378 Chandler. — On the Arrangement of the Vascular Strands 
unusual path in the cortex, an upward curved course being that most 
commonly adopted. 
It is unnecessary to describe in any detail the later processes in 
the elaboration of the vascular system. The cauline strands increase 
in number by repeated fraction, and the mature dictyostelic condition 
is very quickly attained. 
In one or two plants examined it was noticed that small strands 
which ended blindly in the ground-tissue at a higher level were occasionally 
given off from the cauline system. Similar strands were also found to 
occur in Polypodium aureum . 
Aspidium Tsus-Simense, Hk. 
The transitional processes of this fern are of a very simple character 
and will be described merely in outline. Passing upwards in the young 
plant, we find the root-strand changing into the protostelic condition in the 
normal way, though the completion of the change is much longer deferred 
than is generally the case. Internal phloem is differentiated just before 
the exit of the first leaf-trace, which is formed in the usual simple manner. 
Fig. 165 represents the cauline system after the completion of the first 
trace. It will be seen that although the gap is not yet repaired, a pocket 
of fundamental-tissue has been differentiated in the phloem at the centre of 
the amphiphloic strand. The vascular tissue later bulges outwards at 
a point opposite the gap, the protruding portion, in which the xylem is seen 
to be arranged in two distinct tracts, soon passing outwards as the second 
leaf-trace. The inner and outer parts of the ground-tissue become con- 
tinuous after the exit of the leaf-trace. The curved vascular band 
subsequently becomes more strongly convex, and, as before, the ‘ back ’ 
of the arc separates from the lateral portions as we pass upwards, to supply 
the third leaf with a single petiolar bundle. The later formation of the 
first double leaf-trace is illustrated in Figs. 170, 171. 
The mature vascular organization is the result of the elaboration, along 
ordinary lines, of the simple dictyostelic structure obtaining at this level, 
and need be described at no further length. 
Polypodium aureum, L. 
The dorsiventral rhizome of this fern creeps along the surface of the 
ground, bearing upon its upper surface two rows of leaves which are more 
or less arranged in pairs. The internodes between consecutive pairs are of 
considerable length. The change from the radial to the dorsiventral habit 
is well shown in a fairly young rhizome, in which the conical base is seen 
to bend sharply at right angles as it rapidly increases in diameter. The 
adult horizontal position of the rhizome is soon reached. 
