THE SPORE-PRODUCING MEMBERS 697 



The recognition of the spore-producing members as a category of 

 parts, probably distinct in origin from the bracts, though often supported 

 on them, having a uniform function, and showing, whether as simple 

 sporangia or as sporangiophores, similarities of position, raises the question 

 whether the two types of spore-producing members are genetically connected ; 

 it is necessary to enquire whether there is any structural indication of an 

 evolutionary progression having taken place from the simple sporangial 

 sac to a septate state, and thus of the origination of the stalked sporangio- 

 phore with vascular supply from the single sporangium. In the first 

 instance it is to be recognised that such a progression cannot rightly 

 be negatived on d priori grounds ; for - it has been shown that septation 

 of sporangia has occurred in well-authenticated cases (p. 120), while 

 biological probability would favour such amplification in homosporous 

 forms (p. in). The structural evidence showing that septation has Igken 

 place may be derived both from the septate and from the non-septate forms, 

 but no consecutive demonstration is to be obtained from comparison of the 

 representatives of any one phylum. On the one hand the occurrence of 

 sterile cells and tissue-tracts has been described at length in simple 

 sporangia, and it is specially worthy of note that it is in the largest 

 of them (Isoetes, p. 318, Lepidostrobus, p. 323) that the nearest approach 

 to a septate state is found : in the megasporangium of Isoetes the 

 sporangium is technically septate, for each spore-mother-cell may be 

 completely partitioned off by tracts of sterile tissue (Fig. 320). Such a 

 condition, which only appears relatively late in the individual development 

 of Isoetes, is comparable with that of a young synangium of Equisetum 

 or of Kaulfussia^ inasmuch as in these also the archesporial cells are 

 found isolated in sterile tissue (Fig. 206 A) : the fact that the condi- 

 tion of isolation is seen earlier in the individual development of these 

 sporangiophores is in complete accord with their greater morphological 

 advance : a less advanced state is, however, seen in Tmesipteris (Fig. 230 B), 

 in which the septum and sporogenous groups are at first indistinguishable 

 from one another, but differentiate after the tissue has attained a consider- 

 able bulk. If the individual development be rightly held as an indication 

 of the evolutionary progression in the race, then the sporangiophore 

 in the cases quoted would find its evolutionary prototype in larger 

 non-septate sporangia, such as those seen in the Lycopods, from which 

 the condition in Tmesipteris would be less far advanced than that of 

 Equisetum or of Kaulfussia. Such a comparison comes with special 

 force in those cases where, as in the Psilotaceae and Sphenophylls, the 

 position of the sporangiophore is identical with that of the Lycopod 

 sporangium. 



already existent in the individual, but the substitution of two related centres of initiation 

 in place of one, while their near proximity may lead to a more or less common 

 upgrowth with consequent cohesion at the base. 

 1 Studies, iii. pi. viii., and Fig. 37. 



