24 



mous standing, but, on the other hand, there are specimens 

 which, as Williamson has shown, unite such structures 

 with foliage referred to ferns of the genus Sphenopteris. 1 

 Williamson suggests that inasmuch as the living Stangeria 

 among the Cycads combines an exogenous stem with fern- 

 like leaves, the same may have been the case in the Carboni- 

 ferous. If so, the problem as to their position can be 

 determined in each case only by the discovery of their 

 fructification. 



In Bertrand and Renault's recent elaborate memoir on 

 Poroxylon, these botanists have shown that this genus pos- 

 sesses an exogenous stem of some complexity. It has a 

 distinct pith, not Sternbergian, with gum canals, an inner 

 or centripetal layer at first in distinct bundles of scalari- 

 form and punctated fibres, a true radiating woody zone of 

 multiporous fibres, with numerous medullary rays, and a 

 cambium layer, two layers of inner bark, and an outer 

 suberous bark. The leaves are petiolate and simple, and 

 have a single vascular bundle at base, forking in the blade, 

 in the manner of Nceggerathia. From these and other 

 more minute characters in the distribution of the tissues, 

 the authors conclude that Poroxylon may be placed between 

 the Dyploxyloid Sigillarice and the Cycads, as probably a 

 low Gymnospermous type. They refer to three species of 

 Poroxylon P. Edvardsti, P. Boyseti and P. Stephanensis. 



Medullosa of Cotta presents several thick woody cylinders 

 twisted together, and with detached star-shaped or radiat- 

 ing bundles of fibres in the pith. The woody tissue of 

 Medullosa is said to resemble that of Palceoxylon, which is, 

 however, a subgenus of Dadoxylon, and allied to the 

 Conifers. 



Colpoxylon has a thin woody cylinder and much thicker 

 bark than the preceding, and simple bundles in the pith. 



Gycadeoxylon has several concentric circles of fibrous 

 tissue, with cellular tissue between them, somewhat in the 

 manner of Gnetacese, and with no fibrous bundles in the 



1 Transactions Royal Society. 



