PTERIDOPHYTA 



125 



and fleshy {fig. 917, _p), when it is developed underground and 

 contains no chlorophyll. It is always known as the xn-o- 

 thalUum. 



In the heteroporous forms there are two kinds of prothallia 

 formed, one from the microspore, and the other from the 

 macrospore. In neither case do we get the prothallium entirely 

 set free from the spore. They are not very strongly developed, 

 only sufficiently so indeed to 



permit of the production of Fig. 883. 



the sexual organs. The pro- 

 thallium from the microspore 

 gives rise to antheridia ; that 

 from the macrospore to arche- 

 gonia. In some of the iso- 

 sporous forms the prothallia, 

 which are all alike as to their 

 vegetative features, are mo- 

 ncecious, producing both an- 

 theridia and archegonia ; in 

 others, as in the equisetums, 

 they are dioecious, only bear- 

 ing one or other of them. 



The antheridia are either 

 immersed in the substance of 

 the prothallium, as in tiie 

 tuberous forms, or project 

 from the surface as in the 

 flattened ones. They are 

 superficial always in their 

 origin, and consist of a wall 



of a single layer of cells, which /^^V;, 883. Gametophyte produced by the 



encloses a number of spirally macrospore of Salvinin. pro. Protliallium 



•IT -T J. ^ ^1 -1 bearing a, young sporopliyte. After 



coiled ciliated antherozoids pringsheim. 

 {fig. 860, b). The archegonia 



are also superficial, having a venter embedded in the tissue of 

 the prothallium, and a slightly projecting neck consisting of only 

 a small number of cells {fig. 897). The structure is otherwise 

 like that of the Bryophj'tes. Fertilisation is effected in the 

 same way as in the latter group. The number of archegonia 

 produced often depends upon whether or no the oospheres of 

 the first- formed ones become fertilised. If not, others are 

 developed. 



Vegetative reproduct ion of the gametophyte is not uncommon 



