FILIOES. 



373 



the macrosporangia usually containing only one macrospore. 

 Sporangia in clusters, enclosed in modified leaves or 

 "fruits." 



Order Filices, the true Ferns. Tlie protliallia of the Ferns are 

 green thallus-like structures, growing upon the surface of the ground. 



Pig. 257<z.— Longitudinal section of the apex of the root of PteHs hastata. », 

 apical cell ; o, o, epidermis ; e, cortical tissue ; c-c, c-c, the primary fihro-vascular 

 bundles ; n, m, I, A, the root-cap ; k, A, daughter-cells recently cut off from the apical 

 cell.— After Nageli and Leitgeb. 



' and composed at first of but a single row of cells, but later of extended 

 layers of cells. Thuy are monoecious, and bear tlieir antheridia on the 

 basal portion of thf- ander surface, wliile the archegonia are found near 

 the apical margin of the same surface. After fer- 

 tilization the germ-cell divides into four parts, the 

 uppermost one (or two) of which becomes the foot, 

 or organ which remains in contact with the prothal- 

 lium ; one of the other parts develops into the first 

 root, and the other into the first leaf. The young 

 plant is thus formed on the under side of the pro- 

 thallium, from which it grows up as shown in Figs. 

 256 and 355. 



The stems of Ferns are mostly short, or slender 

 and creeping in our species, but in the tropics they 

 are often of considerable height and thickness, 

 some tree-ferns attaining the height of 34 metres 

 or more (80 feet or more). They increase in length 

 only, and this takes place by the continued division of an apical cell) 

 They contain flat fibro-vascular bundles (Fig. 357, A and B), wliich are' 

 usually disposed in a single circle, as seen in a cross-section, but iu 

 some cases there are bundles in the medullary portion also. On ac- 

 count of the presence of thick masses of thick-walled cells, (scleren- 



Fig. 257*.— Portion 

 of under surface of a 

 leaf of Polypodium. 

 showing sori. — From 

 Le Maout and I)e- 

 caisne. 



