Evans: NOTES ON GENUS HERBERTA 199 
same distinction between cortical and central regions is apparent. 
There is perhaps a tendency for the cortical region to be thinner 
and for the cell-walls of the central region to be less thickened 
than in the secondary stems, but there are no essential différences 
between the two. 
The leaf-cells in their more important features have long been . 
familiar to students of the Hepaticae. Although forming a single 
layer as in most of the Jungermanniaceae the cells show a differ- 
entiation into elongated cells and more or less isodiametric cells 
(see, for example, PLATE 8, FIG. 4). The elongated cells form a 
median band extending from the line of insertion into the basal 
portion of the leaf. Somewhere below the sinus the band, or 
“vitta,’’ as Stephani terms it, forks, one branch passing into each 
division. Here they may extend to the extreme apices or stop 
at a variable distance below them. The isodiametric cells form 
the rest of the leaf and are divided into three patches by the vitta 
and its branches, the two lateral patches extending from the base 
into the divisions on their outer sides and the median patch from 
the forking of the vitta into the divisions on their inner sides. 
Apparently the first allusion to the vitta is found in the original 
description of H. dicrana (Tayl.) Trevis.* In a critical note, 
quoted from Taylor, a “nerve’’ is spoken of which runs out into 
the divisions. Gottsche afterward described the vitta in other 
species, and Stephani lays especial emphasis on it in his recent 
monograph. In his opinion the vitta yields some of the best 
differential characters in distinguishing species. He considers 
the basal portion (below the forking) to be constant in size for a 
given species, and he finds an equal constancy in the length of the 
branches. Unfortunately it is not always easy to determine the — 
exact lateral boundaries of the vitta or the points where the 
branches terminate in the divisions of the leaves. Although the 
median cells of the basal portion are markedly different from the 
cells near the margin of the leaf, there is sometimes a gradual 
transition between the vitta and the marginal portion, and a 
similar transition may exist between the cells of the vitta and the 
marginal and apical cells of the divisions. Two observers, in 
consequence, might obtain different results in measuring the same 
* Syn. Hep. 239. 1845. 
