Scott and Hill.—Structure of Isoetes Hystrix. 417 
stem it becomes very markedly triangular, as seen in trans¬ 
verse section (cf. Plate XXIII, Figs. 13 and 14), the angles 
growing more prominent towards the base of the stem. The 
proportionate length of the cylindrical a$d triquetrous portions 
of the stele, though showing individual variations, remains 
approximately constant in the young and old stems. The 
transition from the one form to the other is gradual, and the 
exact limit therefore arbitrary, but roughly speaking, the upper, 
cylindrical portion occupies from about one-third to two-fifths 
of the whole length, while the lower, more or less triangular 
region, extends through the remaining two-thirds or three- 
fifths. A precisely corresponding change in the form of the 
stele takes place in the two-furrowed species, only here the 
basal region is, in transverse section, an elongated ellipse, 
instead of a triangle (v. Mohl, 1 . c., Figs. 7 and 8). The pro¬ 
jecting arms correspond in position to the furrows, and are due 
to the abutment of successive root-bases. As new roots arise 
in acropetal succession, the triangular region (in the three- 
furrowed species) gradually encroaches on the cylindrical, 
keeping pace with the general growth in length, and thus 
maintaining an approximately constant relation between the 
two regions. Thus every part of the stele is cylindrical in its 
young condition, and only assumes the triquetrous form with 
advancing age, as it becomes the seat of root-formation. 
The stele gives off numerous leaf-traces, which start almost 
horizontally, and then, bending upwards, pass out one into 
each leaf. The whole stele bears leaf-traces, but they are' 
usually only to be recognized on the upper, cylindrical portion ; 
for lower down, the old traces have become more or less com¬ 
pletely obliterated, and their continuity has been interrupted 
by the excessive growth of the secondary tissues through 
which they pass (cf. Hofmeister, 1852, p. 149). 
Before going on to consider the differentiation of the tissues, 
it is desirable to say something as to the apical growth of the 
stem as a whole, a point on which there has been much 
difference of opinion, and which is not yet cleared up. Hof¬ 
meister, as is well known, attributed the apical growth to 
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