The Nature and Origin of Stijmles. 27 



ceive to develop under conditions somewhat more primitive than 

 is the case with aerial buds. But in both these the recapitulation 

 of the development of leaf-forms ma3^ be traced with a consider- 

 able degree of confidence, from the primitive sheathing protective 

 scale to the most highly differentiated and complex of modern 

 leaf-forms. 



It is at this point that the fragmentary geological evidence 

 sheds its strongest light on the problem under consideration. In 

 the Cretaceous and Tertiary floras which preceded the modern, 

 the present degree of differentiation had not as yet been attained 

 and but few modern species made their appearance before the 

 close of the Tertiary.* The species, however, which immediately 

 preceded those which now exist were very closely related to them, 

 being their immediate ancestors, and differed from them only in 

 showing a somewhat lower degree of differentiation, and their 

 leaf-forms are accordingly more primitive than those of the ex- 

 isting species which have descended from them. 



Now it is a well-established fact that the lower leaves of young 

 branches and shoots, and especially of those which spring from 

 the stumps of felled trees, are frequently unlike the adult forms 

 which occur higher up and bear a close resemblance to the fossil 

 leaves of extinct species, so close indeed, as oftentimes to be in- 

 distinguishable from them. This is strong evidence in favor of 

 the doctrine that the lower foliar organs represent not reduced 

 leaves, as botanists have commonly supposed,f but the primitive 

 foliar organs, and that in an ascending series from the lowest scale 

 to the mature adult leaves of the upper part of the stem, giving 

 a more or less perfect summary of the phylogenetic development 

 of the foliar organ from the most primitive type upward to the 

 most highly differentiated. J In other words, a single stem may 

 represent the whole phylogeny of the foliar organs of its type. 

 It is true that there are simple leaf-forms which have become so 



* Our modern species of Corylus are recorded from the Eocene by Professor 

 J. S. Newbury. Later Extinct Floras of North America. Ann. N. Y. Lye. 

 Nat. Hist. 9 : 59-6i). 1868. 



t See DeCandolle. Org. Veg. 2 : 212. 1827. 



J "Most modern botanists now regard the varying forms of leaf seen on young 

 shoots and near the base of trees as valuable hints at the probable stages 

 through which the final forms have passed in the history of their development." 

 Professor L. F. Ward. Proc. U. S. Nat. Mus. 11 :41. 1888. 



