102 STRUCTURE OF FERN STEMS. 
vascular bundles proceeding to the leaves, by the fall of which 
organs the scars have been produced. 
Internal Structure of Fern Stems.— Upon making a transverse 
section of a Tree-fern it presents, as we have already briefly 
noticed (see page 75), the following parts :—Thus in the centre, 
when young, a parenchyma (jig. 178, m), the cells of which have 
thin walls ; but in old stems this central parenchyma is destroyed, 
so that the stem becomes hollow. Towards the outside of this 
parenchyma, and just within the rind, we find the so-called 
wood (fibro-vascular bundles), arranged in the form of irregular, 
sinuous, or wavy plates, v, v, v. These masses of wood have 
generally openings between them, by means of which the 
parenchyma beneath the rind and that of the centre of the stem 
communicate ; but in other cases these woody masses or plates 
touch each other at their margins, and thus form a continuous 
circle within the rind. These masses, as already noticed, consist 
Fie. 203. Fie. 204. 
Fig. 203. Rhizome of Male Fern (Aspidium Filixr-mas), marked externally by 
rhomboidal scars, which present dark-coloured projections, ¢.——Fig. 204. 
Vertical section of the dichotomous or forked stem of a Tree-fern. 
of closed fibro-vascular bundles, the vessels of which are chiefly 
scalariform in their character ; these are situated in the centre 
of the bundles, where they may be readily distinguished by 
their pale colour (fig. 178, v, v, v). External to them are 
usually a few layers of parenchymatous cells, which contain 
starch in the winter, and amongst which are situated some wide 
sieve or lattice-cells. The whole is surrounded by a single 
layer of cells, the walls of which are usually more or less lignified 
and dark-coloured, thus constituting the tissue termed scleren- 
chyma, and forming what has been called the bwndle-sheath. 
The tissues external to the fibro-vascular bundles constitute 
collectively what has been termed the rind (fig. 178, e). 
Growth by Terminal Buds.—We have already stated that 
Tree-ferns have no branches (fig. 15). This absence of branches 
arises from their having, like Palms, no provision for lateral 
buds: hence the cylindrical form of stem which is common to 
them as with the stems generally of Monocotyledons. For the 
