FILICINEAE. HOMOSPOROUS FILICINEAE. 219 



invested with the permanent wall of the spore, and the wall of the mother-cell 

 and of the compartments is then absorbed. Spores of the form given in Fig. 166 are 

 said to be bilateral, in contradistinction to those which are formed after the four 

 nuclei were arranged tetrahedrally in the mother-cell, and have therefore a rounded 

 tetrahedral form ; the latter only are said to occur in the Hymenophyllaceae, Osmun- 

 daceae and Cyatheaceae, in the other families sometimes the one kind, sometimes the 

 other is found. The mode of formation of the walls still requires explanation in 

 some points. 



The development of the sporangia in the other Filicineae agrees with the above 

 description in the main points, that the sporangium ; roceeds from a single epidermal 

 cell, and that it forms a tetrahedral 1 archesporium, which first gives off the tapetal 

 cells and then divides to form the spore-mother-cell. But there are several variations 

 in points connected with the position of the sporangia. For instance, in Ceratopleris 

 they are not collected into sori and are not placed on a placenta, but are formed 

 one by one, while the sporophyll is still rolled up, from the superficial cells of the 

 under side of the leaf in acropetal succession, which is however interrupted by 

 intercalations. In the Hymenophyllaceae the placenta is formed by the extremity 

 of a vein which is prolonged beyond the surface of the leaf, and on which the 

 sporangia arise in basipetal succession, as in the Polypodiaceae. The sorus is 

 surrounded by a cup-shaped indusium. In Aneimia and Lygodium the sporangia 

 are formed singly from cells of the margin of the leaf, and the arrangement 

 of the cells also is somewhat different. The first step in the formation of the 

 sporangium is the arching outwards of a cell of the leaf-margin. An anticlinal 

 wall appears after the formation of the basal (periclinal) wall of this cell, then 

 a wall inclined in the opposite direction to the anticlinal wall, then a wall parallel 

 to the first wall, then a periclinal wall (parallel to the margin of the leaf) which 

 separates off an outer parietal cell and the archesporium ; this has not the form 

 of a tetrahedron, but more nearly that of a quadrant of a cylinder, the difference 

 from the ordinary condition being due to the origin of the sporangium on the 

 margin of the leaf. The annulus in the Schizaeaceae is formed from -a group of 

 cells with thickened walls which lies on the apex of the sporangium. The sporangia 

 open by a longitudinal fissure, at right angles therefore to the annulus (in the Poly- 

 podiaceae by a transverse fissure). The peculiar formation of the indusium of 

 Lygodium has been already mentioned. The sporangia of the Osmundaceae ac- 

 cording to Prantl follow the customary course of development, and are collected 

 into sori without indusia. 



a. Histology 2 . The epidermis of the leaves of Ferns is distinguished by the 

 presence of chlorophyll in its cells, while that substance is usually absent from the 

 epidermal cells of phanerogamous land-plants, and by the peculiar construction of its 

 stomata. Fig. 167 shows that in Pterisflabellata a small cell is taken out of an epidermal 

 cell by a wall which is curved in the shape of a watch -glass ; this cell is either itself the 

 mother-cell of the stoma, or the mother cell is cut off from it by a second anticlinal 



1 With regard to the Schizaeaceae see below. 



2 De Bary, Vergleich. Anatomic, pp. 181, 289, 355, etc. [Polonie, Ueber d. Zusammensetz. 

 d. Leitbiindel d. Gefasskrypt. (Jahrb. Kon. Gart. Berlin, II, 1883). Terletzki, Anat. d. Vegeta- 

 tionsorgane von Struthiopteris u. Pteris (Pringsheim's Jahrb. XV, 1884).] 



