102 
Ford . — The A natomy of 
xylem. When present it is in a more or less median position, 
and on the outer edge of the xylem. In a few cases, but 
these are very rare, the xylem seems to touch the pericycle 
directly ; as a general rule small parenchymatous cells are 
interposed between the two tissues. 
Scattered parenchymatous cells are also found lying round 
the tracheids between these elements and the sieve-tubes 
(Fig. 4, pa). On the side of the xylem which is away from 
the protoxylem, and on this side only, Thomae has figured 
and described the sieve-tube zone. Beyond this again, and 
between this zone and the pericycle, he has figured a layer of 
protophloem-elements, though he has not described them in 
his account of Ceratopteris . He does however mention and 
describe protophloem-elements in his general description of 
the vascular elements in the petioles of Ferns. This layer 
of protophloem he figures as being made up of conspicuous 
thick-walled elements, and the sieve-tubes in his drawing also 
give one the impression of being thick-walled. The sieve- 
tubes are however essentially large thin-walled elements (Fig. 
4, si) and are present on both sides of the xylem. A layer of 
small crushed elements are found (Fig. 4, pp) on one side of 
the xylem only, viz. on the side turned away from the proto- 
xylem, the individual cells being inconspicuous and not par- 
ticularly thick-walled. This inconspicuous layer probably 
represents the protophloem, which forms so noticeable a feature 
in Thomae’s figure 1 . 
In regard to the structure of the young petiole there is 
not much to be said. The petioles of the first two or three 
leaves are more or less circular in outline, and contain a single 
central stele. Petioles produced at a later stage are reniform 
in shape with two steles— -one at each corner. A third and 
still older stage shows petioles with a triangular outline, the 
steles, three in number, being found at the angles. As the plant 
increases in age the individual bundles divide frequently and 
anastomose, this being the case in the petioles of quite old plants. 
2. Lamina. The leaves of Ceratopteris thalictroides are 
1 Thomae, 1886, p. 151 . 
