CHAPTER XIV 



THE GAMETOPHYTE, AND SEXUAL ORGANS 



The germination of the spore produces in every normal cycle of Ferns that 

 alternate somatic phase which is called the Gametophyte. It might be anti- 

 cipated that, as in the sporophyte, it also would provide features of value for 

 comparison, in respect of external form and of internal structure. It has 

 been seen how the sporophyte has definite form, with axis and appendicular 

 organs produced in regular sequence. Also that it shows a high degree of 

 differentiation of tissues. It has been remarked that the comparative value 

 of the vascular system of the sporophyte depends upon its "phyletic inertia": 

 that is, upon the fact that it shows so high a degree of conservatism in its 

 construction, that comparisons can be effectively drawn not only between 

 plants closely related, but also between those of wider affinity. But the 

 constituent tissues of the gametophyte in the Filicales show a much lower 

 scale of differentiation, while its external form, which is without any regular 

 sequence or relation of parts, is frequently very simple, and is liable to be 

 directly impressed in high degree by the external circumstances under which 

 it develops. These facts detract heavily from the value of the gametophyte 

 for comparative purposes, so far as its vegetative characters are concerned. 

 Comparison is therefore thrown back upon the sexual organs, ox gametangia, 

 as its most reliable features. Nevertheless certain somatic characters of the 

 gametophyte .may be made use of, and these will be considered first. It 

 should, however, be borne in mind that knowledge of the prothalli of Ferns 

 is far less complete than that of the sporophyte-plant, and present views 

 may be subject to drastic revision as the facts are more fully disclosed by 

 further investigation. 



The Cordate Type 



The commonest type of prothallus for the Leptosporangiate Ferns is 

 that seen in Dryopteris (Fig. 265). It is characterised by the presence of 

 a single growing point, which is deeply set between two more strongly 

 growing lateral lobes. Each of these consists of a single layer of cells, 

 while the central region is more massive, and it is attached to the soil by 

 numerous rhizoids. In adult prothalli an acropetal succession of arche- 

 gonia borne on the lower face of this cushion leads to the apex itself: while 

 the antheridia, which are commonly borne on the same prothallus, are 

 scattered irregularly over the basal region of the cushion, and of the lateral 



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