CHAP. III. 
FERNS. 
259 
and in that case are usually unbranched, of the same thick- 
ness at the upper and lower ends, and grow exclusively 
at the apex. The surface of the stem is often hairy or 
shaggy, sometimes spiny, and in all cases is more or less 
copiously furnished with callous points, which render it rough 
like shagreen leather, or covered with roots, sometimes en- 
tangled into a compact layer much thicker than the trunk 
itself, and appearing to be the extension of the callous 
points. 
The anatomy of tree ferns has been skilfully elucidated by 
Mohl, to whose treatise upon the subject [Martins, Plant. 
Crypt. Bras. p. 40.) the reader is referred for the details of 
their curious organisation. I must content myself with a very 
general statement. The trunk is covered with a hard rind, 
occupying the place of bark, two or three lines thick, and 
consisting of hard brown parenchymatous and prosenchy- 
matous tissue, the latter, if present, being on the inside. 
Within the rind is a mass of parenchymatous thinner-sided 
tissue, which is analogous to the horizontal cellular system of 
exogens and endogens. The wood is formed by concave or 
sinuous plates, whose section has a lunate or wavy form, and 
which are closely arranged in a circle next to the rind, en- 
closing a column of parenchyma, just as the wedges of wood 
in exogens enclose a similar column of pith; and in like 
manner there are openings between the plates, through which 
the subcortical and medullary parenchymas communicate. 
Each plate consists externally of several layers of hard brown 
prosenchyma, next within which is a pale stratum of thin- 
sided parenchyma, and in the centre of all is a soft pale mass 
of trachenchyma, consisting of large scalariform and spiral 
vessels (sometimes line in diameter) mixed with soft 
parenchyma. Externally the stem is marked with long, or 
rhomboidal scars, the surface of which is broken into nume- 
rous hard ragged projections which represent the broken 
communication between the trunk and the leaves, by the fall 
of which the scars are produced. Next the apex of a trunk 
the scars are always arranged with great regularity, but 
towards the lower part of the stem they become much longer, 
irregular in form, and are separated by deep furrows ; from 
s 2 
