XV] ULODENDRON 133 



same arrangement and size as the dots which are found on 

 typical Ulodendron scars. He interprets the ring surrounding 

 the umbilicus as the remains of the primary wood and the 

 small strands as leaf-traces supplying the branch. 



In the diagrammatic section shown in fig. 158 the outer 

 cortex of the main stem is represented by oc 1; this consists of 

 secondary tissue. The corresponding tissue in the branch is 

 seen at oc 2. The stele of the stem is shown at Tr. St. and 

 that of the branch at Br. St. ; It, It, mark the position of the 

 leaf-traces. If we assume the branch to be detached along 

 the line LS, the depression would show numerous spirally 

 arranged dots representing the points of exit of leaf-traces and 

 the vascular axis would be exposed in the umbilicus. This 

 explanation appears to me to be in harmony with the surface- 

 features of Ulodendron scars on both Bothrodendron and 

 Lepidodendron stems. The occasional occurrence of leaf- 

 cushions on a portion of a Ulodendron scar is a difficulty on the 

 cladoptosis hypothesis. Assuming that true leaf-cushions occur, 

 their presence may, as Watson suggests, be due to the folding 

 back of a piece of the outer cortex of the branch which has 

 been " crushed down on to the area of the scar^" 



Since this account was written a note has been published 

 by M. Renier^ in which he describes a specimen of Bothro- 

 dendron from Liege, one face of which shows a projecting Ulo- 

 dendroid scar with an excentric umbilicus. On the other face 

 is a dichotomously branched shoot with surface-features corres- 

 ponding to those on the scar; the evidence that the scar 

 represents the base of the branch is described as indisputable. 



Stur' held the view that the depressions on Ulodendron stems 

 represent the places of attachment of special shoots comparable 

 with the bulbils of Lycopodium Selago, or, it may be added, 

 with the short branches occasionally produced on Cycas stems. 

 If the depressions were formed by- the pressure of the bases 

 of cones, it is clear that the size of the cavity must be an index 

 of the diameter of the cone. The larger Ulodendron scars 

 exceed in diameter the base of any known lepidodendroid stro- 

 bilus. Another obvious difficulty, which has not been over- 



1 Watson (08) p. 10. ^ Renier (08). ' Star (75) A. Heft ii. 



