EVOLUTIONARY TENDENCIES AMONG GYMNOSPERMS 413 



less directly to the conditions of living. The most ancient gymno- 

 spcrms possessed ample, fernlike leaves, and under appropriate con- 

 ditions this type of leaf persisted, as in the troj)ical cycads of today. 

 The conifers, however, have develojX'd a very different type of leaf, 

 one that was well under way among the Cordaitales, and which reaches 

 an extreme expression in small and rigid needles or concrescent scales. 

 This cannot be regarded as the result of a general tendency among 

 gymnosperms, quite unrelated to conditions of living, such as is shown 

 by the persistent progressive changes of other structures. The leaf 

 is too variable a structure, and too closely related in its work to exter- 

 nal conditions to permit such an explanation of its changes. 



It would be interesting to know the conditions in which needles 

 and concrescent disks were established; but in the absence of any 

 such knowledge, the sharply contrasted geographical distribution 

 of Cycadales and Coniferales may suggest that the conditions of 

 change were associated with the evolution of the land areas and of 

 the climate of the temperate regions. 



THE STROBILUS 



The Cycadotilicales are the only gymnosperms without strobili. 

 Although the sporophylls differ more or less from the fernlike foliage 

 leaves or their branches, they are not aggregated into a strobilus 

 distinct from the rest of the shoot. The organization of a strobilus 

 by the shortening of the sporophyll-bearing shoot is a conspicuous 

 feature of gymnosperms, and it must have been derived from the 

 condition observed among the Cycadofilicales. 



The Cordaitales were the first gymnosperms to produce strobili, 

 and this is one of their conspicuous contrasts with Cycadofilicales. 

 The record of the structure of their strobili is meager, but it shows 

 several tendencies in strobilus-formation. Of primary importance 

 is the fact that the strobili are monosporangiate, and this monospo- 

 rangiate character prevails throughout the Ginkgoales and Coniferales. 

 Among the Gnetales, a group probably related to the general conif- 

 erophytc phylum, bisporangiate strobili occur in Welwitschia. If this 

 connection be accepted, therefore, these bisporangiate strobili have 

 been derived from monosporangiate strobili. It is not necessary 

 to associate in one genetic connection all of the bisporangiate gymno- 



