3 g8 SCIENCE PROGRESS. 



the wood, and thus there would be left a series of ribs 

 projecting into the pith cavity. Sand or mud filling- up the 

 hollow pith would be moulded on to the surface of the 

 woody ribs and intervening depressions in the primary 

 medullary rays, and thus the surface of the pith cast would 

 present the appearance of narrow grooves alternating with 

 broader ridges. 



Our knowledge of the internal structure of Calamites is 

 chiefly derived from the minute examination of fragments 

 of small twigs or branches preserved in the calcareous coal 

 balls of Lancashire and Yorkshire, and from the investiea- 

 tions of French palaeobotanists, founded on similar fragments 

 from the siliceous nodules of Autun and other districts. 

 When Adolphe Brongniart realised the fact that such Coal- 

 Measure plants as Calamites and Sigillaria increased in 

 thickness, in a manner strictly comparable to the growth of 

 phanerogamous stems and roots, he was not unnaturally 

 led to look upon the discovery as a fatal objection to these 

 genera being included among the Vascular Cryptogams. 

 He accordingly separated the Calamariece into two distinct 

 groups : the Calamite<z, closely allied to the Equiselacece, 

 and the Calamodendi'ece, included in the Gymnosperms on 

 account of their secondary growth in thickness. 1 This 

 twofold division of calamitean plants was adopted by most 

 writers, and is still retained by one or two French authors. 

 Since Brongniart's time it has been established that 

 secondary growth in thickness does not in itself afford a 

 satisfactory basis of classification. At the present day such 

 a manner of growth is exceptional among Vascular Crypto- 

 gams ; in such forms as it exists it is only on a small scale ; 

 but we have in this circumstance simply an illustration of 

 the methods of development and altered conditions in the 

 vegetable kingdom. In the vast forest which in Coal- 

 Measure times stretched over wide areas reclaimed from 

 the Carboniferous sea, the most abundant and conspicuous 

 trees were arborescent Pteridophytes ; with these were 

 associated a few gymnospermous types, but we have abso- 



1 Williamson (4). 



