The Present Position of Palaeozoic Botan}-. 215 



ong"inally to the iiucellus might easil}' acciuire an extension into the 

 integument, as observed in one or two cases bj' Miss S topes (1904). 

 On the whole I should regard the Trigonocarpon type of seed as nearer 

 to tliat of recent Cycads than any member of the Larjenosfoma group. 

 It is extreme!}' unlikely that any of the direct ancestors, either of 

 Cycadaceae or other Cycadophyta, have as yet come under observa- 

 tions, but it is quite conceivable that one or other line of Cj^cadean 

 descent may have passed through a stage not unlike that represented 

 by the Neuropterideae. though probably without the polj'stelic modi- 

 fication of the stem often, but not alwaj'S. found in that group. 



It must be borne in mind that in Mesozoic times the Cycado- 

 phj'ta were a vast class of plants, embracing a great variety of 

 organization, and it is not improbable that their various lines may 

 have sprung from the plexus of Palaeozoic Pteridospermeae at diffe- 

 rent points. 



2. The Relation of the Pteridospermeae to the Cordaiteae. 



In many respects these two classes of Palaeozoic Spermopliyta 

 are so widely different that a comparison scarcely seems possible in 

 the present state of our knowledge. In habit there is no resemblance, 

 the Cordaitean trees having much more in common with Conifers such 

 as Agafhis than with the Fern-like Pteridosperms. The specialized 

 sporophylls of the Cordaiteae, grouped in definite catkins or cones, 

 are also totally different from the compound, scarcely modified leaves, 

 which bore the seeds and pollen-sacs of Pteridosperms. The Cor- 

 daiteae, in fact, are on a far higher level, and have always been 

 justly recognized as true Gymnosperms. Yet, on present evidence, 

 they were as ancient as the Pteridosperms themselves, a consideration 

 which shows how enormously far back the history of Seed- plants 

 must extend, and imposes a certain reserve on our phylogenetic spe- 

 culations. 



There are, however, clear indications of affinity between Pterido- 

 spermeae and Cordaiteae, different as they appear. The strongest 

 mark of this affinity is in the seeds, which are essentially of the same 

 type in both groups. A year or two ago we were inclined to regard 

 the bilateral or radial symmetry as a basis of distinction, but we now 

 have good evidence for the occurrence of the bilateral as well as the 

 radial form among Pteridosperms, and we are not able, at the present 

 time, to assign constant diagnostic characters to the seeds of the two 

 classes. The pollen-chamber, the double vascular system and probably 

 the free nucellus, are characters met with in members of both groups. 



Anatomically there is a great difference between the two, if we 

 base our comparison on the typical Cordaites-stem, as described by 



