42 Farmer. — On Isoetes lacustris , L. 
at its interior, which are continuous above with the phloem of 
the leaf-traces. Russow partly for this reason, has regarded 
the prismatic layer as representing a bast or phloem, but quite 
apart from the fact that it is produced internally to the cam- 
bium, and would thus, from the point of view of its position, 
be anomalous, its structure is remarkably complex and hetero- 
geneous. Hegelmaier 2 briefly indicated the zones observable 
in the tissue under consideration and assigned the cause of the 
appearance to alternating layers of clear and starch-filled cells. 
Transverse sections taken through the central part of the stem 
show that the prismatic layer which surrounds the central 
xylem is a band of varying thickness, being especially inter- 
rupted by the bundles which pass out to the roots. The zone- 
like arrangement consists in alternations of tubular thin-walled 
cell-rows of varying thickness, whose cell-contents are clear 
and watery, with others, w T hose cells are wider in the radial 
direction, and filled densely with starch. Occupying a middle 
position in the latter zone, is embedded an irregular ring of 
cells whose walls are thickened like those of the tracheids, but 
these too, unlike the latter, often contain protoplasm and 
starch. (The dark bands in PI. VI. Figs. 26 and 27.) The 
number of the zones or bands varies with the thickness of the 
stem, and therefore presumably with its age, and also with 
the height at which the section is cut. The longitudinal sec- 
tion is especially instructive in explaining the relation which 
exists between the number of the zones at different heights. 
The bands are seen to converge towards the axile xylem in 
the order of their position ; the outermost one being of course 
inserted highest. Their relative position naturally suggests 
some connection between their number and the periods of 
active growth, but I am unable to say if such a connection 
really exists. Fig. 27, which is a photograph of a section of 
/. velata , for the material of which I am indebted to the kind- 
ness of Prof. Pirotta, shows the longitudinal arrangement of 
the bands through part of their length. 
1 Russow, Vergl. Unters. p. 139. 
2 Hegelmaier, loc. cit. 
