294 THE GAMETOPHYTE, AND SEXUAL ORGANS [CH. 



shown in the table. In all cases it appears that the opercular cells are 

 extruded bodily when the antheridium opens. 



It is in the persistence of these parallels in so many cases, in respect of 

 the various parts of the sporophyte and of the gametophyte, that the cogency 

 of the conclusion lies, that an evolutionary progression has taken place from a 

 more massive to a more delicate structure in the Filicales. But the facts 

 from a single part may be misleading. For instance the regularly segmenting 

 apical cell in stem and root of the Ophioglossaceae accords ill with their 

 typically eusporangiate sporangia, and their massive antheridia. A further 

 warning against pressing these comparisons unduly is found in Equisetum. 

 Von Goebel shows extreme divergence of form and segmentation of its 

 antheridia. When borne on a massive region of the thallus the antheridium 

 is of the type usual for the Eusporangiates (Fig. 282,/). But when borne 

 on the end of a prothallial filament it shows the segmentation typical of 

 Leptosporangiates (Fig. 282, d, e), with which the regular segmentation at 

 the apex of stem and root of Equisetum would agree. But such exceptions 

 do not vitiate the comparisons where, as in the great majority of cases, the 

 correspondence in structure actually exists. 



It remains to estimate the value of the characters of the gametophyte 

 for purposes of phyletic argument. Taking first the vegetative features: 

 they are chiefly negative. The structure is uniformly parenchymatous, with 

 the minimum of differentiation, and very few features are permanent by 

 inheritance. The form is very plastic under varied conditions of lighting 

 and moisture. It is the absence of phyletic inertia which more than anything 

 else detracts from the value of the vegetative characters of the gametophyte 

 for purposes of comparison. Its instability of structure, as well as its relative 

 simplicity, place it far behind the sporophyte as a source of trustworthy 

 material for phyletic argument. 



It is possible to refer most and perhaps all of its types to an origin from 

 the simple or branched filament. This structure is actually seen in the fila- 

 mentous prothallus of Schizaea and Trichomanes, both genera of relatively 

 primitive position. In both of them the gametangia may take the place of 

 the end of a filament, a point which is suggestive of comparison with 

 certain Algae. TricJioinanes and HyinenopJiyllmn both suggest progressive 

 steps to a flattened vegetative expanse a single layer of cells in thickness. 

 Localisation of segmentation gives rise to definite apical activity, and the 

 establishment of this may be seen in the ontogeny of many prothalli. In 

 the great majority both of Eusporangiate and Leptosporangiate Ferns that 

 localisation is distal, and the result is the symmetrical prothallus of the 

 textbooks. But such cases as Anemia, Vittaria, Pteris longifolia (Fig. 267), 

 and many others, show that the localisation may be at some indefinite point 



