FILICINEM. 



427 



Hhe fool 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. 



i 



i^lG. 297. — Adiantutn Capillus-Veneris ; vertical longitudinal 

 section through the prothallium // and the young Fern E; 

 h root-hairs, a archegonia of the prothallium, b the first leaf, 

 tv the first root of the young plant (X about 10). 



Fig. 298. — AdiaiitHtn CapiUtis-Venerts ; the pro- 

 thallium // seen from below with the young Fern 

 attached to it ; b its first leaf; -w' 7u" its first and 

 second roots ; h root-hairs of the prothallium (x about 

 30) • 



The mature Fern is, in some Hymenophyllacese, a small delicate plant, not 

 much exceeding in dimensions the larger Muscineae; 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 aqm'lmd), or climbs up rocks and stems; in some it ascends obliquely 

 {e.g. Aspidium Filtx-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 P fen's 

 aquilina). 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 Hymenophyllaceae 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, 



