362 ORGANOGRAPHY. 



cells called sperm-cells, se, in which are developed spiral ciliated 

 filaments, sp, termed antherozoids or spermatozoids. The arche- 

 gonia {fig. 789) are little cellular papillae of a somewhat oval 

 form, with a canal in the centre leading to a cell called the 

 germ-cell, which is contained in a cavity called the embryo-sac. 

 Before impregnation a minute corpuscle, which is termed the 

 embryonal corpuscle, may be also observed in the germ- cell. 

 Impregnation takes place by the contact of the antherozoids with 

 the corpuscle, and this after fertilization forms the primordial 

 cell from the development of which ultimately the plant with 

 fronds bearing capsules is produced. 



The Ferns are thus seen to exhibit in their growth two stages ; 

 in the first of which the spore produces a thalloid expansion 

 resembling the permanent state of the HepaticacccB {figs. 811 and 

 813); and in the second, peculiar bodies are formed upon the 

 surface of the prothallium, by the action of which there is ulti- 

 mately produced a new plant resembling the one from which the 

 spore was originally derived. Thus, Ferns exhibit an instance 

 of what has been called cdternation of generations. 



2. Equisetacbje or Horse-tails. — In these plants the fully 

 developed fructification is borne in cone-like or club-shaped 

 masses at the termination of the stem-like branches {fig. 11). 

 Each mass is composed of a number of peltate stalked scales, on 

 the under surface of which numerous spore-cases, called thecaov 

 capsules, are arranged {fig. 790). These capsules, when ripe, 



Fig. 790. Fig. 791. Fig. 792. 



Fig. 790. Peltate stalked scale of a species of Horse-tail (Equisetitm), bear- 

 ing on its lower surface a number of capsules. Fig. 791. Spore of a 



Horse-tail furnished with two elaters, which are wound round it. The 



ebiters are terminated at each end by a club-shaped expansion. Fig. 



792. The same spore in a dry state, showing the elaters iu au uncoiled 

 condition. 



open by a longitudinal fissure on their inner surface, and thus 

 set free the contained spores. These spores, like those of Ferns, 

 are commonly regarded as gemmcB or rudimentary buds. 



The spores present a very curious structure ; they are little 

 rounded or somewhat oval bodies, and are regarded by Henfrey 

 as only possessing one true coat, in consequence of their outer 

 coat splitting up in a spiral direction so as to form two elastic 

 appendages which are attached by their middle to the spores 

 and terminated at each end by a club-shaped expansion {figs. 



