Ceratopteris thalictroides , (L.) 105 
some places and from the pericycle in others. They are 
always present at the two ends of the bundle, lying between 
the xylem and the pericycle (Fig. 6, pa). 
The pericycle and endodermis are easily distinguished. 
The small strands of the stem (Fig. 7) are often very 
feebly developed. In passing through a series of transverse 
sections from the apex downwards towards the base of the 
stem, it is found that at some levels individual bundles 
suddenly end somewhat abruptly. At other levels again 
these small bundles are seen to be uniting with each other or 
dividing. Their arrangement, where this can be determined 
with any certainty, seems to be bicollateral, but the phloem is 
often indistinguishable. * 
The young Stem. The structure of the young stem can 
only be studied in microtome sections. A transverse section, 
taken from the middle or near the base of the stem of a 
young plant 2-4 inches in height, shows that the main mass of 
the stem-tissue consists of loose large-celled parenchyma, in 
which the vascular bundles lie embedded. As in the case of 
the mature stem, the younger one is polystelic, each strand 
being furnished with a separate endodermis and pericycle. 
The number of strands present varies from two to five, and 
no clearly marked difference in size is met with, as in the case 
of the older plants ; the division into an outer ring of large 
steles and an inner group of small ones does not occur in the 
young stem. Each stele is bicollateral, but here again, owing 
to the incomplete differentiation of the tissues, the sieve-tubes 
have often not yet acquired their characteristic appearance. 
y. The Course of the Vascular Strands in the Stem. I have 
attempted to make out the course and fate of the steles in the 
stem of Ceratopteris plants of different ages, but it is only in 
the young stems that it has been possible to find any degree 
of regularity in the behaviour of individual strands. The 
result given in text-figure 1 was arrived at by tracing the 
course of each bundle in the stem of a young plant in a series 
of transverse sections from the base upwards and towards the 
apex. From this diagrammatic figure it will be seen that 
