264 THE SPORE-PRODUCING ORGANS [CH. 



Comparing the numbers in this table we see first how great are the 

 differences in spore-output per sporangium within the FiHcales: secondly, 

 that in many examples where the figures are moderate the actual number 

 of spores is approximately constant for a given species, and often for a genus: 

 but that where deviations occur in the same species, as in Cyathea dealbata, 

 Platyzoma, or Ceratopteris, or within genera, as in Todea, Trichomanes, Pellaea 

 or Clieilanthes, the differences suggest the omission of one of the synchronous 

 divisions prior to the tetrad-division. The larger numbers of spores 

 are associated with those Ferns which on other grounds are held to be 

 relatively primitive. Comparison with early fossils shows that high figures 

 ruled in them also : for instance in Zygopteris there was a spore-output that 

 cannot have been less than 500 to 1000 (Fig. 199), while Stauropteris and 

 P teridotheca also show evidence of large figures. These plants are on general 

 grounds of comparison related to such living Ferns as are placed at the head 

 of the list. It may then be held that in the more primitive Ferns, and 

 especially in those types which are represented in the Primary Rocks, the 

 spore-output was relatively large. 



Caution should, however, be observed in the use of these estimates as 

 a strictly arithmetical guide. It cannot be truly stated that those Ferns 

 which have the largest spore-numbers per sporangium are the most primitive 

 of all. OpJiioglossiun penduluju which stands first on the list with the high 

 estimate of about 15,000 is not on that account alone to be held as the most 

 primitive type named. Reasons will be advanced later for the view that 

 Botrychium is a more primitive type, though its estimated output is only 

 1500 to 2000. But both belong to a very primitive family, as their relatively 

 high spore-output would indicate. It is, in fact, as an indication of general 

 relationship that the numbers can be correctly used, rather than as an exact 

 guide. In this way the fact that " Polypodiaceous " Ferns, w^iich constitute 

 the vast bulk of present-day species, have numbers ranging from 64 down- 

 wards is impressive by its constancy. The only exceptions which have so 

 far come to light are Jamesonia scalajHs, where the number may reach 72, 

 and Cheiropleuria, where the number appears constantly to approximate to 

 128. On the other hand some P'erns show marked instability as regards the 

 lower figures: for instance, Cyathea dealbata, 8 — 16; NotJiolaena sinnata, 

 16 — 32 ; Ceratopteris^ 16 — 32. In all of these the variation seems to turn upon 

 one division more or less in the sporogenous tissue. The most remarkable 

 case of all is Notholaena affinis, which has sporangia with small spores 

 ranging to 64 in number, and others with large spores ranging from 12 to 24. 

 The nearest parallel is found in Platyzovia, which has large sporangia with 

 few large spores ranging up to 16 in number, and smaller sporangia with 

 32 spores of smaller size (Fig. 258). These facts suggest initial heterospory, 

 but they have not yet been tested by cultivation of the prothalli. 



