230 MORPHOLOGY OF GYMNOSPERMS 



their occurrence a character of the first importance. For example, 

 the leaves of Thuja occidentalis are extremely reduced, but the leaf 

 gaps are none the less in evidence; and the same is true of all the 

 numerous conifers examined (77). The conditions, therefore, which 

 resulted in the small leaves of conifers, did not result in obliterating 

 the evidence of their derivation from large-leaved forms. As Jeffrey 

 has stated it (77), the small leaves of conifers are of ecological rather 

 than of phylogenetic interest. This is a good illustration of what 

 he calls cenogenetic and palingenetic characters; the small leaves 

 of Lycopodiales are doubtless palingenetic, but the small leaves of 

 conifers are cenogenetic. An old feature may reappear, therefore, 

 in a recent form, without indicating any phylogenetic continuity. 

 This fact is of great importance, for it compels a decision, in every 

 occurrence of a primitive feature, whether it is really the retention of 

 an ancient character (paUngenetic) or the reappearance of an ancient 

 character (cenogenetic). This decision must generally be a matter 

 of opinion, but it is often made probable by its association with other 

 characters. 



As has been stated, in the more primitive groups of gymnosperms 

 the so-called double leaf trace occurs, and this is naturally regarded 

 by many as a primitive feature. Among the Pinaceae the double 

 leaf trace is characteristic of the Abietineae, and is not a feature of 

 the Cupressineae at least. Whatever the details may be for the other 

 tribes, if this is a primitive character retained by the older types of 

 gymnosperms, it has begun to disappear among the Pinaceae. From 

 this point of view, one could predicate the presence of the double 

 leaf trace in Araucarineae and its disappearance from Taxodineae. 

 Sewaiud (117) has called attention to the fact that the persistence of 

 leaf traces, even in old trunks, is characteristic of araucarians. Chau- 

 VEAUD (83) has dissented from the view that the double leaf trace 

 is a primitive feature, calling attention to the fact that the trace con- 

 nected with the cotyledons of Pinus and Abies is single, and in the 

 course of ontogeny splits into two for the later leaves. He concludes 

 that the double leaf trace is a secondary formation. The cotyle- 

 donary vascular strand has been found to be single in all the Pinaceae 

 investigated, except in Araucarineae, and except that occasionally 

 in a species of Cupressus (C. torulosa) there are found two separate 



