3 o STUDIES IN FOSSIL BOTANY 



there are distinct traces of transverse walls in these 

 elements. This, however, is very inconstant, and where 

 these apparent transverse walls appear, they seem to 

 be perfect septa, and not merely annular ridges such as 

 we are accustomed to find in vessels, marking the 

 limits of their constituent cells. I am disposed to 

 think that the elements of the wood in Calamites were 

 occasionally septate, unlike true tracheides, but there is no 

 proof that they ever arose by cell-fusion, and we cannot 

 therefore regard them as vessels in De Bary's sense. 

 There is evidence that the pits, which were either scalari- 

 form or of the rounder, multiseriate type, were closed. 



The disposition of the pits is of some interest. 

 Throughout the whole of the secondary wood they are 

 limited to the radial walls. The general arrangement was 

 thus very much like that in the Coniferae of the present 

 day, and the mechanism of water-conduction must have 

 been similar. The cells of the medullary rays have 

 this peculiarity, that they are generally longer vertically 

 than in any other direction, and thus the rays have not 

 the muriform appearance which is general, though not 

 universal, among recent plants. Some of the rays are 

 very small, and may even consist of a single radial 

 row of cells. 



I have already given the principal facts about the 

 primary cortex. In the few cases in which we find 

 the cortex well preserved in an old stem, there is an 

 enormous development of periderm, as shown by a 

 specimen in the Williamson Collection, which has 

 secondary wood two inches thick, and bark of even 

 greater thickness. 1 



1 Figured in Williamson, " Organisation of Fossil Plants of the Coal- 



