428 Scott and Hill.—Structure of Isoetes Hystrix. 
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out the order for ourselves in I. Hystrix , and as the published 
accounts are not altogether clear, we have thought it worth 
while to illustrate the subject by the two accompanying 
diagrams (Figs. 11 and 12 in the text), which were made from 
camera-lucida drawings, so that the position of the roots is 
exact. The relative age of the roots could be determined by 
their state of development, the oldest root-traces having their 
vascular tissues more or less obliterated, while the youngest 
were still wholly meristematic, and all intermediate stages 
were represented. Both sec¬ 
tions pass through the region 
of the root-bases. Fig. 12 is 
from a transverse section 
through the base of the stem, 
while Fig. 11 represents a 
tangential section, from the 
side of the stem, cut at right 
angles to one of the furrows. 
The succession of the roots 
of Isoetes , like that of most 
other adventitious roots, is an 
acropetal one. Yet new roots 
are constantly arising on the 
base of the stem. The ap¬ 
parent contradiction is thus 
explained : the roots, in our 
species, are arranged in three 
sets, corresponding to the 
three furrows. We will fix 
our attention on one set. The roots are arranged in several 
parallel series, on each side of the median line of the 
furrow (F in the diagrams); the whole set of roots extends 
from near the centre of the basal surface of the stem outwards, 
and for a certain distance up the flank. In each series the 
succession of the roots is acropetal, the oldest being nearest 
the centre of the base of the stem, and the youngest highest 
up on the flank. But, at the same time, new series are being 
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Fig. 11. Diagram of the root-succession 
in /. Hystrix , as shown in a tangential 
section of the stem, at right angles to a 
furrow. 
