324 CALAMITES. [CH. 



small projection at the upper end of each ridge is a cast of a 

 depression or canal which existed in the medullary tissue of 

 the living plant. There have been various suggestions as to 

 the meaning of these oval projections ; several writers have 

 referred to them as the points of attachments of roots or other 

 appendages, but Williamson proved them to be the casts 

 of canal-like gaps which traversed the upper ends of prin- 

 cipal medullary rays in a horizontal direction. In a tangential 

 section of a Calamite stem the summit of each primary 

 medullary ray often contains a group of smaller elements which 

 are in process of disorganisation, and in some cases these 

 cells give place to an oval and somewhat irregular canal. 

 In the diagrammatic tangential section represented in fig. 83, A 

 the upper end of each ray is perforated by a large oval 

 space, which has been formed as the result of the breaking 

 down of a horizontal band of cells. Williamson designated 

 these spaces infranodal canals. While proving that they had 

 nothing to do with the attachment of lateral members, 

 he suggested that they might be concerned with secretion; 

 but their physiological significance is still a matter of specu- 

 lation. The casts of infranodal canals are especially large and 

 conspicuous in the subgenus Arthrodendron, a form of Calamite 

 characterised by certain histological features to be referred to 

 later. Williamson^ originally regarded the presence of infra- 

 nodal canals as one of the distinguishing features of Arthro- 

 dendron, but they occur also in the casts of the commoner 

 type Arthropitys. As a rule we have only the cast of the 

 inner ends of the infranodal canals preserved as slight pro- 

 jections like those in fig. 83,-4; but in one exceptionally in- 

 teresting pith-cast described by Williamson, these casts of the 

 infranodal canals have been preserved as slender spoke-like 

 columns radiating from the upper ends of the ridges of the 

 infranodal region of a pith-cast. 



This specimen, which was figured by Williamson^ in two of 

 his papers, and by LyelP in the fifth edition of his Elementary 



' Williamsou (71), p. 507. 



- Williamson (7P), PI. i. fig. 1; (78), PI. xxi. fig. 31. 



2 Lyell (55), p. 368. 



