1900] SPOROPHYVLLS AND SPORANGIA OF ISOETES 227 
of the vegetative organs. The center of the stem is occupied 
by a mass of short spiral and reticulated tracheids interspersed 
especially near the periphery with less numerous parenchymatous 
cells. The peripheral parenchyma is not sufficiently aggregated 
or continuous to form a xylem sheath such as occurs in the 
leaves. The xylem region is surrounded by a ring of tabular 
cells of glistening white appearance, thick-walled and empty 
towards the center but thin-walled towards the outside and more 
or less banded with incomplete layers of starch-containing cells. 
The cells are arranged in pretty regular radial rows, whether 
examined in longitudinal or transverse section. This ring is 
usually designated the prismatic layer, and very frequently, after 
Russow (1), the phloem. Russow claimed to have traced a con- 
tinuity between the prismatic layer and the phloem of the leaf. 
I have not been able to satisfy myself of any organic continuity, 
but even did it exist it seems to me very questionable whether 
that would be a sufficient reason to justify Russow’s view. No 
clearly defined sieve-tubes, the essential elements of the phloem, 
have ever been found either in the stem or in the leaf; and 
besides the inner cells of the prismatic zone are known to become 
secondarily thickened and transformed into xylem tracheids. 
The cells marked 0 in jig. 5 are in this process of transformation. 
Does not this indicate, if not a xylem character, at least the 
undifferentiated nature of the prismatic cells? A transforma- 
tion of phloem into xylem would be, to say the least, an anomaly. 
In view of these difficulties to which may be added another— 
the relation to the cambium —it seems better to drop this appli- 
cation of the word phloem until its justification shall be estab- 
lished on Physiological grounds. A small portion of the prismatic 
layer is shown in i ge 
Immediately outside the prismatic layer and indistinguishable 
from it €xcept in the staining and size of the cells, is a zone of 
meristem which by its active division gives origin outwardly to 
an immense mass of cortex, and internally adds slowly to the 
prismatic layer. This zone is the so-called cambium. Its cells 
contain deeply staining plasmic contents in addition to starch 
