24 STUDIES IN FOSSIL BOTANY 



innermost elements, being the most disorganised, were the 

 earliest formed ; and we thus obtain a proof that the de- 

 velopment of the primary wood was strictly centrifugal. 



The elements of the primary wood, other than those 

 in the canal, present no special interest. We find the 

 spiral and annular tracheae of the protoxylem replaced 

 by other elements (scalariform or pitted) as we get 

 beyond the canal. I have spoken of these bundles as 

 collateral. In the very best preparations, we can satisfy 

 ourselves that on the inner side of the vascular bundle 

 there are no traces of phloem ; the canal abuts directly 

 on the pith. On the other hand, in very fortunate 

 cases, where the preservation is exceptionally good, we 

 find, on the outer side of the wood, strands of delicate 

 tissue made up of small thin-walled elements ; these 

 we can only interpret as phloem -groups ; we find, 

 further, between these groups and the wood, remnants of 

 cells of the cambium itself. Some traces of these tissues 

 will be recognised in Fig. 4, immediately outside the 

 secondary wood. 1 



We thus see that the structure of the young stem 

 of a Calamite is in all essentials similar to that of an 

 Equisetaceous stem. Surrounding the usually fistular 

 pith, interrupted at every node by a persistent diaphragm 

 (see Fig. 7), we have a ring of collateral vascular 

 bundles, with centrifugally developed wood ; at the inner 

 border of each is a canal containing the disorganised 

 tracheae of the protoxylem. 2 Thus, the Calamite, so 



1 For special figures of the phloem and cambium, see Williamson and 

 Scott, ' ' Further Observations on the Organisation of the Fossil Plants of 

 the Coal-measures," Phil. Trans, vol. 185, B, 1894, Plate Ixxviii. Figs. 

 12-14. 



2 In a Calamarian stem of Lower Carboniferous age, from the Burntisland 



