McNicol. — On Cavity Parenchyma and Tyloses in Ferns. 403 
the points of resistance of the lignified parts of the vessel. The continued 
expansion of the parenchymatous processes within the lumina of the vessels 
results in the breaking up of the vessels, which thus early become non- 
functional. Frequently nuclei are distinctly seen in the bladder-like 
swellings in the vessels ; the swellings are finally cut off from the original 
cell by a cross-partition. After breaking up one spiral vessel the parenchyma 
continues its growth, forming new processes which penetrate adjoining 
vessels, and these in turn become broken up. In Microlepia most of the 
spiral vessels become broken up, but in some Ferns the parenchyma destroys 
only a few of the first-formed vessels. Their place is thus taken by irregular 
parenchyma cells, frequently with large intercellular spaces between them, 
which form strands running the length of the petiole. Very frequently 
remains of the disintegrated vessels are seen in the spaces between the cells 
of the cavity parenchyma as small pieces of lignified thickening, sometimes 
two or three connected turns of a spiral, but generally small fragments con- 
siderably crushed. Such remains are to be noticed in the part of the 
parenchyma most recently formed ; they are nearly always to be seen in 
the deep constrictions of the cells which have not ceased to expand, and they 
frequently give the cell itself a superficial appearance of being lignified at 
these points ; in reality the tissue in this plant remains unlignified, though 
in various other Ferns lignification of the cavity-parenchyma cells does occur. 
In Fig. 1 three cavity-parenchyma cells are seen in cross-section at cp ; they 
have replaced the protoxylem and have the appearance of extending a con- 
siderable distance into the phloem ph. The tissue is about one-fifth the 
"width of the whole bundle. The metaxylem, which consists of a single row 
of tracheids, is seen at xy. 
Cavity parenchyma has been recorded by G Wynne- Vaughan (’ 03 ) in 
Davallia , another member of the same sub-order. I examined D . Griffithiana , 
which has an elongated bundle, the xylem being surrounded by phloem 
about three cells deep ; a group of cavity-parenchyma cells was seen at 
each end of the bundle, but the tissue, the cells of which are lignified in this 
species, did not occupy so large a space as in Microlepia. In N ephrolepis 
the vascular bundle is somewhat T-shaped or triangular, and the cavity 
parenchyma appears in the positions marked by stars in the Text-fig. 
1 A, p. 404, opposite the three protoxylem groups. In longitudinal sections 
Nephrolepis shows very clearly the tylose-like swellings which occur in 
the spiral vessels. The Text-figs. 2, 3, 4, 5, show different stages of the 
ingrowth of the parenchyma. 
Text-fig. 2 shows three cells of the xylem sheath which have formed 
small swellings in a vessel. Each cell had several swellings. Fig. 3 shows 
a stage in which the swellings have elongated, and have almost closed up 
the lumen of the vessel ; Fig. 4 shows one swelling which has itself formed 
a swelling without being cut off from the original xylem-sheath cell by 
