LAURENTIAN AND EARLY PALEOZOIC. 



43 



larly or spirally arranged. The transverse swellings of the stem 

 show no difference of structure, except that the tubes or cells may be 

 a little more tortuous, and a transverse film of coaly matter extends 

 from the outer coaly envelope inwardly. This may perhaps be 

 caused by some accident of preservation. The outer coaly layer 

 shows tubes similar to those of the stem.* The horizontal or oblique 

 flexures of the large tubes seem to be mainly in the vicinity of the 

 radial openings, and it is in entering these that they have been seen 

 to branch." 



The conclusions arrived at by Prof. Penhallow are as follows : 

 " 1. The plant was not truly exogenous, and the appearance of 



rings is independent of the causes which determine the layers of 



growth in exogenous plants. 



" 2. The plant was possessed of no true bark. Whatever cortical 



layer was present was in all probability a modification of the general 



structure, f 



" 3. An intimate relation exists between the large tubular cells 

 and the myceloid filaments, the latter being a system of small 

 branches from the former ; the branching being determined chiefly in 

 certain special openings which simulate medullary rays. 



"4. The specimens examined exhibit no evidence of special de- 

 cay, and the structure throughout is of a normal character. 



" 5. The primary structure consists of large tubular cells without 

 apparent terminations, and devoid of structural markings, with 

 which is associated a secondary structure of myceloid filaments aris- 

 ing from the former. 



" 6. The structure of Nematophyton as a whole is unique ; at least 

 there is no plant of modern type with which it is comparable. 

 Nevertheless, the loose character of the entire structure ; the inter- 

 minable cells ; their interlacing ; and, finally, their branching into a 

 secondary series of smaller filaments, point with considerable force to 

 the true relationship of the stem as being with Algae or other Thallo- 

 phytes rather than with Grymnosperms. A more recent examination 



* It is possible that these tubes may be merely part of the stem at- 

 tached to the bark, which seems to me to indicate the same dense cellular 

 structure seen in the bark of Lepidodendra, etc. 



f On these points I would reserve the considerations : 1. That there 

 must have been some relation between the mode of growth of these great 

 stems and their concentric rings ; and, 2. That the evidence of a bark is 

 as strong as in the case of any Palaeozoic tree in which the bark is, as 

 usual, carbonised. 



