342 VASCULAR CRYPTOGAMS. 



Like many thalloid Hepaticae, the prothailia of Ferns also produce adventitious 

 shoots from single marginal cells ; this happens with especial profusion in Osmunda, 

 where the adventitious shoots become detached, and play the part of vegetative 

 organs of reproduction. 



The prothailia show a tendency to be dioecious, which is manifested in the 

 fact that all the spores from a sporangium sometimes produce prothailia bearing 

 antheridia only (as in Osjiiunda regalis) ; while in other cases the archegonia 

 appear later and in smaller numbers, and are fertilised by the antheridia of younger 

 prothailia. 



The Antheridia are, speaking morphologically, trichomes; they are produced in 

 the same manner as the root-hairs, as outgrowths of the marginal or superficial cells 

 of the prothailia ; in the Hymenophyllaceae they are also produced on the protonemal 

 filaments. The projection is usually separated from the mother-cell by a septum, 

 and swells up spherically at once or after the formation of a pedicel. In some 

 cases the mother-cells of the antherozoids are formed at once in this globular cell ; 

 but it usually undergoes still further divisions ^ in consequence of which the wall of 

 the antheridium consists of a single layer of cells surrounding the central cell. The 

 cells of this wall form grains of chlorophyll on their inner face, while the central cell 

 of the antheridium divides further into the mother-cells of the antherozoids, which, 

 however, are not numerous. The dehiscence of the ripe antheridium is the con- 

 sequence of a rapid absorption of water in the parietal cells, which swell up violently 

 and compress the contents of the central cell till the antheridium is ruptured at the 

 apex. The antherozoid- cells thus escape, and out of each of them is set free an 

 antherozoid coiled spirally three or four times. The finer anterior end of each 

 antherozoid is provided with a number of cilia ; the thicker posterior end often 

 drags with it a vesicle furnished with colourless granules, which subsequently falls off 

 and remains at rest, while the filament alone continues in motion. Strasburger states 

 that this vesicle is formed from a central part of the contents of the mother-cell, 

 the parietal protoplasm of which forms the filament and its cilia. The vesicle is 

 hence properly not a part of the antherozoid ; it is only attached to it, and swells 

 up strongly in water by endosmose, as is shown in Fig. 253. 



The Archegonium arises from a single superficial cell of the pro thallium, which 

 is at first only slightly arched and is divided by a wall parallel to the upper surface. 

 The lower of the cells thus formed is the central cell of the archegonium : the upper 



^ These divisions take place in a very remarkable manner. In the hemispherical mother-cell of 

 the antheridium oi Aneimia hirta, an arched wall arises, by which it is divided into an inner hemi- 

 spherical cell, and an outer one which covers the former like a bell ; the latter is then split up by a 

 transverse annular wall into an upper lid-like and a lower hollow cylindrical cell. The same thing 

 occurs in Ceratopteris ; in other cases, as in Asplenium alatum, a funnel-shaped wall is formed in 

 the hemispherical mother-cell of the antheridium, and with the end of the funnel above the wall 

 of the mother-cell ; the upper part of this is cut off by a level septum as a covering cell ; two, 

 or even three, funnel-shaped walls may be formed in succession, so that the parietal layer of the 

 antheridium consists of two or three superposed funnel-shaped cells and a covering cell (as in 

 Fig- 253). The mode of formation of the antheridium-wall is quite different in Osmunda, where it 

 consists below of two or three cells, upon which rest several of the upper cells which result from 

 the division of the stigmatic cell (Kny, /. c). 



