X] 



ARTHROPITYS. 



313 



result of active growth, and not merely the expression of the 

 tangential stretching of the stem consequent on secondary 

 thickening. 



A glance at the complete transverse section of the stem, — 

 of which a small portion is shown in fig. 74 A, — suggests the 

 existence of annual rings in the wood, but this appearance of 

 rings is merely the result of compression. The secondary wood 

 of a Calamite does not exhibit any regular zones of growth 

 comparable with the annual rings of our forest trees. 



Fig. 75. Longitudinal tangential section near the inner edge of the wood of 

 the Calamite of fig. 74. 

 X, X, secondary xylem and medullary rays ; in, principal medullary 

 ray. From a section in the Binney Collection, x 50. 



Before passing to other examples of Calamitean stems, refer- 

 ence may be made to the sections shown in figs. 75 and 76, which 

 illustrate some further points in the structure of Binney's stems. 

 In fig. 75 the xylem tracheids are shown at a?, and between 

 them the secondary medullary rays present the appearance of 



