PALEOBOTANY BERRY. 329 



characters by which several hundred species have been distinguished 

 and utilized for stratigraphic purposes. The perfection of preserva- 

 tion of these trunks surfaces indicates an absence from the Car- 

 boniferous forests of the numerous parasitic and epiphytic forms 

 which crowd tree trunks in the present day tropical forest. 



The essential features of the surface markings are of considerable 

 importance. The leaf cushions or bolsters were crowded in spiral 

 arrangement with angles above and below, and rounded sides (rhom- 

 bic), always longer than broad and truncated above the middle, 

 where the abcission of the leaf occurred. The leaf scar is subcircular 

 and shows three scars — a central one representing the leaf trace and 

 laterals on each side known as parachnoi. Just above the leaf scar 

 is a small triangular print left by the ligule, while below the leaf 

 scar there are frequently two rounded prints or depressions on 

 either side of the keel which, like the parachnoi, are tracts of thin- 

 walled tissue which functioned as aerating tissue. Other markings 

 of an ornamental character are frequently present on the bolsters. 

 With the decay of the cortex the characteristic Lepidodendron fea- 

 tures gradually become obliterated. If merely the epidermis is gone 

 the resulting features are those of Bergeria, once thought to repre- 

 sent an independent genus. If decay has removed part of the cortex 

 the forms are known as Aspidaria, and if all of the outer cortex is 

 gone showing the imbricated leaf traces it is known as Knorria. 



Anatomically the stem is monostelic with centripetal primary 

 wood, which may extend to the center or inclose a pith. There 

 is usually a considerable development of centrifugal secondary 

 wood consisting of scaliform tracheids and medullary rays. Pri- 

 mary phloem has been recognized but there is some doubt regarding 

 the presence of secondary phloem or of any persistent cambium. In 

 forms that have not been demonstrated to have formed secondary 

 wood, secondary thickening takes place in the outer cortex through 

 the development of a periderm. 



Leaf traces, collateral in structure, pass off from the stele without 

 leaving any gaps and pass obliquely through the cortex to the leaves, 

 each leaf receiving a single bundle. The cortex is of great thick- 

 ness and variable according to age and species. Usually there is 

 an inner parenchymatous zone poorly preserved. The outer cortex 

 consists of thicker walled elements, usually with an enormous de- 

 velopment of phelloderm, which served for the lack of mechanical 

 tissue in the vascular cylinder. 



The leaves had a single central vascular bundle surrounded by a 

 sheath of transfusion tissue and the stomata are commonly restricted 

 to two deep grooves on the lower surface. What corresponds to the 

 roots in higher plants are peculiar organs known as Stigmaria. 

 which as casts or impressions are among the commonest coal measure 



