43 $ Scott and Hill.—Structure of Isoetes Hystrix . 
well known, and is illustrated, somewhat diagrammatically, in 
Fig. 30. 
The position of the stele is from the first somewhat eccen¬ 
tric ; a large intercellular space arises in the cortex, by 
breaking down of its internal layers, on one side of the stele, 
while, on the opposite side, continuity between central cylinder 
and cortex is maintained. As regards the external tissues, 
the chief point of interest is the presence of short cells—the 
‘ trichome-initials ’ of Bruchmann (1874, p. 566)—in the super¬ 
ficial layer, a point in which the root of Isoetes agrees with 
that of Lycopodium . It is from these cells only that the 
root-hairs, which are very abundant in I. Hystrix , take their 
rise. 
The structure of the stele is in all cases monarch. There 
is a single group of xylem, with a single band of phloem, 
forming a horseshoe on one side of the wood (Figs. 30, 31). 
The orientation of the tissues is such that the xylem of all the 
roots, where they spring from the stem, is turned towards the 
centre-line of the furrow ; the phloem, as well as the inter¬ 
cellular cavity, which always lies on the phloem-side, are 
directed towards the margins of the furrow. 
The stele is surrounded by an endodermis of the usual 
structure, which is clearly of common origin with the inner 
cortex. The pericycle is incomplete on the side opposite the 
middle of the phloem. 
The wood consists for the most part of densely spiral or 
annular tracheides, somewhat reticulated in places. The first 
tracheide to be differentiated lies on the side of the xylem 
remote from the phloem, as shown in Fig. 32. From this 
point the development advances outwards, though not in per¬ 
fectly regular succession, until the xylem is complete. The 
protoxylem in the roots of the adult plant is, as a rule, separ¬ 
ated by the pericycle from the endodermis ; in the first root 
it appears, according to Bruchmann (1874, p. 565) and Farmer 
(1890, PI. V, Fig. 4) to be in contact with the latter layer. 
The protophloem is recognizable before the differentiation 
of the xylem begins, and forms an arc, in immediate contact 
