EvAXs: 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 differences 

 bet\veen 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. 



