The Primary Structure of the Stem of Catamites. 165 



in form, and probably belong to the category of scleren- 

 chymatous fibres. 



From this account of the primary structure of this type 

 of Calamitean stem, it will be seen that the specimens how 

 described differ in many respects from those described and 

 figured by Williamson in 1878. As already stated, the 

 tissues are much more differentiated, and that in nearly 

 every part of the stem. In the pith, we have the elements 

 at the periphery smaller than those in the centre, and the 

 occasional occurrence of larger elements with black con- 

 tents, may be indicative of other differences. In the stele, 

 we have phloem strands accompanying the carinal canals, 

 to the walls of which the torn vessels still adhere, and there 

 is a sharp distinction between the stele and the cortical 

 tissues. The latter again are distinguishable into two 

 zones, and within each there are considerable histological 

 differences, which add to the complexity of the whole, and 

 make it a very different structure from the "primitive 

 generalised parenchyma " of Williamson's specimens. My 

 own impression is, that these differences are due to the fact 

 that the latter appear to be in an earlier stage of develop- 

 ment than those under treatment, which seem to present 

 the primary structure fully matured and ready for the 

 initiation of secondary thickening. It is possible, however, 

 that the two sets of specimens do not belong to the same 

 type of Catamites > and that this is the explanation of the 

 want of agreement between them. 



An interesting question in connection with the fossil 

 plants of the Coal Measures is the degree of correspondence 

 between the size of a stem and the extent of the develop- 

 ment it has undergone. The preparations under considera- 

 tion appear to throw a little light upon it. The transverse 

 section of the stem which has been described in detail, 

 measures, as already stated, -^ inch by -^ inch. But my 

 collection includes others smaller than this, in which 



