52 Dr. W. C. Williamson on 



Type of Lepidodendron Mundum. 



My sections of this small and somewhat rare type vary 

 somewhat in the numerical relation of the several tissues. 

 Nevertheless they virtually tell the same story as the 

 preceding ones have done. Thus, commencing with my 

 smallest twig, with a cortical diameter of I mm., we have a 

 primary xylem consisting of but 26 tracheids. In C.N. 

 405b, where the cortex has enlarged to 2*5 mm., the 

 tracheids have increased to 83. In 415a the cortex has 

 enlarged to 47 mm. ; the tracheids now number at least 123, 

 and in 413, where the cortex has a mean diameter of y$ mm. 

 and the Primary Xylem cylinder has reached its largest 

 dimensions of 1*5 mm., the tracheids exhibit a corresponding 

 increase to 190 in number. We thus discover that, as we 

 descend from the smallest, i.e., the uppermost, twigs to the 

 larger and lower branches, we have the same enlargement 

 of the primary stele as a whole and in the number of its 

 component tracheids that we have seen in the types pre- 

 viously considered. The equal dichotomy of 416a, also 

 shews that in Mundum we have exactly the same dichotomy 

 as characterises the arborescent form. Like conditions are 

 seen in C.N. 412, in the combined cylinders of which we 

 have at least 154 tracheids. Of unequal dichotomies I have 

 as yet seen no trace in L. Mundum. (Table VI.) 



Type of Lepidodendron parvulum. 



So far as I can judge from the limited number of speci- 

 mens of this type which I possess, it is the smallest of the 

 Lepidodendra yet discovered. I have included them here 

 because, though the smallest, they present precisely the 

 same form of equal dichotomy as we find in the largest of 

 the arborescent groups. The diameter of my sections> 

 C.N. 12, 420, and 421, including their leaf-cushions, does not 

 exceed 4 mm. But besides these I have five transverse 



