726 Gwynne- Vaughan.— Observations on the 
rubiginosa. I agree with Lachmann ( 1 . c., p. 130) in thinking 
that the course of the root-stele depends chiefly upon the 
different tensions set up in the cortex of the stem, incident 
upon the varying energy of its terminal growth at the time 
of the development of the root. 
The most striking feature relating to the root-stele is 
that it possesses no cortex of its own during the greater 
part of its course through the ground-tissue of the stem. The 
cortex of the stem runs without break or interruption right up 
to the endodermis of the root. According to Van Tieghem 1 
this is due to the fact that the definitive apical cell of the 
root arises in the endodermis so very near to the apex that 
the cortical tissue lying without it has at that point only 
attained three or four layers in thickness. The apical cell 
is subsequently kept in this position by divisions in the sub- 
jacent pericycle, which keep pace with the expansion of the 
surrounding cortex and result in the formation of a c root- 
pedicel.’ 
Histology of the Vascular System. 
In the Cyatheaceae and Polypodiaceae the endodermis is 
always exceptionally well-marked and characteristic. It 
always stands out with great clearness in sections treated with 
phloroglucin, because the radial walls of its cells are lignified 
and stain red. In all cases examined the endodermis and 
pericycle on both sides of the stele appear to arise from the 
division of a common cell-layer. The first-formed elements 
of the xylem in the petiolar bundles are small annular or 
spiral tracheides, and they are always grouped into well- 
defined strands situated on the morphologically internal side 
of the xylem (Figs. 24, 2$ y 26, flrx.). The metaxylem some- 
times closes up in front of the protoxylem elements, so that 
they appear to be immersed in the substance of the xylem ; 
but nevertheless, as regards these two orders, it may be taken 
as a general statement that the protoxylems in the petiole 
1 Van Tieghem et Duliot, Recherches comparatives sur l’origine des membres 
endogenes, Ann. des Sc. Nat., S6r. 7, t. viii, p. 540, 1888. 
