94 STUDIES IN FOSSIL BOTANY 



formation in the inner cortex (Fig. 37, h). In older 

 stems, the primary cortex had been cast off altogether. 

 Sometimes traces of its disorganised tissues remain, 

 but ultimately the primary cortex was replaced by a 

 regular scale - bark, formed by successive layers of 

 periderm (Fig. 38). This formation cut deeper and 

 deeper into the cortex, until it reached the phloem 

 itself, just as in an oak-tree, for example, at the 

 present day. So it appears that the secondary tissue 

 lying outside the wood consists in its inner part 

 of the phloem, and in its outer part of the periderm, 

 which was formed by a phellogen, arising first in the 

 cortex and afterwards in the phloem itself. 



The species Sphenophyllum plurifoliatum is based on 

 petrified specimens ; it is not improbable that it may 

 prove to be identical with the contemporary S. myrio- 

 phyllum, Cr£pin, known in the form of impressions. 

 The species agree in the large number of linear leaves 

 or leaf-segments in a verticil, and in the large diameter 

 (about 1 cm.) attained by the main stems. 1 



I must now say something as to another form of 

 Sphenophyllum, known by the name of Sphenophyllum 

 insigne. This species was described by Dr. Williamson 

 as long ago as 1874. It is a very interesting form, 

 because it comes from some of the oldest Carboniferous 

 strata, namely from the Burntisland beds, belonging to 

 the Calciferous Sandstone series, at a horizon far below 

 the Coal-measures. In this species, the stem attains a 

 diameter of almost a centimetre — a rather large size for 

 a Sphenophyllum. In its general anatomy, the stem, 



1 See Zeiller, Bassin houiller de Valenciennes, Flore fossile, p. 422, PI. 

 lxii. Figs. 2-4. 



