49 2 Tans ley and Lulham. — A Study of the 
of the compensation-strand, it forms a gutter, open on the dorsal side, which 
does not close for some time. The leaf-trace close to its origin is an 
arch whose ends are curved inwards and then backwards towards the 
general curve of the arch. The whole thickness of the arch is involved 
in this curvature, so that the filling of ground tissue extends into the 
lateral concavities. The cross-section of the trace is very much like 
that of the solenostelic species of Gleichenia ( G . pectinata) as figured 
by Boodle l . 
The connexion of the inner cylinder with the outer at the node takes 
place as before towards the end of the node, just as the extremities of 
the wings are departing. Tracheids from the inner cylinder supply the 
whole of the backwardly directed limbs of the leaf-trace, and also of 
course fill up the dorsal gap in the outer cylinder formed by the departure 
of the trace. As in the simpler case just described, when the trace is 
asymmetrical, i. e. turned slightly towards the median dorsal line of the 
stele, the connexion is first made below that wing (the dorsal one) which 
goes off first. 
(3) The adult plants . 
We now come to the ‘adult’ plants, with steles varying from 5-2 
to 1*6 mm. in diameter. The term ‘adult’ may be considered ambiguous 
in a case like this, where the degree of complication attained varies within 
such wide limits, and is no doubt wholly determined by external conditions. 
The term may, however, be fairly applied to plants with leaves and 
leaf-bundles showing their full complication, i. e. in which the forwardly 
directed limbs of the lateral loops are present (Seward, ’ 99 , Plate XIX, 
Fig. 31, &c.). 
The complication of the vascular system of the rhizome varies, 
however, from the case in which there are two siphonostelic cylinders only 
(Seward, ’ 99 ), through various cases marked by increasing complexity of the 
third cylinder, to the case in which there are three complete siphonostelic 
cylinders (Wigglesworth, ’ 02 ). This increasing complication is pretty 
strictly correlated with the diameter of the rhizome, which also determines 
the diameter of the outer cylinder and that of the leaf-trace. In the 
large leaf-traces the lateral loops are much more extensively developed 
than in the smaller ones, and it is this last feature no doubt which 
is connected with the increased complexity of the vascular system of 
the rhizome. 
The ‘ adult ’ rhizomes may be divided into two groups. First, those 
in which the connexion of the second cylinder with the first occurs relatively 
far back in the node, just below the point at which the backward curl 
Boodle (’01 B), Figs. 26, 27. 
