaves, which are more or less 
appressed to the stem. The leaves in fact are in a spiral and conform to 
the one third arrangement. It will be seen at a glance that the leaves are 
not all exposed to the same external conditions. Those, for example, in the 
two ranks which we saw from above, are turned toward the light, and are, 
therefore, well placed for carrying on photosynthesis ; those in the third ran^, 
however, are practically cut off from the light. Probably this difference in 
environment has been a potent factor in bringing about a diversity in the 
leaves, those in the third rank being different in form and usually much 
smaller than the others. For the sake of convenience in description, the 
leaves of the third rank are spoken of as “underleaves” while those of the 
other two ranks are called “ side-leaves ” or simply “ leaves.” In certain of 
our species the underleaves are so small that they can be demonstrated only 
by careful dissection; in a few species they are absent altogether. Even in 
the last case, however, the leaves are closer together on the upper surface 
of the stem than on the lower, so that they do not conform to the one half 
arrangement, which we would naturally expect with only two longitudinal 
ranks of leaves. 
The leaves exhibit a much greater diversity of form than the under- 
leaves, and this manifests itself in pecularities of the margin, in lobing or 
