728 Gwynne- Vaughan.— Observations on the 
irregularly (Fig. 23). In Cyathea Brunonis , Alsophila excelsa , 
Dicksonia culcita , and in several other dictyostelic Ferns with 
large meristeles, the leaf-trace protoxylems are decurrent 
along the margins of the leaf-gaps and even for some distance 
below the gap itself, but the differentiation of the main mass 
of xyJem is irregular, or sometimes more or less centripetal. 
These observations are of course not exhaustive, but, so far 
as they go, it appears that whenever definite protoxylem 
strands consisting of spiral and annular elements do occur in 
the stem they are in relation, directly or indirectly, to the 
decurrent protoxylems of the leaf-trace. On the other hand, 
the small scalariform tracheides, which are the first to be 
formed at the periphery of the xylem of the stem, may in 
many cases be regarded as constituting a definite exarch 
protoxylem proper to the stem itself. 
The phloem is generally separated from the xylem by 
a continuous layer of parenchyma (the ‘ vasal-parenchym * of 
Strasburger), although sieve-tubes are occasionally to be found 
in direct contact with the tracheides. The parenchymatous 
cells of the phloem generally contain less starch and more 
proteid matter than those of the xylem or those of the above- 
mentioned ‘ xylem-sheath.’ This distinction, however, is very 
variable, and seems to depend upon such factors as the season, 
the condition of growth, &c. In the case of the Ferns, there- 
fore, it seems advisable to give no greater importance to the 
terms phloem-parenchyma, xylem-parenchyma, and xylem- 
sheath than as indicating certain definite topographical regions 
in a common vascular ground-tissue. Cavity-parenchyma is 
very generally present in the petioles of the Cyatheaceae and 
Polypodiaceae, occurring at points just in front of the proto- 
xylem strands (Figs. 24, 25, 2 6). It consists of rather large 
cells, the longitudinal walls of which are transversely plicate 
on the side facing the xylem. These cells have living contents 
and thin cellulose walls, except in Loxsoma Cunning hamii 1 , 
where they become reticulately thickened and lignified. In 
Dicksonia apiifolia, Davallia hirta , and D. N ovae-Z elandiae , 
1 Gwynne- Vaughan, 1 . c., p. 87, Fig. 11. 
