The Carboniferous Arborescent Lepidodendra. 37 



of the branch into two smaller ones is yet visible externally, 

 but such a separation speedily manifests itself. Before this 

 latter stage is reached the two transverse sections of the 

 divided Primary Xylem cylinder now appear as two 

 crescents, the two horns of each of which approach one 

 another; at this point the cells of the two medulla are in 

 direct continuity with those of the inner cortex. Several of 

 my sections demonstrate that, for a very short space, this 

 crescentic condition is a permanent one, and approximates to 

 what Du Bary speaks of as " the Foliar gap " in Ferns 

 {Comparative Anatomy of Phanerogams and Ferns, English 

 Translation, p. 283). But at a slightly higher point the horns 

 of each of these two crescents rapidly converge, and become 

 united into two circular cylinders differing in no respects, 

 save size, and number of internal parts, from that of 

 the parent stem. Sachs designates each portion of a 

 stem or branch immediately below a dichotomy, the basis 

 of bifurcation. Yet higher there occurs sooner or later a 

 second dichotomy at the summit of each of the two smaller 

 branches produced by the first one. What takes place in 

 this is a mere repetition of what had occurred lower down, 

 and each of the four resultant younger branches has, as 

 before, though diminished in diameter, exactly the same 

 type of organisation as the two " bases of bifurcation " from 

 which they sprang. We thus see that, however numerous 

 the dichotomies, they are all produced in exactly the same 

 way ; and from the base of the parent branch up to the 

 smallest terminal twig of the full-grown tree, no structural 

 changes are introduced,save certain secondary ones produced 

 by growth processes, which begin to manifest themselves at 

 the base of that trunk. 



It will be readily understood that at each of this 

 ascending series of divisions the number of the cells and 

 vessels of the bark, Primary Xylem strand, and medulla 



