370 West . — A Contribution to the Study of the Marattiaceae . 
eighth leaf-trace dichotomizes near its base (Text-fig. 4, A, l.t . 8), whilst the 
traces of later leaves depart as two distinct strands, which, instead of arising 
from the base of the gap, as in the case of the single trace of the earlier 
leaves, arise from the sides of the gap, usually at different levels (PL XXI, 
Fig. 1 A, l.t. 1-4). Further branching and occasional anastomosing of the 
strands of the leaf-trace occur in the cortex of the stem. 
The siphonostelic condition is maintained for a short time only ; sooner 
or later, owing to the crowding of the spirally arranged leaf-traces, the gap 
above one leaf-trace fails to be repaired till after the exit of the trace of the 
next leaf; in this way a simple dictyostele is produced (PL XXI, Fig. 1 a). 
The change in the direction of growth, to which reference has already 
been made, takes place in the specimen under discussion at the level of the 
apex of the gap made by the departure of the traces of the third leaf 
represented in the large model; this region is indicated in PL XXI, Fig. 1 A, 
by the two arrows. 
Correlated with the change in the direction of growth there is an 
important difference in the organization of the vascular tissues, the radial 
configuration of the stem-stele being replaced by one in which dorsiventrality 
is well marked. This change is indicated at an early date by the formation 
of a large gap (y.g. in PL XXI, Fig. 1 A) in a position where one would 
expect to find a gap formed by the departure ' of the traces of leaf 5. 
However, the traces of this leaf, instead of continuing the spiral in which 
the earlier leaf-traces are arranged, depart from the stem-stele at a point 
almost opposite the exit of the preceding leaf-trace. In this connexion 
it is interesting to find that the commissural strand, after leaving the inner 
surface of the vascular cylinder above the gap of leaf 4, takes a sharp 
bend (indicated by a x in PL XXI, Fig. 1 a) towards the upper end of 
this lai'ge ventral gap before continuing its course towards the apex of the 
next leaf-gap above (i. e. the gap made by the departure of the traces of the 
fifth leaf), which it helps to close. A root-trace (PL XXI, Fig. 1 A, r.t. 1) 
fuses with this commissural strand immediately below the point where the 
latter joins the inner surface of the stelar cylinder. 
The stem, meanwhile, steadily increases in diameter, the dictyostele 
opening out to a corresponding degree ; at about this stage the latter 
becomes fractionated, perforations, other than leaf-gaps, occurring in the 
stelar cylinder. 
On the dorsal and lateral surfaces of the dictyostele the leaf-traces 
become more crowded and more complex, frequently leaving the stem-stele 
as six or more separate strands, among which the two main laterals can 
readily be recognized by their stronger development (cf. Farmer and Hill, 
29 , p. 378). The strands of the leaf-trace depart from the basal and lateral 
margins of the foliar gaps, anastomosing at irregular intervals with the 
strands which can still be regarded as belonging to the original dictyostele 
