378 Stiles .— The Structure of the Aerial Shoots of 
elements have scalariform thickenings, and gradually increase in size from 
the protoxylem inwards. Transitions between spiral and scalariform 
thickenings occur. As in the lowest part of the aerial stem, parenchyma 
and fibrous elements are usually not present in the xylem except in the 
very young stems below the growing point, where the elements that will 
form the metaxylem are as yet undifferentiated. 
Extending round the xylem is a mass of thin-walled tissue, presumably 
of the nature of phloem. No lignification of the elements, such as occurs in 
Tmesipterisl could be noticed, but in some cases, though by no means 
in all, the phloem cells were lignified slightly at the corners, as in 
P. triquetrum. 2 
This phloem tissue appears to be composed of two kinds of elements. 
In longitudinal section all the cells appear elongated, but some are much 
longer than the others and contain abundant contents, among the latter 
being numerous highly refractive globules, no doubt similar to those occur¬ 
ring in the sieve-tubes of Ferns described by Poirault 3 (Fig. 7). They are 
quite obvious in unstained sections, and show up very distinctly in sections 
stained with ruthenium red. The elements containing these globules 
would presumably correspond to sieve-tubes, but not the slightest sign of 
callus could be found on staining with London blue or corallin soda, 
though this apparent absence might be due to the use of spirit material. 
The cross walls of these elements were sometimes very oblique, and some¬ 
times practically at right angles to the longitudinal walls; in some cases 
the highly refringent globules were observed to be clustered against the 
wall, but in no case were sieve-plates observed. Nuclei were generally, but 
not always, absent from these cells, and in one case a disorganizing nucleus 
was observed. 
Associated with these cells, and as a rule outnumbering them, are 
other elements of about the same diameter, but not so long. In these 
a large nucleus is always present, which often, but not always, presents 
a more or less elongated shape. From this account it will be seen that on 
the whole the phloem of P. flctccidum is very similar to that of P. tri- 
quetrum. 4 In the phloem of Tmesipteris , however, Miss Sykes 5 records 
sieve-tubes only, and these differ from those of Psitotum in having lignified 
walls and numerous lateral sieve-plates on them. 
Surrounding the stele is the endodermis, which, as in the triquetrous 
region of the stem, forms a well-defined layer; its radial walls show their 
lignification plainly when stained with methyl violet and Bismarck brown. 
Immediately outside the bundle, and completely surrounding it, is 
a cylindrical band of rather large cells, which often have thick walls. The 
1 Sykes, M. G. (' 08 ), p. 70. 2 Ford, S. O. (’ 04 ), p. 593. 
3 Poirault, G. (’ 93 ), p. 139. 4 Ford, S. O., 1 . c. 
5 Sykes, M. G., l.c. 
