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Endodermis in the Stem of Hippuris . 
cambial strands of the leaf-traces, and thirdly to keep pace with the rapid 
increase in size of the young cortex consequent upon the formation of 
the air-canals in the internodes. For this reason the identity of the original 
periblem caps and their constituent cells is more difficult to trace in the 
node, though the contour of the original cells can as a rule be recognized 
by the thickness of their walls (see Fig. 5). 
It has already been indicated that the number of circles of air-canals 
is related to the number of layers of periblem, being one fewer ; thus, in 
the specimen figured with five periblem caps there are four rings of canals 
(Fig. 3). In the older parts of the stem, however, additional rings can 
be observed, and it is in studying the modes of formation of these additional 
intercellular spaces that the part played by the plerome in contributing to 
the inner region of the cortex, including the endodermis, becomes clear. 
It is necessary therefore to return and to consider in some detail 
the fate of the most internal layer of the periblem. As described above, the 
first dividing-wall runs tangentially across the cell from one air-canal 
to another (Fig. 3). Further segmentation takes place in the inner and 
larger of the two cells so formed, and in this the next wall is placed in 
an obliquely radial direction, with its outer end usually based upon an 
air- canal. Where the inner end of this wall intersects the wall of the outer- 
1 
most plerome cell a new intercellular space arises, and thus is initiated a fifth 
ring of air-canals (Fig. 4, ac. 5 ). It is obvious that cells bounding these canals 
on the inner side must be derived from the plerome, and this can be readily 
confirmed. Moreover, it not infrequently happens that in the development 
of the fourth ring of air-canals the separating cell-walls of the two cells 
bounding it on its inner side may split apart, thus permitting the enlargement 
of a plerome cell which thus becomes the inner boundary of the canal. The 
whole plerome is at this stage actively enlarging, but the outermost cells 
divide for the most part by radial walls, and also increase in size in a tangen- 
tial direction. They thus form a fairly well defined layer distinct from the 
rest of the plerome. Although no regular sequence can be traced in their 
divisions, they sooner or later divide by tangential walls into inner and 
outer cells (Fig. 6). This stage can be traced for some distance down the 
stem. Both layers may undergo subdivision by radial walls. The cells of 
the outer layer eventually enlarge and become rounded off as intercellular 
spaces develop between them. Eventually the cells of the inner layer 
undergo a tangential division, and the relative position of the two layers of 
cells so formed remains unaltered in the mature stem. The innermost layer 
is the endodermis, and later develops the cuticularized folded band on 
the radial walls. 
Considerable variation may be seen from the process just described. 
Some of the divisions may be omitted or, in other cases, may be increased 
in number, but on the whole the general sequence of development in 
H 
