118 BRITISH EOCENE FLORA. 



Macrosporangia take the place of the staminal ones, each containing a macrospore clothed 

 in a double membrane, similar in construction to seeds already described, and more 

 complex than those of any existing Conifer. The Corddites were the most developed 

 of the extinct " Progymnosperms," and nearly equalled true Gymnosperms in the 

 relative perfection of their organism. Though highly specialised, this betrays a decided 

 approach to Ginkgo, not only in the form of the leaves, which are broad, blunt, and some- 

 times bilobed, but also in their organs of reproduction, more especially the seeds named 

 Cardiocarpus, which almost exactly reproduce those of Ginkgo. 



The " Progymnosperms " are not entirely extinct, for one order, the Cycadacese, has 

 survived with little variation to the present day, and the living species preserve, almost 

 unmodified, the characteristics of their remote ancestors of Carboniferous age. Their 

 general growth and external appearance were briefly described in our Introduction, 1 but 

 the probable course of the evolution of the Gymnosperms cannot be sketched without 

 more particular reference to the details of their internal structure. 



The Cycadacese are allied to true or acicular-leaved Coniferse mainly through their 

 reproductive organs, whilst in many other respects they are far less developed. Their 

 relative inferiority is especially apparent when the internal structure of their stems is con- 

 trasted, for these correspond in plan with such primitive types as Poroxylon and Corddites^ 

 and differ remarkably from the truly exogenous stems of the higher Gymnosperms. A 

 fully matured stem of Cycas is composed firstly of central pith; secondly, a single 

 cylinder or zone of primary wood without exterior rings of growth ; thirdly, one or more 

 regions of liberian parenchyma and an equal number of woody cylinders ; fourthly, the 

 cortical parenchyma ; and fifthly, the hypoderma or zone of increase. 2 The permanent 

 bases of the petioles contribute to the latter and make an external covering of consider- 

 able thickness. Both the woody zones are traversed by medullary rays, but the second 

 is engendered later than the first, and has a different origin. The fibrous elements of 

 the woody zones are marked with rows of obliquely elliptic areolations, differing, according 

 to Saporta and Marion, alike from those of true or cone-bearing Coniferse and from other 

 " Progymnosperms," but recalling somewhat more the structure of Ginkgo and the Taxeae. 

 The general plan has little in common with that of the Coniferae or Dicotyledons ; but, 

 due allowance being made for the branching habit and presumably more rapid growth, 

 bears a striking analogy with that of Corddites. 



The leaves of Cycadaceae are, according to Sachs, of two kinds ; the one described as 

 " dry, brown, hairy, sessile, leathery scales, of comparatively small size," the other large 

 and pinnate, that is, provided with distinct segments inserted on the sides of a usually 

 simple support. In the forms both of the scale-leaves, and of the modified leaves which 

 support the sexual organs, Saporta and Marion see reasons to infer that the now pinnate 

 foliage-leaves must have been originally simple or merely fimbriated along the margins, 



1 Ante, p. 14. 



2 See Carruthers' " Memoir on Cycadean Stems," ' Trans. Linn. Soc.,' vol. xxvi, p. 675. 



