58 Dr W. C. Williamson on 



elongation of all the primary structures. In cases like 

 L. Selaginoides, where we find this secondary tissue-system 

 even in very young twigs, it must have followed very quickly 

 all the primary growths. But in the type of L. Wunschianum, 

 as well apparently as in L. Harcourtii, we find branches of 

 considerable dimensions in which it has not yet made its 

 appearance. In the anomalous branch of the former type 

 (C.N. 450), which has still a diameter of 23 centimetres, 

 though so much of its outer cortex has disappeared, its 

 Primary Xylem cylinder has a diameter of 36*5 millimetres 

 and its central medulla one of 24 mm. These latter internal 

 tissue-systems are invested by a very thin ring of secondary 

 xylem, which obviously began as a unilateral crescent like 

 those of L. Selaginoides. The importance of this specimen 

 resides in the fact that the transverse section of its Primary 

 Xylem cylinder is composed of more than 19,000 tracheids. 

 Contrasting the stele of this stem with C.N. 452, which 

 latter obviously came from the base of a tree two feet 

 in diameter, its exceptional condition becomes manifest. 

 Whilst these conditions unequivocally give support to the 

 views held by Solms-Laubach, and those who agree with him, 

 they are not free from difficulties of their own. Assuming 

 that these enormous developments of primary tissues origi- 

 nated at the base of the primary stem, close to a growing 

 point, what must the diameter of that point have been ? I 

 can only conclude that we still have much more to learn 

 about the growth and development of these remarkable trees. 

 If many of the Lepidodendra started their career from 

 growing points of such dimensions, it is remarkable that no 

 trace of any such growths have been discovered, and it is no 

 reply to this difficulty to affirm that we must not expect to 

 see such young structures in a fossil state. In fact, examples 

 of smaller dimensions are far from uncommon. My cabinet 

 contains a fine example enclosed in a nodule of clay ironstone, 

 the central axis of which is but one millimetre in thickness. 



