Cer at op ter is thalictroides, (L.) ioi 
imperfectly differentiated sieve-tubes on the one hand and 
ordinary parenchymatous cells on the other. 
In the mature steles, the sieve-tubes in transverse sections 
are unmistakable, for they are large elements, and have 
characteristic granules adhering to their walls. Each bundle 
(Fig. 4) is surrounded by a ring of brown thick-walled cells 
(Fig. 4, scl). The walls of those cells which lie immediately 
next to the bundle are fairly thick, but in the cells of the 
succeeding layers the thickening becomes less until the tissue 
gradually merges into the surrounding parenchyma. The 
endodermis ( e ) is easily distinguished by the thickening of the 
radial walls. This thickening of the radial endodermal walls 
is a very constant feature in Ceratopteris , for it is found in the 
bundles of the stem, roots, and leaves of young, as well as of 
mature, plants. 
Within the endodermis a single layer of cells is found (Fig. 
4,/*?), the individual cells of which are large and are superposed 
with great regularity on those of the endodermis, the radial 
walls of the two layers being in direct continuation with each 
other. This arrangement, which is not an uncommon one in 
other Ferns, e. g. in Loxsoma 1 , is found in a large number 
of the bundles of Ceratopteris . I have been unable to 
determine definitely whether this relation between the endo- 
dermis and the pericycle is due to the fact that they have 
had a common origin, though this seems highly probable. If 
this be the case, then the inner layer must be regarded as 
a false and not as a true pericycle. 
The xylem occupies the centre of the bundle and consists 
of a group of scalariform tracheids, the number varying with 
the age and size of the bundle. In the larger steles small 
areas of parenchymatous cells are found scattered among the 
tracheids; in the smaller bundles these are absent. The 
protoxylem consists of a small group of spirally thickened 
elements. In some of the smaller bundles, and especially in 
those in a young plant where the individual elements are very 
small, it is often almost impossible to make out the proto- 
1 Gwynne- Vaughan, 1901, p. 78 . 
