PTEBIDOPHYTA— FILIOINiE 



133 



The sporangia occur usually in groups on the lower or dorsal 

 surface of the sporophylls (fig. 890), or in some cases in a band 

 along their margins, being situated on a kind of placental 

 outgrowth. They are sometimes quite exposed to the air, 

 sometimes partially protected by hairs or paraphyses which 

 arise among them from the placenta. Generally, however, the 

 sorus is more or less covered by a definite membrane called the 

 vndiisium, which springs from the epidermis of the leaf. Where 

 the sorus is marginal, as in Pteris, the edge of the leaf curves 

 over it, forming a false indusium, which sometimes is supple- 

 mented by a kind of membrane springing from the leaf along 

 the inner side of the sorus. 



Fig. 890. 



Fig. 891. 



Fuf. 889. A portion of a frond of the common Polypody (Polypodium vuU 

 gave), showing two sori springing from its veins. The sorl are nalced, 

 and consist of a number of sporangia or capsnles, sp, in which the spores 



are contained. Fig. 890. Portion of a frond of the Male-fern {Aspidium 



FlUx-mas), with two sori, s, s, covered by an indusium. Fig. 891. 



Portion of a frond of the Royal or Flowering-fern (Osmunda regnlis), 

 with its sporangia or capsules arranged in a spiked manner on a branched 

 rachis. 



The sorus (fig. 892) consists of a number of sporangia, often 

 mixed with a number of hairs or paraphyses, which are some- 

 times glandular. Each sporangium is an ovoid or globular 

 body placed upon a long stalk. It originates from a single 

 superficial cell of the placenta, which grows outwards and 

 becomes divided into two. The upper one gives rise to the 

 body of the sporangium, the lower one to the stalk. As the 

 upper one grows, there are formed in it three oblique walls, which 

 cut out of it a somewhat tetrahedral cell with its base uppermost. 

 A further wall is formed across its base, so that the structure 

 consists of a wall and a central cell. Divisions parallel to the 



