252 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
thallus, so that differences in size are not of much value in distin- 
guishing the species. The thickness of the walls is also rather incon- 
stant: in some cases the walls are distinctly and uniformly thick- 
ened; in others they are exceedingly thin. In fact, a great deal of 
variation in this respect is often found in a single species under 
different environmental conditions. Trigones, as a rule, are not 
present, but in certain species they form a more or less conspicuous 
feature of the epidermal cells. Scattered cells containing oil bodies 
occur in certain species and are sometimes distinguished from the 
other epidermal cells by their smaller size, as well as by their contents. 
The epidermal pores on the vegetative thallus are of the simple 
type usual in the Operculatae. The opening is surrounded by a hya- 
line membrane representing the vestiges of a ring of disorganized 
cells, and around this are narrow and specialized cells arranged in 
‘adiating and concentric series. The pores vary in relative abun- 
dance, in size, and in number of specialized cells by which the open- 
ings are surrounded, and a great deal of variation is often to be found 
in a single species. In an average case each pore is surrounded by six 
radiating series of cells with three in each series, the outer cells being 
scarcely different from the ordinary epidermal cells. In some species 
the walls separating the radiating series of cells are more or less 
thickened, and this condition may be so marked that the pores acquire 
a stellate appearance, similar to what is found in most members of 
Leitgeb’s Astroporae. Pores of this type, however, are very excep- 
tional; it is much more usual for the radial walls to be only slightly 
thickened or even thin throughout. In most cases the pores project 
but slightly above the dorsal surface of the thallus. 
The green tissue is built up on the Reboula type! and incloses 
several layers of air chambers separated by partitions one cell thick. 
In some species the tissue is very loose, the air chambers being large; 
in other species the tissue is more or less compact. When the tissue 
is loose the dorsal chambers may not be subdivided at all, and each 
chamber under these conditions has its epidermal pore. This is the 
case, for example, in A. tenella, A. ludwigii, and A. palmeri. Even 
when the tissue is loose, however, the dorsal chambers may be some- 
what subdivided by supplementary partitions, as in A. linden- 
bergiana and A. californica, and some of the chambers seem on this 
account to be destitute of pores. When the tissue is compact the sub- 
division of the dorsal chambers is carried much further, so that nar- 
row and canal-like secondary chambers are formed, only a few of 
which show pores. A. elegans and A. bolanderi are good examples 
of this type. When a thallus is studied in cross section, the secondary 
partitions look as if they were free filaments and they have some- 
*See Evans, Bull. Torrey Club 45: 285, 1918. 
