FERNS. 



357 



like a bract ^ Sori are not usually formed upon all the leaves of the mature plant ; 

 sometimes groups of fertile and sterile leaves alternate in regular succession, as in 

 Striithiopteris germanica. In some cases the sori are uniformly distributed over the 

 whole of the lamina, in others they are connected with definite portions of it. The 

 fertile leaves may be in other respects like the sterile ones, or 

 they may be strikingly different from them ; and this difference 

 is not unfrequently occasioned by the partial or entire failure of 

 development of the mesophyll between and near the fertile 

 veins ; the fertile leaf, or the fertile part of the leaf, then ap- 

 pears like a spike or panicle furnished with sporangia {e. g. 

 Osmunda, Aneimia). The sporangia generally arise from the 

 epidermis of the veins of the leaf, and especially on the under 

 side of the lamina ; but in the Acrostichacece they spring both 

 from the veins and from the mesophyll ; in Olfersia they cover 

 both surfaces of the leaf at the sides of the mid-rib, or in 

 Acrostichum only the under side. When, as is usually the 

 case, the veins are the only parts that bear the sporangia, the 

 fertile veins may be like the sterile ones, or may undergo a 

 variety of changes at the spots where they bear the sori; 

 they may be swollen into a cushion (forming a receptacle), or they may project 

 beyond the margin of the leaf, as in the Hymenophyllacese. The sorus may be 



FIG. 265.— Under side 

 of a lacinia of a leaf of 

 Aspidiwn Filix-nias, 

 with eight indusia i 



(X2). 



Fig. ■ifA.—Aspidium. Filix-ntas. A transverse section of a leaf with a sorus consisting of the sporangia s and the 

 indusium ii; right and left in the mesophyll of the leaf are two small fibro-vascular bundles, the sheath of which shows 

 the dark brown thickenings on the walls that face inwards. B a young sporangium, its annulus standing vertically to 

 the plane of the paper, r its apical cell ; in the interior four cells are seen resulting from the division of the central cell ; 

 C lateral view of a nearly ripe sporangium, rr its annulus, d the stalked gland peculiar to this species ; within the sporan- 

 gium are seen the young spores already formed. 



seated on the end of a vein, which then frequently puts out two branches in the angle 

 of which is placed the sorus, or it may be formed on the back and below the ends 



^ Athough these points of structure are employed in systematic botany as characters of families, 

 their morphology is at present but little known. A history of the development of the sori of Ma- 

 rattia, Kaulfussia, and Dan^a, consisting of so-called united sporangia, is an especial desideratum. 



