The Carboniferous Arborescent Lepidodendra. 57 



have been larger than my minute one referred to above ; 

 and as the independent roots of a Stigmaria coalesce at the 

 base of an aerial stem, the diameter of the coalesced units of 

 these organs is very much less than the aggregate of these 

 four roots at a lower point where each root was independent 

 of its neighbours.* Hence we are driven to the conclusion 

 that the base of a Lepidodendroid stem in its youngest state 

 can scarcely have had a diameter approaching to 24 mm. 

 Now assuming, that the number of leaves and their asso- 

 ciated leaf-traces, at the base of that stem, was always 

 greater than at any point at a higher level where the num- 

 ber of the leaves encompassing a branch began to be reduced 

 to one-half by each succeeding equal dichotomy, it again 

 becomes difficult to account for the 6500 tracheids existing 

 at the base of the stem of my L. Wunschianum on Solms- 

 Laubach's hypothesis. Since, as we have already seen, 

 additional leaves could not have been intercalated into any of 

 the diagonal lines as the stems grew, whatever their number 

 that number was fixed, and they must all have been present 

 at the base of the stem at the outset of its growth. 



The magnificent Lepidodendroid stem figured in Vol. III., 

 page 203, of Lindley and Hutton's Fossil Flora of Great 

 Britain seems to shew that throughout the 39 feet inter- 

 vening between the base of the stem and its first bifurcation 

 its diameter underwent but little change. Such being the 

 case, we may fairly conclude that the number of the leaves 

 and leaf-bases clothing its cortex, would, in like manner, 

 undergo no material alteration. But the first dichotomy 

 inevitably altered these conditions. Each of the two 

 resulting branches would divide between them whatever 

 organs existed, external or internal; and, as we have already 

 seen, a similar division would recur at each successive 

 dichotomy. The secondary xylem growth, commencing 

 at the base of the primary stem, would follow the vertical 



* See the Figs. I and 5 of my Monograph. 



