4 BOTANICAL GAZETTE [JULY 
form the elongated cells of the axial vascular bundle (a b, figs. 
6-9). Of the cells arising thus, one of the four first formed in 
the procambium of section II (¢7, figs. 6-9) develops without 
further division by longitudinal walls into the large trachea of 
each side of the bundle. Whether transverse anticlines are 
formed in this in its later development could not be made out 
with certainty, but it agrees apparently with the trachea of the 
same bundle in Marsilia in remaining the full length of the 
segment. As the bundle progresses in its development the 
outer layer of the cells formed from the procambium becomes 
specialized as an endodermis or bundle sheath (0 5, figs. 8, 9). 
The fact that the first wall formed in the procambium of section 
I does not correspond exactly with the halving anticline in the 
outer end of this section (fig. 5) makes it easy to distinguish 
procambium and ground meristem at this point (figs. 6-8). The 
leaf of Pilularia thus forms an exception to the general rule 
holding in the ferns, that the endodermis is formed from the 
ground meristem surrounding the bundle (Haberlandt, 796, p. 
336); and agrees rather with the Juncacee and Cyperacee which 
have been studied by Haberlandt. I am inclined also to believe 
that Marsilia agrees with Pilularia in this respect, though it was 
not possible to determine this with absolute certainty as in the 
present case. 
We may now turn to follow briefly the development of the 
ground meristem layer. The two primary cells of this layer 
in section I and the single cell of each of the other three main 
divisions of the segment divide by a single pericline in each. 
The inner layer of cells thus formed, ten in number, constitutes 
the mesophyll layer (m 9, figs. 7-9), which remains of this num- 
ber (as seen in cross section) until maturity, being one cell 
thick and having no tannin sacs like those of Marsilia. Of the 
ten outer cells (pc, figs. 7-9), of which there are at this time 
several in the length of a segment, each gives rise to its part of 
one of the ten longitudinal partitions which separate the ten 
longitudinal air canals (a ¢, figs. 7,9), in the leaf of Pilularia 
(Bischoff, ’28). These air canals arise very early as small 
