HETEROSPORY AND THE SEED-HABIT 705 



type of the sporangia of other Lycopodiales, falls away, but the details 

 of fertilisation and of embryogeny which follow are still unknown. 1 The 

 nearest approach to a seed-like habit among the living Lycopods is seen 

 in Selaginella apus and rupestris? Here fertilisation occurs while the spores 

 are unshed, and the sporangia still attached to the strobilus : in S. rupestris 

 the connection is maintained with the parent plant until the embryo has 

 produced cotyledons and a root. Thus the Lycopodiales, both fossil and 

 modern, show approaches to a seed-habit, though it is doubtful whether 

 that habit was ever firmly established among them, or persists in the 

 form of any of the Seed-Plants of the present day. 3 The condition now 

 so fully demonstrated for the Pteridosperms shows that a Seed-Habit was 

 definitely acquired along another quite distinct phyletic line. 4 These 

 large-leaved types, bearing their large seeds of Cycad-like character dis- 

 tributed on fronds effective also for assimilating purposes, probably sprang 

 from the same stock as the Ferns, and it is especially with the Botryo- 

 pterideae and the Osmundaceae that they show the nearest analogies. 

 Thus the Seed-Habit appears to have been initiated certainly in two 

 distinct phyla, and it is not improbable that it may have been repeatedly 

 initiated within either or both of them. 



The establishment of a Seed-Habit does not necessarily bring immediate 

 reduction of the supporting system in its train : but it has frequently 

 happened that such reduction follows. The fact that the large seeds of 

 Neuropteris heterophylla are borne on a rachis bearing characteristic 

 vegetative pinnae shows that a correlative reduction is not obligatory. 

 But on the other hand, a reduced state of the sporophylls does usually 

 accompany the seed-habit : in Lyginodendron the female fructification is 

 described as being borne on the rachis of fertile fronds which differed 

 from the sterile foliage in the reduced leaf-area : and this applies also in 

 some degree to the male sporophylls as well. From such minor degree 

 of reduction of the megasporophyll to that condition seen in Cycas is 

 no great step, and from this the sequence through the Cycads gives 

 very convincing evidence of further reduction. 5 It seems not im- 

 probable that in Cycadeoidea a still further step in reduction has been 

 taken, so that while many of the sporophylls appear as minute sterile 

 scales, those which are fertile exist merely as radio-symmetric pedicels, 

 each bearing a single terminal ovule. 6 The microsporophylls show a 

 series of reductions in less prominent degree, but without any strict 

 parallelism with the megasporophylls : thus in Cycadeoidea where the 



1 See Scott, Progresses Rei Bot., i., p. 171. 

 -Miss F. Lyon, Bot. Gaz., vol. xxxii., pp. 182-3. 



3 See SeMjard and Ford, "The Araucariaceae, Recent and Extinct," Phil. Trans., 

 Series B, vol. 198, p. 305, etc. 



4 See Scott, Progressus Rei Bot., i., pp. 190-212, where the literature is quoted. 

 5 Engler and Prantl, Nat. Pflanzen., II. i., Fig. 7. 



tj This is the opinion of Wieland, American Fossil Cycads, p. 230, etc. 



2 Y 



