LIGULATE WOLFFIAS OP THE UNITED STATES. 109 
but two cells high in the interior. The large cavities 
formed by these walls and the two epidermal layers 
are termed "air cavities" by Hegelmaier, and the gas 
they contain enables the plant to float. This cavernous 
tissue extends from one-half to two-thirds the length of the 
frond, from the base. After a long, careful search I was 
unable to find any openings connecting these cavities with 
the exterior, so it is hardly probable that the gas in them is 
free to circulate with the air at the surface of the water. 
All searches for stomata were fruitless. 
The epidermal cells are usually hexagonal and slightly 
elongated in the direction of the long axis of the frond. 
The air cavities are of the same shape and similarly length- 
ened. Both the upper and lower walls of the pouch are two 
cells thick except in the costa, which is increased to several 
cells in thickness. In the stipe and costa the cells are 
much elongated, prism-form with either square or wedge- 
shape ends, arranged in chains. From the point where the 
stipe joins on the frond these chains of elongated cells 
spread out flabelliform and the cells gradually shortening 
and broadening merge into the regular hexagonal form in 
the epidermis (pi. 65, f. 9). However, a few chains go 
direct to the base of the pocket and there join, through the 
matrix, with the stipes of the young fronds, forming a 
continuous chain of these ligneous-like cells throughout 
the generations (pi. 65, f. 11), In view of this fact the 
costa perhaps ought properly to be considered the axis of 
the plant and the frond merely as epidermal tiT>ue modified 
to perform the functions of obtaining and utilizing food. 
Quite regularly distributed over both surface-* are epider- 
mal cells, somewhat smaller than those surrounding them, 
with granular yellow contents ; they occur a}>o in the elon- 
gtited cells of the costa. These are the so-called pigment- 
cells of Hegelmaier. Their function is as vet unexplained 
(pi. 65, f.8). 
The occurrence of this species in this isolated point, far 
away from its first reported habitat, separated from it by 
