- 86 BOTANICAL GAZETTE [aucusT | 
almost completely eliminated. In vascular anatomy therefore, the 4 
Cycadales have retained more ancient features than any other liv-_ 
ing group. 
The vascular condition among Gnetales can hardly be spoken of 4 
in connection with general tendencies; but the appearance of true 4 
vessels associated with the tracheids of the secondary wood is too | 
important to omit. In any event, these true vessels of the secondarill 3 
wood suggest that in the evolution of the vascular cylinder the original - 
tracheids of secondary wood are finally and gradually replaced by 
true vessels. ia 
sperms possessed ample, fern-like leaves, and under appropriate z 
conditions this type of leaf persisted, as in the tropical cycads of — 
today. The conifers, however, have developed a very different — 
type of leaf, one that was well under way among the Cordaitales, and 3 
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 3 
other structures. The leaf is too variable a structure, and too closely E 
related in its work to external conditions to permit such a an explana-_ 
tion of its changes. 4 
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 chan 
were associated with the evolution of the land areas and of the climate 
of the temperate regions. 
THE STROBILUS 
The Cycadofilicales are the only gymnosperms ca. strobili. 
Although the sporophylls differ more or less from the fern-like ” 
foliage leaves or their branches, they are not aggregated into a strobilus 
distinct from the rest of the shoot. The organization of a strobilus 
