40 
Scott . — On a Palaeozoic Fern , the 
The stem has a very characteristic transverse section, owing to the form 
of the stele, which represents a five-rayed star with the rays or arms of 
unequal length (PL II, Phots. 11-13) ; each arm of the stele corresponds 
to the insertion of a leaf-trace ; the phyllotaxis is §, and the longest arm is 
that which is next going to give off a trace, while the shortest arm is that 
which has last done so. The most prominent arms are truncate, the broad 
end assuming a bicornute form. The main mass of the wood consists of 
a wide zone of large scalariform tracheides ; this zone, the peripheral xylem, 
encloses a central mass of tissue, which, like the stele as a whole, is stellate, 
having narrow protrusions which pass out into the projecting arms. Up to 
1900 this central tissue was described as a pith, with the tacit assumption 
that it constituted a mass of parenchyma. This, however, has been shown 
not to be the case (Scott, ’00, p. 381, Fig. 98 1 ). The central tissue con- 
tains numerous tracheides, much smaller than those of the main zone of 
wood. They are accompanied by parenchyma and are ranged in an 
irregular ring round the centre of the stele : from this central ring radial 
tracts of small tracheides extend outwards, up the middle of each arm of 
the stele ; it is among the internal tracheides of the arms that the proto- 
xylem of the stem is situated. Groups of small tracheides also occur 
externally at the extremities of the more prominent xylem-arms ; where ' 
the arm is truncate or bicornute they form two groups, one at each angle. 
The nature of these external groups of small xylem-elements will be dis- 
cussed below ; they are connected at certain points with the internal 
tracheides of the xylem-arms. 
The phloem surrounds and follows the outline of the xylem. It con- 
tains a single or double band of large elements, no doubt the sieve-tubes, 
which, however, are for the most part limited to the bays of the stele, only 
extending round the xylem-arms where the latter are at their shortest, 
having just given off a leaf-trace. 
Immediately surrounding the stele is a comparatively narrow zone of 
inner cortex consisting of thin-walled parenchyma ; beyond this comes the 
broad and dense outer cortex, succeeded on the outside by a few layers of 
delicate hypoderma and the epidermis bearing multicellular hairs. 
The stem bears appendages of four kinds : (1) the large foliage-leaves, 
of which only the bases are shown ; (2) the 4 axillary shoots ’ of Stenzel ; 
(3) the small scale-leaves or aphlebiae ; (4) the adventitious roots. 
The bases of the foliage-leaves are of large size and have a marked 
effect on the outline of the stem ; they are given off at short intervals. The 
whole form of the stele is, as we have seen, dependent on the course of the 
bundles supplying these leaves and their axillary shoots. The latter, the 
nature of which has been somewhat disputed of late (Kidston, TO), are 
a most characteristic feature, peculiar to the Zygopteris Grayi type among 
1 p. 309, Fig. 1 16 in the second edition. 
