The Carboniferous Arborescent Lepidodendra. 41 



densely packed as we approach the centre of this tissue 

 system, where the tracheids are frequently interspersed 

 amongst the cells of a peculiar form of barred parenchyma 

 occupying the position of the medullary cells of all the other 

 Lepidodendra. This renders the counting of these tracheids 

 almost an impossibility and the measurement of the quasi- 

 medulla an absolute one. 



The Table I. shews that in ten sections of this plant the 

 diameter of the cortex advances from nine and a half 

 millimetres to ninety-two in C.N. 1922D. That in the same 

 series the diameter of the Primary Xylem cylinder increases 

 from 1 "5 mm. in C.N. 337 to 6 mm. in C.N. 1922D ; and that 

 whilst in 337, 335, and 337A, no trace of a secondary xylem 

 has yet made its appearance, we find a thin crescent of this 

 tissue-strand in C.N. 339, it has steadily increased to 14*5 in 

 C.N. 1922D. We thus see that throughout these fourteen 

 sections the increase in the diameter of the branch as a 

 whole has been accompanied by a corresponding enlarge- 

 ment of the succession of tissue systems that occupy the 

 interior of the branch. 



Here also we have in C.N. 340 and 340A two examples of 

 equal dichotomies, and one, 1922D of an unequal Dicho- 

 tomy, but of the type in which, though one of the two 

 resultant branches is very much smaller than the others, 

 there is no difference in the details of their organisations. 

 This dichotomy occurs in the largest section of the Selagi- 

 noides type that I have yet discovered. 



Tyfie of Lepidodendron brevifolinm. 



This Burntisland plant is primarily important because 

 of the extraordinary profusion and exquisite preservation 

 of the youngest twigs in various stages of their development, 

 of which some portions of the stratum in which they 

 are imbedded are almost entirely composed. Another 



