258 SCIENCE PROGRESS 



Let us now turn to the phylogenetic aspect of this subject. 

 For this purpose I must refer to the habit and morphological 

 structure of certain plants occurring chiefly in the Palaeozoic 

 formation of the coal measures. These are the " Seed-ferns," of 

 which we now know several forms. The stems known as 

 Lyginodendron, Heterangiwn, and Medullosa bore large Fern- 

 like fronds of usually very complex organisation. Some of 

 these leaves were purely vegetative, others bore the repro- 

 ductive organs, and with, as a rule, their green laminae greatly 

 reduced or quite absent. In some cases, however, the fertile 

 leaves or sporophylls appear to have had their laminae but 

 little, if at all, reduced. The fossil known as Crossotheca is 

 the fertile portion of the foliage of Lyginodendron ; it is that 

 part of the foliar organ bearing the male sporangia, and these 

 are situated on the lower surface of round, flat subdivisions 

 of the much-divided leaf. Lyginodendron stems are also known 

 to have borne reduced l fertile fronds which produced large 

 seeds, or, more strictly speaking, bodies intermediate 2 between 

 sporangia and seeds, known as Lagenostoma, at the end of short 

 subdivisions of the frond-pinnae. 



Neuropteris, the foliage of some of the stems known as 

 Medullosa, also bore similar large seed-like bodies. Pecopteris 

 Pluckeneti is a scarcely modified vegetative frond bearing 

 hundreds of seed-like structures along the margins of the 

 pinnae. All these plants were probably derived from true 

 Ferns or Fern-like plants bearing on their leaves a single 

 kind of sporangium only, as in Marattia. The few facts already 

 stated are sufficient to show that at this period of the world's 

 history there was a group of plants bearing large, complex 

 Fern-like sporophylls of two kinds, some producing male 

 sporangia, others seed-like organs. Whether the same indi- 

 vidual plant bore both kinds of sporophyll, and, if so, how they 

 were respectively situated with regard to each other, is not 

 at present known. It is quite likely that the plants were 

 dioecious, the male and female organs occurring on separate 

 plants. 



1 i.e. as regards the production of green laminae. 



2 These bodies are not really seeds, as that term is understood when applied to 

 the corresponding organs in flowering-plants, for the reason that they do not 

 contain embryos, the effects of fertilisation, if not that process itself, only being 

 reached after the organ had fallen to the ground. 



