EVANS—-THE NORTH AMERICAN SPECIES OF ASTERELLA. 253 
times been described and figured as such. It is doubtful, however, 
if actual filaments ever occur, even in the vicinity of the pores, al- 
though marginal cells of partitions may project as teeth. Cells con- 
taining oil bodies are scattered among the green cells, even when 
they do not occur in the epidermis. 
The compact tissue forms a distinct keel in the median portion of 
the thallus and thins out more or less gradually on the sides, disap- 
pearing altogether at some distance from the margin. The keel 
formed is usually broad and rounded, but is narrow and sharp in A. 
lindenbergiana, where it constitutes one of the distinctive features 
of the species. The compact tissue is composed of uniform paren- 
chyma, except for scattered cells with oil bodies. The walls are some- 
times thin throughout, but are usually thickened to a greater or less 
extent and then show crowded and minute pits of the simple type. 
In certain species mycorhiza is usually present, and it sometimes occu- 
pies a broad median strand, elliptical in section. The walls of the 
cells containing the mycorhiza often, but not always, show a distinct 
purple pigmentation; otherwise the compact tissue is colorless or 
nearly so. In A. californica scattered slime cells may usually be ob- 
served, not only in the compact tissue but also in the partitions be- 
tween air spaces. These cells vary greatly in abundance, and are 
sometimes absent altogether. 
The ventral scales, as in most of the Marchantiales, are arranged 
in two longitudinal rows. As a rule the scales of one row alternate 
with those of the other, but this relationship is not always apparent. 
In specialized regions, such as the basal portion of a ventral branch 
or the terminal portion of a branch bearing a female receptacle, the 
arrangement tends to be irregular, and it is sometimes difficult to dis- 
tinguish the two rows clearly. The scales often yield valuable char- 
acters in distinguishing species, but in considering them a certain 
amount of variability must be assumed and it is unwise to draw con- 
clusions from too limited a number of examples. The scales are 
often exceedingly fragile, especially on plants developed in the shade 
or under moist conditions, and it is necessary to dissect them one by 
one from a mature thallus in order to secure an adequate idea of 
their form and structure. The division of the scales into basal por- 
tion and appendage or appendages is usually well marked, although 
the transition is sometimes very gradual. The basal portion is ovate 
to lunulate and is composed of cells distinctly smaller than those of 
the appendages. In most cases the marginal cells are considerably 
smaller and more irregular than those of the median portion. The 
margin itself is either entire or more or less denticulate, the teeth 
often representing the short and irregular stalks of slime papillae. 
Scattered about among the other cells are occasional cells containing 
