214 



THIRD GROUP.— VASCULAR CRYPTOGAMS. 



once or repeatedly pinnate. The rudiment of the leaf is formed by the archinp; 



outwards of a single cell at the growing point of the stem. The growing point of the 



young leaf is either occupied from 

 the first by a group of marginal cells, 

 as in the cotyledon, or at first by a 

 two-sided wedge-shaped apical cell, 

 which forms segments for a time in 

 the plane of the leaf (Fig. i6i), and 

 is then replaced by a group of cells, 

 in the same manner as in the pro- 

 thallia, by the formation of a peri- 

 clinal and other walls. The branches 

 of the leaf, the pinnae, &c. are next 

 formed ; a group of cells below the 

 growing point of the leaf grows 

 more vigorously, and so commences 

 the formation of a pinnate leaf (Fig. 

 i6i at Z)'. 



The root. The stem, as it grows, 

 is as a rule constantly putting forth 

 new roots in acropetal succession, and 

 in the creeping species these roots 



secure the plant to the surface on which it grows. In Pteris aquilina the new roots 



Fig. i6i. Tip of a leaf of Co-atopteris Thalictroides. S the apical 

 cell of the leaf, L rudiment of a lateral lobe (lacinia) of the leaf which has 

 no apical cell. After Kny. 



. 162. Apical region of fern-roots. A longitudinal section through the extremity of a root of Pteris hastata ; 

 , cell, k segment of the same reaching to the root-cap, k, /, m, n layers of the root-cap, c segments of the root, 

 /erse section through the apical cell and the adjoining segments of the root oi Aspidiiim Filix-fetiiitia. After 

 Nageli and Leitgeb. 



appear from immediately below the apex, and in this species and in Aspidium Filix-vias 

 they also grow at a very early period from the adventitious buds of the leaf-stalk. It 



1 [Bower, Comp. morph. of the leaf in the Vase. Crypt, and Gymnosp. (Phil. Trans. 1884), 

 regards ihe whole leaf in Vascular Cryptogams from apex to base as an axis, Xh& fhyllopodium, which 

 may be branched or unbranched. In most homosporons Ferns the phyllopodium shows a two-sided 

 apical cell and monopodial branching with not infrequent dichotomy in the higher series of the 

 branching; but in the Hymenophylleae the branching is chiefly, if not exclusively, dichotomous. In 

 Osmundaceae the phyllopodium has a three-sided conical apical cell and the branching is monopodia).] 



