48 Farmer.— On Isoetes lacustris , L. 
cells become clear and marked off from the four rays of 
parenchyma which always connect the central bundle with the 
periphery of the leaf (PL VI. Fig. 23). Division is not so 
regular in the lower part of the leaf, where the sheathing 
portion is formed as a wing-like outgrowth, which is rendered 
more evident as the base of the leaf becomes elevated above 
the level of the stem. The bundle of the leaf in its subulate 
portion pursues a straight course, and, bending out behind 
the insertion of the ligule, where also its character slightly 
changes, it finally curves inwards again below it, and passes as 
a trace into the stem. It is rudimentary in structure, but of 
very constant form, and is collateral, both in its course in the 
leaf and also after entering the stem, thus indicating a relation- 
ship with the higher Ferns and with the Phanerogams. 
In the portion of the leaf near the apex, the lignified portion 
of the xylem is reduced to a single tracheid, which is sur- 
rounded by six or eight parenchymatous cells, representing 
the rest of the xylem, which occasionally suffer more or less 
complete lignification, especially in the lower part of the 
leaf. I am unable to confirm Russow’s statements as to the 
development of the xylem ; I do not find, at least in /. lacustris , 
that there is any definite direction in which the fresh xylem- 
elements are produced. Sometimes they are formed centri- 
petally, sometimes in the opposite direction, and occasionally 
they spread unequally in the lateral direction. 
The phloem is represented by a few cells which occur at the 
outer flanks of the xylem, and the protophloem tissues, where 
they can be distinguished, occur as distorted elements at the 
outer side of the wood. This slight tendency to encroach 
around the xylem is of interest, as pointing to an approach to 
the concentric type of the Fern-bundles proper ; it is shown in 
PI. V. Fig. 15, which was drawn with the camera lucida. I 
failed entirely to discover any sieve-plates in the phloem either 
of I. lacustris or /. velata ; possibly, however, they might be 
found in a more pronounced terrestrial form. The elements 
of the xylem become more numerous as the bundle bends out 
behind the ligule, and at the same time much shorter ; a fact 
