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 4 friort 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. 111). The structural evidence showing that septation has. taken 
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 (Jsce¢es, p. 318, Lepidostrobus, p. 323) that the nearest approach 
to a septate state is found: in the megasporangium of Jsoefes 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 Jsoetes, is comparable with that of a young synangium of .Agudsetum 
or of Kaxéfussia,’ 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 Zmesipteris (Fig. 2308), 
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 Zmes%pteris 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 proximily may lead to a more or less common 
upgrowth with consequent cohesion at the base, 
1 Studies, iii, pl. viii., and Fig. 37. 
