PILI CI NE^. 
427 
The fooi is an organ by which the embryo attaches itself to the tissue of the 
prothallium, in order to draw nourishment from it while the first roots and leaves are 
being formed. The first parts of the stem and the roots and leaves, which are now 
developed in succession from the embryo, are very small, and remain so ; those 
which are formed later are gradually larger. The leaves become constantly more 
complex in form, and the structure of the stem more intricate as the new portions 
formed by its growth in length increase in diameter. The first parts of the stem, 
like the first leaf-stalks, contain each only one axial fibro-vascular bundle ; the later 
ones a larger number when both stem and leaf-stalk have attained a considerable 
thickness. In this manner the Fern continues to gain strength, not by subsequent 
increase of size of the embryonic structures, but by each successive part attaining 
a more considerable size and development than the preceding ones ; until at length 
a kind of stationary condition is arrived at in which the newly-formed organs are 
nearly similar to the preceding ones. The following observations refer especially to 
this mature condition of Ferns. 
Fig. 2gj.—Adianti0n Capilltis-Veneris ; vertical longitudinal 
section through tlie prothallium pp and the young Fern E ; 
h root-hairs, a archegonia of the prothaüiuni, b the first leaf, 
w the first root of the young plant {X about 10). 
Fig. 298. — Adianiu7n CapiUiis-Veneris ; the pro- 
thallium pp seen from below with the young Fern 
attached to it ; b its first leaf ; -w' -w" its first and 
second roots ; h root-hairs of the prothallium (X about 
30). 
The mature Fern is, in some Hymenophyllaceae, a small delicate plant, not 
much exceeding in dimensions the larger Muscinese ; in other sections the fully 
grown plants attain the size of considerable shrubs; some species, natives of the 
Tropics and of the Southern Hemisphere, assume even a palm-like habit, and are 
called Tree-ferns. The stem creeps on or beneath the ground (as in Polypodium 
and Pieris aqmltnd), or climbs up rocks and stems; in some it ascends obliquely 
{e.g. Aspidium Filix-mas)', in Tree-ferns it rises up vertically in the form of a 
column. The roots are usually very numerous ; in Tree-ferns the stem is often 
entirely covered by a dense mantle of them. They arise on the stem in acro- 
petal succession ; sometimes close to the growing apex of the stem (as in Pieris 
aquilmd). When the internodes remain very short, and the stem is entirely covered 
with the bases of the leaves, the roots arise, as in Aspidium Filix-mas, from the leaf- 
stalks. In many Hymenophyllacese which have no true roots, branches of the stem 
assume a root-like structure. In creeping and climbing species the leaves are sepa- 
rated by distinct internodes which are sometimes very long; in thick, ascending, 
