SPHENOPHYLLEAE 89 



We will now go on to describe the internal structure. 

 The anatomy of the stem is exceedingly remarkable, 

 and quite unlike that of the stem in any other group 

 of plants. It was made known, in the first instance, by 

 the investigations of M. Renault 1 on the French 

 specimens. This observer was so fortunate as to find 

 the internal organisation preserved, in specimens which 

 at the same time showed the external characters of 

 definite species, and this is one reason why Spheno- 

 phyllum. has become one of the best -known fossil 

 genera. One anatomical feature is constant in all the 

 forms examined. We always find, in the middle of the 

 stem, a solid strand of primary wood without pith, and 

 this strand is either triarch or hexarch in structure, 

 and was always centripetally developed, as shown by 

 the position of the spiral elements at the prominent 

 angles (see Figs. 36-38, 40, and 41, B). 



First of all, we will confine ourselves to the primary 

 structure, as shown, for example, in the species from the 

 Lower Coal-measures of Lancashire and Yorkshire, 

 named Sphenophyllum plurifoliatum' 1 (Figs. 37 and 38). 

 The primary wood, as seen in transverse section, is tri- 

 angular ; it consists entirely of tracheae, without pith or 

 conjunctive parenchyma. At the angles, narrow spiral 

 or reticulate elements are found, marking, no doubt, the 

 starting-points of the development. The more internal 

 part of the wood consists of large, pitted tracheae. In 

 this case the structure is clearly triarch, like that of so 



1 See his Cours de botanique fossils, vols. ii. and iv. , and the earlier 

 papers there cited. 



2 See Williamson and Scott, "Further Observations on the Fossil 

 Plants of the Coal-measures," Part i., and the earlier memoirs of William- 

 son, there cited. 



