184 W. T. GORDON 
pteris Grayv); and a similar cause may be assigned to the apparent grouping of the 
xylem into certain areas in A. corrugata and A. Brongmart:. In other words, the 
departure of the petiole-trace is beginning to have a greater effect on the stem xylem 
in M. duplex than it had in Diplolabis, and the series from Dzplolabis to Ktapteris 
(Zygopteris Gray), through Metaclepsydropsis duplex and Ankyropteris corrugata 
and Brongniarti, foreshadows a type of petiole-trace departure which will no longer be 
In the Zygopteridez the departure 
seems to have been protostelic in all cases, but the Osmundaceze show the change from 
the one type to the other. MM. duplew is thus, as far as the axis is concerned, distinctly 
intermediate between the Diplolabis type and that of Ankyropteris corrugata ; indeed, 
it holds the same position among the Zygopterideze with four rows of primary pinnee, 
protostelic, but cause a gap in the outer xylem ring. 
that Ankyropteris corrugata does among those with two orthostichies of such 
appendages. 
Dividing up the Zygopteridez according to this criterion, as Kipston and GwyNNE- 
VauGHAN have done, we establish the two stem series: (1) Diplolabis r6mer—Meta- 
clepsydropsis duplec—LKtapteris di-upsilon (Zygopteris Grayt), and (2) Ankyropteris 
To make the intermediate position of M. duplex 
in the first series quite clear, the following table has been inserted :— 
corrugata—A nkyropteris scandens. 
Diplolabis. Metaclepsydropsts. Etapteris, 
Shape of stem xylem in transverse | Circular, with inner | Circular, with inner | Pentagonal, with 
section and outer zones. and outer zones. inner and outer 
zones, 
Type of stele Solid. With conjunctive | With conjunctive 
parenchyma in parenchyma in 
Type of tracheide 
Outer zone 
Inner ,, 
Arms radiating from inner zone . 
Long, pointed, reti- 
culate, 
Short, square-ended, 
reticulate. 
Absent. 
central zone. 
Long, pointed, re- 
ticulate. 
Long, pointed, reti- 
culate or scalari- 
form. 
Slightly developed. 
central zone. 
| . 
Long, pointed, sca- 
laviform. 
Long, pointed, sca- 
lariform. 
Strongly developed. 
ee ee oe oe a 
Unequal dichotomy, 
so-called “‘axil- 
lary” branch. 
Type of branching Equal dichotomy. Equal dichotomy. 
It has been shown that the reticulate type of thickening on the walls of tracheides 
is more primitive than the scalariform type, so that as far as we can judge M. duplex 
occupies a position above Diplolabis in the Zygopterid series ; and the other criteria at — 
our disposal all point to the same conclusion. 
Before passing to a discussion of the systematic position of the petiole, I wish to 
