710 Gwynne- Vaughan. — Observations on the 
be expected to hold good rigidly from leaf to leaf in every 
specimen. This statement applies in particular to the diagram 
given in illustration (Fig. 18). With this reservation, however, 
it is believed that the diagram will serve to represent the 
course of development of the vascular system, not only in the 
Cyatheaceae, but also in most of the solenostelic and dictyo- 
stelic Ferns up to the particular stage that they retain when 
mature. 
The young plant of Alsophila excelsa has its leaves ar- 
ranged radially all round the axis, and it probably grew erect. 
At the very base of the stem the single central cylinder 
possesses a small central strand of xylem, usually with a few 
xylem-parenchyma cells intervening between the tracheides. 
The first leaf-trace may depart without in any way altering 
the structure of this stele or of its xylem-strand, but usually 
the phloem on the adaxial surface of the leaf-trace is prolonged 
a short distance downwards into the substance of the central 
xylem. At the departure of the subsequent leaves this feature 
is much more pronounced, and the phloem thus decurrent 
runs down through the whole length of the internode to meet 
with that decurrent from the leaf below. In the second leaf, 
however, it often falls short of the point of departure of the 
first leaf and ends blindly in the internode. 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 de- 
parture 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 disappear 
before the node below is reached. Higher up it is continuous 
from node to node, and surrounds a progressively increasing 
amount of ground-tissue which is now decurrent with it. The 
vascular system has, in fact, at length 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. 
