The Carboniferous Arborescent Lepidodendra. 43 



In my Section C.N. 502 we have a good example of 

 Unequal dichotomy. A small segment of Tracheids is 

 being detached from the extreme periphery of the Primary 

 Xylem cylinder, but appears to have been shut in by the 

 thick investing secondary one. We thus see that in 

 L. brevifolium both the equal and the unequal dichotomies 

 occur. The former in branches of all ages, from my youngest 

 to my oldest. The unequal one also occurs in one of my 

 most advanced forms. 



Type of Lepidodendron Wunschianum. 

 The Arran Plant. 



This type (Table III.) is one of the most valuable that we 

 possess, because it is the only one in which we can trace the 

 development of the tissues from the youngest twigs, down 

 to stems six feet in circumference, embedded in volcanic 

 ash, obviously not far from, if not actually on, the spot 

 where they had originally grown ; the investing rnck was 

 full of fragments of their twigs and branches. In the 

 British Natural History Museum in Cromwell Road are 

 some fragments of the same plant derived from the quarries 

 of Cragleith, near Edinburgh. 



The youngest twigs (C.N. 428 to 432) are thickly clothed 

 with very short leaves and have a Primary Xylem composed 

 of a solid bundle of tracheids, devoid of all traces of a 

 medulla. Counting the tracheids in this characteristic 

 bundle, the sole representative of the stele in these young 

 growths, I found them to be about 204. This primary 

 bundle has a diameter of little in number more than "j of a 

 millimeter. We next advance to the twig (C.N. 433), which 

 is of much larger dimensions, cortex and leaves having 

 increased much in size, and the central Primary Xylem, still 

 devoid of a medulla, has now a diameter of a trifle under 

 4 mm., but of which, though I found it difficult to count 



