194 SUMMARY OF CURRENT RESEARCHES RELATING TO 



form the leaf-trace. The pith is solid, with no trace of discoid structure, 

 and very variable in size. The secondary wood has the typical Dado- 

 xylon structure ; the medullary rays are one or two cells in thickness. 



(2) The Pitys group with numerous small xylem-strands around a 

 large pith, in which they are more or less deeply imbedded ; their struc- 

 ture is generally mesarch. The leaf-trace passes out as a single strand. 

 The pith is large and may show some approach to discoid structure. 

 The secondary wood has the typical characters of the Pissadendron 

 subgenus of Dadoxylon with rather wide elements ; the larger medullary 

 rays are always pluriseriate. 



(3) The type of Dadoxylon Spemeri with a few small primary xylem- 

 strands scattered at the pith margin and closely applied to the secondary 

 wood, of mesarch structure, and passing out in pairs, each pair consti- 

 tuting a single leaf-trace. The pith is of moderate size and probably 

 not discoid. The secondary wood is of the usual Dadoxylon type but 

 very dense, consisting of small tracheides, with medullary rays generally 

 of one cell in thickness. 



As regards their affinities, the stems referred to Calamopitys are very 

 suggestive of Cycadofilices, owing to the great development of the 

 primary xylem-strands, and the marked similarity to the structure of 

 Lyyinodendron. They differ from the latter in the structure of the petiole 

 which approaches that of a Myeloxylon. The tendency towards an 

 endarch structure in the lower part of their course suggests that this 

 group had advanced further towards the usual stem structure of Gymno- 

 sperms than had Lyyinonendron or even Poroxylon. Of interest also is 

 the narrow-rayed secondary wood, quite Cordaitean or Araucarian in 

 structure. 



Pitys were tall branching trees, a habit which is incompatible with 

 Cycads or Cycadofilices, and the only known family to which they could 

 be referred is the Cordaiteae,- leaves of which have been found at a similar 

 horizon. They differ in having broad medullary rays and a non-discoid 

 pith and in the presence of the primary xylem-strands, and are probably 

 to be regarded as a primitive member of this family, affording a new 

 link between Lyginodendreae (which they resemble in having mesarch 

 xylem-strands) and the true Cordaiteae. 



Dadoxylon Spenceri with its dense wood and double leaf-traces appears 

 to stand near the typical Cordaiteae, and also as AVilliamson pointed out, 

 suggests the recent GinyJco, which may itself have Cordaitean affinities. 

 The primary xylem-strands, though much reduced, have essentially the 

 same structure as in Lyyinodendron. This fossil indicates that in the 

 period of the Upper Carboniferous, stems which had in other respects 

 attained a typically Gymnospermous character, had not quite lost the 

 primitive form of wood, which we can trace back, through the Cycado- 

 filices, to the Ferns. 



Permeability of Cell-walls to Air.* — C. Steinbrinck describes 

 some experiments on the cell-walls of the sporangia of ferns and Selayi- 

 nella, and of the leaves of mosses with reference to their permeability to 

 the passage of air. A historical introduction is followed by a descrip- 



* Flora, xcii. (1903) pp. 102-31 (1 pi.). 



