608 FILICALES 
From this point, therefore, up to the third or fourth leaf, the centre of 
the xylem-strand is occupied by a core of phloem. At the departure of 
about the third or fourth leaf the pericycle follows the phloem down into 
the internode below, so that a few pericyclic cells are now to be found in 
the centre of the core of phloem. At the fifth leaf 
(or sometimes at the fourth) the endodermis also 
is decurrent, giving rise at first to a few cells only 
in the centre of the pericycle, which usually dis- 
appear before the node below is reached. Higher 
up it is continuous from node to node, and sur- 
rounds a progressively increasing amount of ground- 
tissue, which 1s now decurrent with it. The vascular 
system has, in fact, become a solenostele. This 
stage, however, does not last long, for the leaf-gaps 
begin to overlap after the departure of about the 
eighth leaf, and above this point the system 
becomes more and more dictyostelic, although at 
first a complete vascular ring is occasionally to be 
met with. ‘lhe leaf-trace of the first five or six 
leaves consists of a single curved strand. Above 
this point two or three separate strands are given 
off to each leaf, and at about the tenth leaf four 
such strands are present, two arising from each 
side of the leaf-gap. 
The first indication of the internal steles that 
occur in the mature plant is to be found at about 
the tenth leaf. Just below one or both of the 
two upper (adaxial) traces of this leaf the xylem 
of the stem-stele is seen to project slightly inwards, 
so as to form a small ridge on its internal surface, 
which is often continued as such for some distance 
down the stem. Sometimes, however, it separates 
Fic. 338. 
PO eee ae ee off completely so as to produce a small xylem- 
of vascular system of a young — strand lying free within the phloem of the stele, 
plant in) median longitudinal 
section, The xylem is black, which either ends blindly below or eventually fuses 
the phloem lightly shaded, and : ) 
the endodermis is indicated by yp again with the main xylem-strand. ‘These free 
a dotted line, the ground-tissue 
Lon (After Gwynne.  xylem-strands are always present at the subsequent 
: leaf-gaps, and, although still remaining enclosed by 
the same endodermis, they become more and more distinct from the main 
xylem-strand of the stele. Later on they may even separate off from the 
stele altogether in the upper part of their course, only fusing with it 
again at a point lower down. The separation of the small xylem-strands 
from the main stele finally becomes complete throughout, and from 
their starting-point they run as small independent vascular strands ending 
blindly in the central ground-tissue, and having no further communication 
