RELATIONS OF CALAMODENDRON TO CALAMITES. 263 



only corticated and in a more advanced stage of growth. 

 Here^ again^ we have the central cavity^ a, the thin medulla, 

 b, and the vascular wedges c, represented by the same alter- 

 nations of black and white as in fig. 2 ; but by detaching 

 the vascular zone, we have also represented, at b, V , the 

 causes of the alternating ridges and grooves of specimens 

 like fig. I ; at c the exteriors of the vascular wedges project 

 externally as their inner angles project inwardly into the 

 medullary cavity *. At c' a vascular lamina of one of these 

 wedges is seen in radial vertical section, showing the char- 

 acteristic arched arrangement of its vessels where they 

 cross the node /. At A" is one of the infranodal canals 

 passing out from the pith to the bark, through the upper 

 end of each primary medullary ray, as at h, A', and at fig. 2, h, 

 whilst at I, i', as at i, i of fig. 2, we have lines of cellular 

 tissue passing outwards through both wood and bark, being 

 apparently lines of communication, doubtless containing 

 some vessels, between the interior of the plant and each 

 of its verticillately arranged leaves. At k we have the bark 

 with its absolutely smooth, ungrooved, and unconstricted 

 exterior at k', its nodes being prominent, rather than con- 

 stricted, as they are at fig. i,a. 



Independently of the bark which encloses them, we 

 have here a complex series of structures : — a, the fistular 

 cavity; b, medulla; c, vascular wedges; d, internodal 

 canals ; /, node ; g, primary cellular medullary rays, — 

 besides which each vascular wedge, c, is composed of 

 a number of thin, parallel, radiating, vertical laminae of 

 vessels, between which are numerous secondary medullary 



* On the right hand of this figure the vascular zone lias been removed from 

 the interval between the two stars, showing the undulating outline, b, of the 

 very thin medulla, which has adapted itself to the corresponding undulating 

 contours of the medullary angles of the vascular wedges, c, the intervening 

 primary medullary rays, a, and upon which the inorganic cast, fig. i, of the 

 medullary cavity, a, was moulded in its turn. 



