20 MORPHOLOGY OF GYMNOSPERMS 



are three concentric cortical layers of wood, a succession of cortical 

 cambiums appearing, similar to that found in certain genera of living 

 cycads. In this form numerous bundles also occur in the pith, which 

 is another feature of some of the modern cycads. 



Cladoxylon and Colpoxylon are other stem genera with poly- 

 stelic structure as in Medullosa. In the former genus, a devonian 

 form, the vascular structure is complex, the steles often being elon- 

 gated and curved in section, and sometimes fusing, but each stele 

 has its own zone of secondary wood, in which the tracheids are scalari- 

 form. The latter genus is permian, and is interesting in that the 

 stems are protostelic below and polystelic above. 



Recently Scott (68) has described a Sutcliffia (5. insignis) from 

 the Lower Coal-measures, which he regards as "a new type of Medul- 

 loseae." If so, it is like a Medullosa reduced to the Heterangium 

 condition, for it is protostelic and there is only a slight amount of 

 secondary thickening, but from this main axial stele secondary steles 

 become detached at intervals. Moreover, the xylem is exarch, as 

 contrasted with the mesarch condition in Medullosa. However, the 

 leaf traces are numerous, complexly branching, and anastomosing. 

 Such a complex leaf structure connected with a protostelic stem 

 cylinder is said to be a unique combination, and suggests a reduction 

 of the central cylinder. The Medullosa connection is still further 

 suggested by the mucilage canals and reticulate peripheral bands of 

 sclerenchyma. 



Calamopitys is a stem genus of the Devonian and Lower Carbon- 

 iferous that is very suggestive of Lyginodendron, with small pith, few 

 and comparatively large mesarch primary bundles, a thick zone of 

 secondary wood, and a leaf trace that forks on leaving the cylinder. 

 The leaf traces are quite distinct, however, in being concentric in 

 the cortex and in dividing on entering the leaf, so that the petiole 

 contains several bundles. 



Protopitys is a remarkable isolated form of the Devonian and 

 Lower Carboniferous with an abundant development of secondary 

 wood. The large pith (elliptical in section) is surrounded by a con- 

 tinuous zone of primary wood, of unequal thickness but not broken 

 into bundles. 



The preceding stem genera are not all known with certainty to 



