430 Scott and Hill. —Structure of Isoetes Hystrix . 
will be noticed that in Fig. it (which being from the side of 
the stem, shows only the younger roots of those series which 
reach so far) an intermediate row on each side is the longest. 
This is so, because the younger series, towards the middle, are 
not yet carried so far up, while the older series, towards the 
outside, were completed while the whole root-bearing region 
was shorter. 
The older and outer series of each set get pushed further 
and further away from the centre-line of the furrow by 
the growth of new series of roots, and of the cortical tissue 
between them. 
The three prominent arms of the stele, in its lower portion, 
are simply due to the abutment on the vascular cylinder of 
the three sets of roots. Each root, as is well known, arises 
from cortical tissue 1 just outside the cambium. It is thus 
separated at its origin, from the wood of the stele, by all the 
intracambial tissue which has been formed. This tissue 
becomes partly differentiated into wood, so as to connect up 
the new root with the conducting tissues of the plant generally 
(Fig. 15). Each new vertical series of roots necessarily arises 
a little further out from the stele 2 than its predecessor, and 
its abutment adds a little to the length of the arm to which 
it belongs. We thus obtain a kind of sympodium, the 
wood at each angle being built up of the successive root- 
bases. The original cylindrical xylem-cylinder remains, more 
or less unaltered, in the middle of the whole structure. The 
same thing is going on at the base of the stem, as new 
series of roots start there. Their bases join on successively 
to the bottom of the stele. 
It has long been known that the base of the stem of 
Isoetes has a slow downward growth, a fact which much 
exercised the earlier writers. Von Mohl (1845, p. 125) dis¬ 
cusses, and leaves undetermined, the question whether Isoetes 
1 We use the word cortical because the attempt to delimit a pericycle appears 
quite hopeless in the case of the Isoetes- stem. 
2 This will be evident on comparing Text-fig. n with Plate XXIII, Figs. 14 
and 15. 
