FILICES. 



373 



the macrosporangia usually containing only one macrospore. 

 Sporangia in clusters, enclosed in modified leaves or 

 "fruits." 



Order Filices, the true Ferns. The prothallia of the Ferns are 

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



Fig. 257a. Longitudinal section of the apex of the root of Pteris hastata. v, 

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

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

 cell.-After Nageli and Leitgeb. 



and composed at first of but a single row of cell?, but later of extended 

 layers of cells. Th- are monoecious, and bear their antheridia on the 

 basal portion of t 1 ander surface, while 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- 

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

 250 and 255. 



The steins of Ferns are mostly short, or slender 

 and creeping in our species, but in the tropics they 

 are often of considerable height and thickness, Fe'af "of" Poiypodium, 

 some tree-ferns attaining the height of 24 metres Le^Maout^aiid^De- 

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

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

 They contain flat fibro-vascular bundles (Fig. 257, A and B), which are 

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

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

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



