ORGANS OF REPRODUCTION. 



371 



a cavity called the emhryo-sac. After impregnation has taken 

 place, the embryo-cell develops and forms an embryo, from 

 which ultimately, the plant with fronds bearing sporangia 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 Hepaticacece 

 (Jigs. 808 and 810) ; and in the second, peculiar bodies are formed 

 upon the surface of the pro-thallus, by the action of which there 

 is ultimately produced a new plant resembhng the one from 

 which the spore was originally derived. Thus, Ferns exhibit 

 an instance of what is called alternation of generations. 



2. Equisetace^ 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 (Jig. 157). Each mass 

 is composed of a number of peltate stalked scales, on the under 

 surface of which numerous sporangia are arranged (^gr. 787). 

 These sporangia when ripe, open by a longitudinal fissure on 

 their inner surface. Some botanists consider each of these 

 scales bearing sporangia on its under surface as a single organ; 

 and the sporangium is then described, as a stalked " mushroom- 

 shaped body, possessing a number of little pouch-like cases 

 under the overhanging outer portion, and round the stalk; 

 these pouches bursting by a perpendicular slit inwards so as to 

 dischiirge the spores." 



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 the 

 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 are terminated at each end by a club-shaped ex- 



Fig. 787. 



Fig. 788. 



Fig. 789. 



Fig. 787. Peltate stalked scale of a species of Horse-tail (Equisetum), bearing on 

 its lower surface a number of sporangia Fig. 788. Spore ot a Horse-tail fur- 

 nished with two elaters, which are wound round it. The elaters are terminated at 



each end by a club-shaped expansion Fig. 789. The same spore in a dry state, 



showing the elaters in an uncoiled condition. 



pansion (Jigs. 788 and 789). These spiral elastic filaments, 

 which are called elaters, are at first wound round the spore ( Jig. 

 788), but when dry they ultimately uncoil (Jig. 789), and thus 

 appear to assist in the dehiscence of the sporangium, and 

 in the dispersion of the spore to which they are attached. 



BB 2 



