Polycycly. 20 t 
cylinder in turn passes through the Lindsaya- phase in progressing 
from the haplostelic to the solenostelic-type. We also see that the 
local internal dilatation of the cylinder in the region of the node is, 
in the first instance, a reinforcement of the water-conducting 
capacity of the stele immediately in front of the point at which it is 
diminished by the departure of the leaf-trace. Secondarily the 
internal strand is drawn into supplying the recently added portions 
of the elaborated leaf-trace, and in so doing becomes, in the extreme 
case, taxed to such an extent that a third cylinder is developed to 
reinforce the second in precisely the same way as the second 
reinforces the first. If the leaf-trace became more complicated 
still there is no apparent reason why this process should not be 
carried even further, and additional internal cylinders be developed. 
The beginnings of a fourth cylinder are actually formed in 
Saccoloma adiantoides Sw. (Dicksonia Plumieri Hk.), so far as can 
be judged from the account given by Mettenius (’65), though the 
relations are decidedly different from those obtaining in Matonia , 
owing to the radial arrangement and crowding of the leaves. 
Saccoloma is a big fern with an upright cylindrical stem, 1^" thick 
in Mettenius’ specimen, and radially arranged leaves showing a 
divergence of g 8 r . The vascular system (Fig. 62, 63) consists of 
three concentric cylinders, the outermost of which is broken only at 
the leaf-gaps, while the second locally breaks into two or more 
arcs, and the third normally consists of two similar, but of course 
smaller, arcs. 
The leaf-traces are derived entirely from the external cylinder, 
the gaps created by their departure being filled up by the moving 
out of arcs from the second cylinder, which in turn is regenerated 
by the outward passage of strands from the third innermost cylinder. 
This last is reconstituted by the branching of its strands below the 
point at which they begin to move out, so that the beginnings of a 
fourth cylinder are found at certain levels (Fig. 62, B, and Fig. 63 ( 
last figure). The inner cylinders in this case are purely compen¬ 
satory in function, and are not drawn into supplying the leaf-traces 
directly. 
Acrostichum aureum (Chrysodium vulgare ) is an example of a 
fern with a simple type of polycyclic stem—the solenostele possess¬ 
ing a few internal, short, irregularly branching and anastomosing, 
blindly ending accessory strands, one of which fills each leaf-gap 
(Fig. 64)—but with a very complicated leaf-trace and a great 
number of petiolar strands. The internal gap-filling strand takes 
