320 



CALAMITES. 



[CH. 



It is well known how a wound on the branch of a forest tree 

 becoihes gradually overgrown by the activity of the cambium 

 giving rise to a thick callus, which gradually closes over the 



Fig. 80. Diagrammatic sketch of a transverse section of a Calamite twig, 

 showing callus wood. From a specimen in the Cambridge Botanical 

 Laboratory Collection, x ca. 10. 



wounded surface in the form of two lips of wood which finally 

 meet over the middle of the scar. The two lips of callus are 

 clearly shown in the fossil branch arching over the tear in the 

 wood just beyond the ring of carinal canals. The tissue external 

 to the wood represents the imperfectly preserved cortex. A 

 section which was cut parallel to that of fig. 80 shows a con- 

 tinuous band of wood beyond the wound, and the latter has 

 the form of a small triangular gap; this section appears to 

 have passed across the wound where it was narrower and has 

 already been closed over by the callus. The formation of a 

 rather different kind of callus wood has been described by 

 Renault' and by Williamson and Scott ^, in stems where aborted 

 or deciduous branches have been overgrown and sealed up 

 by cambial activity. 



' Eenault (96), p. 91. 



^ Williamson and Scott, loc. cit. p. 893. Vide specimens 133*— 135* in the 

 Williamson Collection. 



