X] VASCULAR SYSTEM. 307 



wavy character of the surface of the specimen is probably the 

 result of shrinking, and does not indicate original surface features. 



In examining sections of calcareous nodules from the coal 

 seams one meets with numerous fragments of small Calamitean 

 twigs with little or no secondary wood ; in some of these there 

 is a small number of carinal canals, in others the canals are 

 much more abundant. The former probably represent the 

 smaller ramifications of a plant, and the latter may be regarded 

 as the young stages of branches capable of developing into 

 stout woody shoots ^ Longitudinal sections of small branches 

 teach us that the xylem elements next the carinal canals are 

 either spiral or reticulate in character, the older tracheids being 

 for the most part of the scalariform type, with bordered pits on 

 the radial walls. This and other histological characters are 

 admirably shown in the illustrations accompanying Williamson 

 and Scott's memoir on Calamites. The student should treat 

 the account of the anatomy of Calamites given in these pages 

 as introductory to the much more complete description by 

 these authors. They thus describe the course of the vascular 

 bundles in a Calamitean branch : — 



"The bundle-system of Calamites bears a general resem- 

 blance to that of Equisetum. A single leaf-trace enters the 

 stem from each leaf, and passes vertically downwards to the 

 next node. In the simplest cases the bundle here forks, its two 

 branches attaching themselves to the alternating bundles which 

 enter the stem at this node. In other cases both the forks 

 attach themselves to the same bundle, so that, in this case, 

 there is no regular alternation. In other cases, again, the 

 bundle runs past one node without forking, and ultimately 

 forms a junction with the traces of the second node below its 

 starting-point. These variations may all occur in the same 

 specimen. The xylem at the node usually forms a continuous 

 ring, for where the regular dichotomous forks of the bundles 

 -are absent their place is usually taken by anastomoses'." 



As in Equisetum, the xylem at the nodes possesses certain 

 characteristic features which distinguish it from the internodal 



1 On this point vide Williamson and Soott (94), p. 869. 



2 Williamson and Scott, loc. cit. p. 876. 



20—2 



