348 CALAMITES. [CH. 



origin of a root from the node of a Calamite stem. The section 

 has passed through a stem in a tangential direction, showing 

 the characteristic arrangement of the vascular bundles x, and 

 principal medullary rays m. The small leaf-traces, t, t, afford 

 another feature characteristic of a Calamite stem. The portion 

 of stem to the right of the figure has been slightly displaced, 

 and between this piece and the root R, one of the ubiquitous 

 Stigmarian appendages, s, has inserted itself. At i? a fairly 

 thick and decorticated root is seen in oblique transverse 

 section ; at the upper end the root tracheids are seen in direct 

 continuity with the xylem of the stem. In the centre of the 

 root is the large solid pith surrounded by twelve bluntly pointed 

 xylem groups, composed in the main of radially disposed scalari- 

 form elements with narrow secondary medullary rays like those 

 in a stem. Between each xylem group there is a broad 

 medullary ray, which tapers rapidly towards the outside, and 

 is soon obliterated by the formation of interfascicular se- 

 condary xylem. At R a portion of another root is seen in 

 transverse section, and R" the inner part of a single xylem 

 group is shown more clearly. The solid pith and the absence 

 of carinal canals are the two most obvious distinguishing 

 features of the roots. 



As Renault points out, roots of Calamites have been figured 

 by some writers^ as examples of stems, but it is usually com- 

 paratively easy to distinguish between roots and stems. On 

 examining the xylem groups more closely, one notices that 

 the apex of each is occupied by a triangular group of centripet- 

 ally-developed primary tracheids, the narrow spiral protoxylem 

 elements occupying the outwardly directed apex. The pro- 

 toxylem apex is usually followed externally by a ray of 

 one or two radially disposed series of parenchymatous cells. 

 This ray is not distinguished in fig. 92 R" from the rows of 

 xylem tracheids. Each xylem group is thus formed partly of 

 centripetal xylem and in part of secondary centrifugal xylem ; 

 the latter is associated with secondary medullary rays, as in 

 stems, and contains a broader ray (fascicular ray of Williamson 



1 E.g. Sohenk (90) in Zittel's Humlbuch, p. 237. 



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