PTEEIDOPHYTA. 209 



portions of a leaf are sometimes rolled up together, the tissues of 

 the leaf being already fully developed and only waiting to expand. 

 The leaves are often excessively divided and compound,- with 

 pinnate branches, and have an epidermis with stomata and a well- 

 developed system of venation. Stipules are only found in Marat- 

 tiacece and Opliioglossacece. 



Very often peculiar hairs or scales (palece, ramenta), dry, brown, 

 flat and broad, are found on stem and leaf. 



The SPORANGIA are small, round capsules, which, in a very large 

 number of Ferns, are formed on the back, but more rarely on the 

 edge of the ordinary foliage-leaves. It is very seldom that there 

 is any difference in form between the barren foliage-leaves and the 

 fertile leaves, as is found for example in Blechnum spicant or 

 Struthiopteris ; or that the fertile part of the leaf is differently 

 constructed from the barren portion of the same leaf, as in the 

 Royal-Fern (Osmunda). In such instances the mesophyll of the 

 fertile parts is poorly developed. 



The sporangia in the Polypodiacece are lens-shaped, with long 

 stalk (Fig. 211 D) : their wall consists of one cell-layer on which 

 a single row of cells, passing vertically over the top (that is along 

 the edge of the sporangium), is developed into the "ring" (annu- 

 lus). The cells of the annulus are very much thickened on the 

 inner and side walls, and are yellowish-brown. The thickened 

 cells, however, do not entirely encircle the sporangium, and on 

 one side, near the stalk, they pass over into large, flat, thin-walled 

 cells. These form a weak point in the wall, and it is here that 

 the sporangium is opened diagonally by the elongation of the 

 annulus. The sporangium of the Polypodiacese opens as it dries. 

 The cells of the annulus are very hygroscopic, and in straighten- 

 ing, the annulus bends back with a jerk, thus ejecting the spores 

 to considerable distances. The cells of the annulus absorb water 

 with great readiness. [The sporangium arises as a single epi- 

 dermal cell, from which a basal stalk-cell is cut off. Three oblique 

 cell-walls, intersecting near the base, are next formed in the upper 

 cell, and a fourth between these and parallel to the free surface ; 

 an inner tetrahedral cell enclosed by four others is thus formed, 

 the outer cells become the wall of the sporangium, while the inner 

 cell, by a series of walls, parallel to its sides, cuts off a layer of 

 cells which eventually form the tapetum, the remaining central 

 cell constituting the archesporium.] 



The SPORES are either oblong and bilateral, or they are tetra- 



