28 The Nature and Origin of Slijyules. 



by reduction but, as an organ cannot be reduced until it has been 

 developed, these are to be looked for above and not below the 

 perfect leaves, and are found in bracts, involucral scales and the 

 parts of floral envelopes, reduction taking place in inverse order to 

 the course of development, and only the most primitive structure, 

 the simple sheath, persists in the petals of most flowers. Re- 

 duced leaves are also common in parasites, and in the flora of 

 desert regions as is well illustrated in some of the Leguminosae 

 of Australia the leaves of which are little more than spines, or 

 are developed into bladeless phyllodia, while in the seedlings the 

 ancestral pinnate or bipinnate forms occur.* 



We thus have shown in each season's growth of a plant, though 

 not clearly in annuals because disguised in the seedling, a more 

 or less complete series of foliar organs which may for illustration 

 be compared with the vertebrate series among animals, the lowest 

 leaf-scales being comparable in degree of development to the sim- 

 ple structures of the fishes and the most highly developed leaves 

 to the complicated ones of mammals. Each leaf in the series is 

 equally perfect for the function it is intended to perform, but the 

 lowest of a lower tj-pe of organization, as are the fishes, and rep- 

 resenting an earlier stage in the phylogenetic series. 



Now in animals we look to the developing egg of the more gen- 

 eralized fishes for the least abbreviated erabryological recapitula- 

 tion of the early development of the vertebrate branch, for in the 

 mammals the early stages are passed through so rapidly and with 

 so many disguises as to be of comparatively small importance in 

 giving the history of the branch, unless viewed in the light of the 

 embryological development of the lower types. So the lower 

 foliar organs of a branch or shoot are embryologically of far 

 greater importance than the upper, for in the beginning of the de- 

 velopment of one of the upper leaves we have but the earl}^ stages 

 of a highl}' organized appendage. These earl^' stages are conse- 

 quently abbreviated and more or less disguised. The formation 

 of the stipules in the growth of the upper leaves is therefore not 

 a salient point in the consideration of our problem though it has 

 had much stress laid upon it, yet it is of interest to note that in 

 general the stipules appear earlier than the leaf-blade, thus giving 

 evidence that they are of more ancient origin. It ma}^ be added, 



*See Sir John Lubbock. On Seedlings. 1: 474. 1892. See also p. 440 as 

 to the similar case of Lathyrus Aphaca. 



