220 MANUAL OP BOTANY 



(Coleocheete) by a small multicellular body developed from the 

 zygote, the cells of which all give rise to zoospores. 



In the Fungi the gametophyte is alvv'ays the prominent form. 

 It bears both gonidia and gametes, but the latter in some cases 

 are not functional. The parasitic habit of life of miany of 

 these plants is attended by a general degradation of both form 

 and structure, which especially marks the reproductive organs. 

 In some cases the production of sexual cells by the gametophyte 

 has disappeared. Such a gametophyte is called a potential one. 

 Its true nature can only be recognised by a comparison with 

 other forms which bear both kinds of reproductive cells. The 

 sporophyte can be seen in such forms SisMucor {fig. 487), where 

 the zygote on germination produces a small promycelium, and 

 doubtfully in certain Ascomycetes, where it may be represented 

 by the ascocarp. A very rudimentary condition of it is shown 

 by Cystopus (fig. 489), where the zygote produces a number of 

 zoospores after a period of rest. 



In the Mosses and their allies the gametophyte is stUl the 

 prominent form, the sporophyte being represi»ted by the 

 complex sporogonium. Above the group of the ThaUophyta, 

 the nature of any phase of the plant body can be recognised by 

 tracing its origin. The gametophyte is always the product of 

 the germination of the spore, and where it has undergone much 

 reduction, this is the only clue to its identity. In the series of 

 forms above the Ferns we iind cases in which it becomes more 

 and more closely attached to the spore, ultimately being alto- 

 gether enclosed in it. 



Those plants which are heterosporous exhibit two forms of 

 gametophyte, one developed from the microspore, the other from 

 the megaspore : both are inconspicuous. 



In the isosporous or homosporous Ferns and in the Horse- 

 tails the gametophyte is always thaUoid. In the former group 

 each gametophyte usually produces gametes which are female 

 and others which are male, the former developed in organs called 

 a/rchegonia, and the latter in others known as antheridia ; in 

 the Horsetails it generally gives rise to one or the other, but 

 not to both. We have thus in the Horsetails potential though 

 not actual heterospory. In the heterosporous Ferns the relative 

 development of the two forms of gametophyte becomes unequal, 

 and this difference may be seen throughout the higher plants. 

 In Salvinia and in the Phanerogams the male gametophyte is 

 a small tubular structure, produced by the outgrowth of the 

 inner coat of the spore. In Salvinia it forms at its apex an 



