The Carboniferous A rborescent Lepidodendra. 47 



The remarkable range of the characteristic features of 

 the Lepipodendroid stems and branches, from the youngest 

 twigs to the arborescent stems, without any changes in their 

 typical organisation save such as are due to growth, makes 

 this type an extremely significant and valuable one. 



Type of Lepidodendron Harcourtii. 



This is the type of which Brongniart received a section, 

 and through an erroneous inference which he drew from it 

 led to so many years of conflict and misinterpretation. No 

 specimen of it has yet been found shewing a trace of 

 Secondary Xylem. It appears probable that in this respect 

 the type resembled L. Wunschianum, viz., that the exogenous, 

 or secondary xylem, only made its appearance atan advanced 

 stage of growth, and that we have as yet obtained no 

 specimen sufficiently advanced to have entered upon that 

 stage. In other respects the type bears prominently upon 

 the questions constituting the main subject of this Memoir. 



The two smallest stems I possess of this type are seen 

 in C.N. 1596A. In this slide are two very small and young 

 sections severally marked A and B. In the former, marked 

 A, the Primary Xylem cylinder has a diameter of 2*6 mm. 

 and contains 681 tracheids. Now, as in the case of most, if 

 not all, of these young Primary Xylems we find here two 

 types of tracheids. That occupying the greater part of 

 the area of the transverse section of the organ consists of 

 large and conspicuous ones, and contrasts with the medulla ; 

 but there is a peripheral series of small, and often of 

 extremely minute, ones. It is these latter which constitute 

 the " Corona " of Professor Bertrand, and from which the 

 leaf-traces are given off. Now in 1596A, section A, we find 

 492 of the larger ones and but 189 of the smaller ones. 

 The effect of this upon the forms of the Corona is shown 

 in my Memoir X I X., Figure 6. What M. Bertrand designates 



