364 Boodle. — Anatomy of the Schizaeaceae . 
p. 97) ; and Prantl (’ 81 , p. 27) evidently referred to this stage 
when he described the protoxylem as ‘ regellos iiber den 
Strangquerschnitt zerstreut.’ 
Many of the conjunctive-parenchyma cells in the xylem 
and also those bounding its periphery contain irregular re- 
fractive bodies, which prove to be silica-nodules. Several 
are shown in Fig. 2 ; the one marked si. is adjacent to the 
nucleus of the cell. They show varied shape and texture, 
but a frequent type has a rod-like general outline, either 
blunt or pointed at both ends, the texture being sometimes 
loose and granular, but more often dense and solid-looking, 
with a botryoidal or mammillated surface. They occur singly 
in a large proportion of the cells mentioned above. The silica- 
nodule, when large, lies with its length along the length of 
the cell which contains it, and appears to be free in the cell 
cavity, but is possibly imbedded in a mucilaginous substance. 
In a piece of old rhizome the silica-nodules were specially 
well-developed, and many attained a length of y 1 ^ mm. They 
can be isolated by boiling a piece of the stele in Schulze’s 
macerating fluid till the tissues are completely dissolved, 
when the sediment consists entirely of silica-nodules. Fig. 5 
is a drawing of a nodule isolated in this way. They are very 
distinct when mounted in methylated spirit or water, but 
almost unnoticeable in strong glycerine or sulphuric acid. 
This of course depends on the refractive index of the liquid. 
Decisive chemical tests were not employed, but it is easily 
shown, by rubbing them between a watch-glass and a glass- 
slide, that they are hard enough to scratch glass, hence it 
may be taken for granted that they consist of silica. The 
siliceous bodies were found to be about equally numerous 
and well developed in the rhizome of a plant grown at Kew 
and of one from Ceylon. Poirault (’ 93 , p. 241) has described 
siliceous bodies of similar appearance in certain epidermal 
cells of the petiole in some species of Marattiaceae. 
The structure of the node will be described later. The 
rhizome of Lygodium has a uniform type of structure in the 
species examined, and as it has been rather fully described in 
