McNicoL — On Cavity Parenchyma and Tyloses in Ferns. 405 
it attains a width about one-fourth or one-fifth that of the bundle. Fig. 2 
is a longitudinal section of Pteris aquilina showing the general appearance 
of a strand of cavity parenchyma cp ; it will be noticed that the strand is 
one or two cells wide, and that the cells themselves are very irregular in 
shape. Fig. 3 shows the expansion ty of the parenchyma cells in a spiral 
vessel. Disintegrated portions of the vessel are seen at px 1 and a nucleus 
at n ; a pitted tracheid occurs at mx. Fig. 4 shows a more advanced 
stage in the cavity-parenchyma formation. Fig. 5 shows a transverse section 
of a single strand of cavity parenchyma. Johnson (’ 03 ) gives an account of 
some tyloses which he found in a preparation of the rhizome of the Bracken 
Fern ; the appearance of the preparation, of which a photograph is given, 
exactly resembles the longitudinal sections of Pteris petiole showing cavity 
parenchyma. The soft cells figured appear to be in contact with the narrow 
elements of the xylem, the wider tracheid being at a greater distance. The 
preparation was made from a small detached piece of material by one 
of the students of his class, and it seems possible that either it may have 
been made from a piece of petiole, the tylose-like cells being cavity 
parenchyma, or that if cut from the rhizome it represents an unusual case 
of continuation of the cavity parenchyma in the rhizome. True tyloses 
formed in the wide pitted tracheids of stem-bundles are not known in recent 
ferns. In Gymnogramme and Cheilanthes there is a single petiolar bundle 
slightly hooked at each end, and in both these genera there is a strand of cavity 
parenchyma on the concave side of each of the hooks. Gymnogramme 
showed swellings of the xylem-sheath cells, one swelling sometimes giving 
rise to another without the formation of a dividing wall. 
Although in a number of Polypodiaceous Ferns cavity parenchyma is 
well developed, it is in the histologically related group of the Cyatheaceae 
or Tree Ferns that the tissue attains its greatest development. 
In Alsophila excelsa the vascular bundles of the petiole are elongated 
and crenulated, the protoxylem groups being situated in the little bays so 
formed. A single petiole has about forty or more groups and consequently 
has an equal number of cavity-parenchyma strands. A single strand attains 
a width of about one-third that of the bundle and the strands thus form 
a most conspicuous feature whether in transverse or longitudinal sections. 
Text-fig. 6, p. 406, shows a transverse section of a petiole of Alsophila 
excelsa : in this section there were to be seen forty-four groups of proto- 
xylem, each having a corresponding strand of cavity parenchyma. Longi- 
tudinal sections (see Fig. 6 in the plate) show that the cells are all quite 
parenchymatous, having thin cellulose walls, distinct nuclei and protoplasmic 
cell contents and that they are irregular in shape and have fairly large inter- 
cellular spaces. They replace many of the spiral elements of the xylem, 
and in transverse sections are seen to extend across the phloem, almost to 
the endodermis at times, as shown in Text-fig. 7 , p. 406. 
