ORGANS OP BEPRODUCTIOX. 



S69 



covered by a thin membranous layer continuous with the epi- 

 dermis, which is called the indusium or involucre, as in Nephro- 

 dium or Lastrcea Filix-mas {fig. 781). Sometimes the sporangia 

 are so densely compacted that, no intervening parenchyma can 

 be distinguished — the latter being destroyed by the excessive 

 development of the former ; in such cases, the sporangia instead 

 of being collected in sori on the back of the fronds, appear as 

 little bodies arranged in a spiked manner on a simple or branched 

 rachis, as in Osmunda {fig. 782). 



Fig. 780. 



Fig 781. 



Fig. 782. 



Tig. 780. A portion of the frond of the common Polypody iPolypodium vidgare) 

 showing two sori springing from its veins. The sori are naked, and consist of a 



number of sporangia, sp, in which the sporps are contained Fig. 781. Portion 



of the frond of the male-fern {Xephrodium Filix-mas), with two sori, s, *, covered 



by an indusium or involucre Fig. 782. Portion of the frond of the Royal or 



Flowering-fern {Osmunda regalis), with its sporangia arranged in a spiked man- 

 ner on a branched rachis. 



The sporangium is a little cellular bag or case {fig. 783, s), usu- 

 ally stalked,/), and more or less completely surrounded by a ring 

 or annulus ; this ring is frequently elastic, and thus causes the 

 bursting of the sporangium when ripe, and the escape of its 

 spores. In some Ferns the ring is imperfect, and in others 

 it is altogether wanting ; hence Ferns provided with a ring are 

 called annulate, whilst those in which it is absent are exannulate. 



The spores have two coats like pollen-grains ; and like 

 them also, the outer coat is either smooth, or furnished with 

 little points, streaks, or reticulations, &c. In germination {fig. 

 784, s) the outer coat bursts, and the inner is protruded in the 

 form of an elongated tube, which ultimately by cell-division, 

 forms a thin, flat, green parenchvmatous expansion, called a pro- 

 thallus or pro-embryo, p, from which one or more radical fibres, r, 

 are commonly produced in its earliest stage. On the under 

 surface of this body, there are soon produced two diflferent 



