COMPARATIVE ANATOMY OF GYMXOSPERMS 309 



in the centripetal cryptogamic wood. On the flanks of the 

 bundle the centripetal wood is continuous with the short-pitted 

 cells of the " transfusion tissue " discovered by Frank in 1864. 

 In the bundles of the adult leaves of most of the living Con- 

 ifer ales there are only the very slightest traces of centripetal 

 wood. Worsdell has reached the interesting general conclusion 

 that the " transfusion tissue which occurs almost universally 

 in the leaves of gymnospermous plants as an auxiliary con- 

 ducting system has been phylogenetically derived from the 

 centripetally formed xylem of the vascular bundle." 



Fig. 112, Z, shows the topography of a cross-section of a 

 branch of Thuja occidentalis. The leaves in this species are 

 extremely reduced, especially those occurring on the upper and 

 lower sides of the flattened branches. It might naturally be 

 expected that under these circumstances the foliar gaps would 

 be obscure or absent, but such is not the case, for subtending 

 the traces, which pass to the specially small leaves on the upper 

 and lower sides of the flattened branch, are two distinct foliar 

 lacunae. An examination of a large number of Conifers, some 

 with a very considerable xerophytic reduction in the size of 

 their leaves, has shown that the presence of foliar gaps is quite 

 constant in the group (Jeffrey 19 ). It is now known that foliar 

 gaps are unfailingly absent in the tubular central cylinder of 

 living and fossil Lycopodiales and Equisetales, while on the 

 other hand they are invariably present in the Filicales. Hence 

 it may be assumed that the Coniferales, much as they resemble 

 the Lycopods in external appearance, are really derived from 

 filiciriean ancestors by adaptation to a xerophytic mode of life. 

 The microphyllous habit is obviously a cenogenetic adaptation, 

 for the structure of the fibrovascular skeleton plainly indicates 

 that the coniferous stock is palingenetically megaphyllous, and 

 thus allied to the Ferns. 



Fig. 112, A A, shows the structure of the root of Pinus 

 Strobus. The cortex and phloem surround a considerable mass 

 of secondary wood, in the center of which may be distinguished 

 the exarch primary wood. This feature is more clearly seen 

 in Fig. 112, BB, which represents the center of the section 

 shown in Fig. 112, A A, more highly magnified. It is an in- 

 teresting fact, to which Van Tieghem 4 has drawn attention, 

 that the mode of growth of the primary wood is the same in all 



