22 



Morphology of Spore-producing Members. [Feb. 16, 



transverse septum, and the apical growth of the sporophyll arrested 

 and taken up by two lateral lobes, the result would be such as is seen 

 in Tmesipteris. This is not a mere imaginative suggestion : it pro- 

 ceeds from the observed fact that the septum in Tmesipteris is undis- 

 tingaishable at first from the sporogenous masses : here, as in other 

 cases, it seems to me probable that a partial sterilisation of a 

 potential archesporium has resulted in a partitioned sporangium. 

 Psilotum itself illustrates, in its abnormal forms, the possible pro- 

 gression from two to four or five locali. It may further be noted, in 

 connexion with the above comparison between Lepidodendron and 

 Tmesipteris, that the vascular tissues of some of the former appear to 

 correspond more closely to those of Tmesipteris than to any other 

 living plant. 



Looking at the whole plants of the Psilotaceae from the point of 

 view above indicated, they are to be regarded as lax strobili, bearing 

 sporangiophores (sporophylls) of rather complex structure. Branch- 

 ing, which is rare in Tmesipteris, is common in Psilotum, and is to be 

 compared with the branching of the strobilus in many species of 

 Lycopodium. In both there are irregularly alternating sterile and 

 fertile zones, not unlike those of some species of Lycopodium (loc. cit., 

 p. 270) ; at the limits of these, arrested sporangia are frequently 

 found. It is not difficult to imagine how such plants as the Psilotaceae 

 may have originated from some strobiloid type, not unlike that of the 

 genus Lycopodium. 



Those who accept the above suggestion will be prepared further to 

 admit the comparison of the synangium of the Psilotaceae with the 

 " fertile frond " of the Ophioglossaceae, which has been made by 

 various ot^er writers — Mettenius, Prantl, Strasburger, Celakovsky. 

 This I believe to be a true homology ; I should, however, add to this 

 the hypothesis that, in either case, we see the result of elaboration 

 in size and form, together with partitioning, of a sporangium of an 

 originally simple Lycopodinous type. In the Psilotaceae the result 

 may be 2 — 5 loculi ; in the Ophioglossaceae the number may be in the 

 simplest cases as low as six, or may rise to many hundreds in the 

 larger species of Ophioglossum or Botrychium. It may further be 

 remarked that every fresh case such as the above, where the de- 

 velopment supports the hypothesis of partial sterilisation of a poten- 

 tial archesporium, and resulting partition, as exemplified among 

 these lowest vascular plants, is of importance. A large body of 

 evidence, to which I am now adding by the investigation of the 

 Marattiaceae, leads me to conclude that such partitioning of originally 

 simpler sporangia has played a very important part in the evolution 

 of vascular plants. 



