198 Evans: NOTES ON GENUS HERBERTA 
old. . According to Goebel* excessive thickening of the cell walls’ 
in the bryophytes, which occurs especially in xerophilous species, 
is significant on account of the great power of imbibition which 
these walls possess. They readily absorb and retain water, and 
the rapidity with which most species of Herberta regain their 
normal appearance and consistency, when a dry tuft is deluged 
with water, is a striking phenomenon. In the cauline portions of 
the plant the cells are all elongated, although they never assume 
a truly prosenchymatous character. The outer or cortical region 
is distinguished from the inner or axial region by its deeper color 
and more strongly thickened walls. In the secondary stems the 
thickening at first takes place in the outermost layer of cells, the 
process continuing until the cell cavities become reduced to narrow 
canals (TEXT FIG. 2). The thickening is largely restricted to the 
outer or superficial walls and the layer presents the appearance 
_ of an epidermis with well-developed cuticle. In some cases the 
entire cortex consists of this outermost layer, the walls of the 
remaining cells showing only a slight degree of thickening (TEXT 
FIG. 26). This, however, is very unusual. In the majority of 
cases two or more additional layers take part in the formation of 
the cortical region; their walls become as markedly thickened as 
those of the outermost layer, the only difference being that the 
thickening i is deposited uniformly on all the walls (TEXT FIG. 3): 
Even in the central region the thickening of the walls is sometimes 
very pronounced, though never so much so as in the cortical region. 
Since, moreover, the central cells are larger than the cortical cells, 
their cavities remain distinctly wider. The cells of the stem are 
everywhere connected by pits, which show clearly in both transverse 
and longitudinal walls (TExT Fics. 3, 4). They are just as evident 
in the superficial layer as elsewhere, the cells of this layer being 
connected with one another both longitudinally and tangentially, — 
as well as with the cells of the layer next within. There is nothing, 
in fact, to indicate any essential difference in function betwee? 
the superficial layer and the other cortical layers. In spite of the 
fact that the outer walls simulate a cuticle so strongly, they have — 
the same powers of imbibition as the other walls and simply form 
part of the imbibing system of the stem. In the rhizomes the 
* See Organographie der Pflanzen, 2d ed. 553. I9rs. 
