EVOLUTION OF COMPOUND LEAVES IN 

 WALNUTS AND HICKORIES 



Variations Showing That Lateral Leaflets Correspond to Stipules 



of Bud-Scales 



O. F. Cook 

 U. S. Dcpavtuicut of Agriculture, Washington, D. C. 



UNUSUAL forms of leaves, in- 

 termediate between the bud-scales 

 and the normally developed 

 foliage, occur in several species of 

 Juglans and Hicoria, including Hi- 

 coria pecan, and may throw light 

 upon the structure and evolution of 

 leaf-forms in this group of plants. 

 Such aberrant leaves have bsen ob- 

 served in several localities, and ap- 

 parently are more frequent on trees 

 growing in rather unfavorable condi- 

 tions, or where the trees have been 

 injured so that latent buds develop 

 out of season. 



Reversion may be defined as the 

 expression of characters that normally 

 are latent, and such characters may 

 contribute to "bud-mutation" or other 

 forms of vegetative variation. Though 

 bud-scales are reckoned as specialized 

 leaves, some of the characters of bud- 

 scales may be primitive, as represent- 

 ing earlier stages of foliar evolution, 

 of the period when the ancestral plants 

 adapted themselves to seasonal changes 

 that made bud-scales necessary. 



The study of leaf-forms should not 

 be restricted by the idea that leaves 

 are mere apj^endages of the stem or 

 axial system, as formerly assumed by 

 many writers on plant anatomy. The 

 axis of the higher plants is not a sim- 

 ple structure but an aggregation of 

 vegetative internodes, each internode 

 representing a part of a metamer or 

 structural unit of a compound organ- 

 ism. Some of the metamers are not 

 represented by vegetative internodes, 

 as bud-scales, bracts, petals, stamens 



and i)istils. The plant body as a whole 

 is a colony of diversified metamers, so 

 that each joint or internode with its 

 leaf or leaf-equivalent is a distinct 

 morphological unit. The develop- 

 ment of the plant is shown by a suc- 

 cession of internode individuals of 

 different forms, produced one from 

 another. The jointed, internodal struc- 

 ture of the plant, and the evolutionary 

 significance of difl:'erent forms of leaves 

 on successive internodes, were recog- 

 nized clearly by Goethe in his poem 

 on the evolution of plants, published 

 in 1790. 



The modern evolutionary study of 

 homology began with Goethe, who 

 connected the two ideas of structural 

 correspondence and community of de- 

 scent. "The same organ," as Goethe 

 said, 'Vhich on the stalk has expanded 

 as leaf and taken a manifold diver- 

 sity of form, now contracts itself in 

 the calyx, expands again in the petal, 

 and contracts in the sex-members, to 

 expand finally as fruit." The need of 

 a general word was noted, "where- 

 with we could designate this so vari- 

 ously metamorphosed organ, and com- 

 pare all of the manifestations of its 

 form." Although Goethe's view of 

 the structural relations was condemned 

 by some botanists of the last century 

 as a "wildly absurd theory," it now 

 is generally accepted, and such terms 

 as internode, metamer and phytomer 

 are in use as names for the "metamor- 

 phosed organ." In the writings of 

 Goethe the diversities of the internode 



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