A TYPE OF PALAEOZOIC PLANTS. 401 



has been described in one specimen as consisting of a single 

 layer of cells. In the great majority of calamitean stems, 

 of which the internal structure is preserved, we have a 

 greater or less development of secondary wood. The 

 additions to the primary bundles were brought about by 

 the activity of a cambium layer, and, as in a typical dicoty- 

 ledonous plant, certain cells of the primary medullary rays 

 became meristematic at an early stage of the plant's growth, 

 and gave rise to interfascicular vascular tissue. The secondary 

 xylem consists of radially disposed tracheae, with scalariform 

 and elliptical pits usually confined to the radial walls. 

 The distribution of the pits is referred to by Williamson 

 and Scott as a fact of considerable interest, " for it indicates 

 that the mechanism for the passage of sap through the 

 wood of Catamites was of the same kind as that existing; in 

 recent Coniferce "} Associated with the tracheae are regular 

 rows of narrow and thinner-walled elements ; these con- 

 stitute the secondary medullary rays, which increase in 

 number as the result of the production of new rows of ray 

 cells, as we pass outwards through the secondary xylem. 

 In describing the manner of development of the inter- 

 fascicular wood, the authors of the recent memoir arrive at 

 the conclusion that the interfascicular tracheae arose "by the 

 elongation of single cells, but that this elongation took 

 place in the cambium cells before the tracheae were cut off 

 from them".* 2 It is extremely rare in the large stems to 

 find any well-preserved cortical tissue ; some few examples 

 are known in which a few crushed phloem elements can be 

 detected, and internal to these the remains of thin-walled 

 cambium cells. In the outer part of the cortex strands of 

 stereome fibres are occasionally found, also scattered gum 

 or mucilage sacs. In the early stages of growth in thick- 

 ness Catamites presents a uniform and simple type of 

 structure, but after a time the primary medullary rays show 

 certain differences in their method of growth, which enable 

 us to institute four fairly well-marked types of stem struc- 

 ture. These mav be classed as follows : — 



1 Williamson and Scott, p. 882. " Ibid., pp. 886-887. 



