80 



The Journal of Heredity 



members served at once as an illustra- 

 tion and a proof of the development 

 of divergent forms from the same an- 

 cestral type. The same idea of meta- 

 merism was applied even to the higher 

 animals, in comparing the skull bones 

 with the vertebrae. 



Though the first thought in homology 

 is of structural correspondence, evolu- 

 tion is the fundamental idea. A dif- 

 ficult or doubtful homology involves 

 the idea of divergent paths of evolu- 

 tion, so far separated that the present 

 diversity has obscured the original 

 identity. But by tracing the evolu- 

 tionary paths something may be learned 

 of the original structure and of the 

 intermediate stages of development. 

 For progress in the study of heredity 

 as well as in breeding and selection, 

 it is desirable to know the evolutionary 

 status of the characters, so that rever- 

 sions and pathological abnormalities 

 may be distinguished from normal di- 

 versities or progressive changes of 

 characters. 



The most primitive forms of leaves 

 may have resembled the cylindrical 

 sheaths of Equisetum, or may have 

 arisen as expanded, sterilized stamens, 

 which is the usual interpretation of the 

 petals and sepals. Dififerent origins 

 may be indicated by the completely 

 different types of leaves that occur 

 on some plants, as the junipers and 

 eucalypts. It is of interest to trace 

 corresponding developments of leaf- 

 structure in the several families of 

 plants to see how far such structural 

 correspondences, or homologies, can be 

 estal)lished. 



Two Primitive Leaf-Elements 



Instead of the customary division 

 of the leaves of higher plants into 

 three structural elements, stipules, 

 petiole and blade, two primary ele- 

 ments may be recognized, the foot and 

 the blade. The foot represents a 



primitive sheathing leaf -base that en- 

 circled the next internode, while the 

 blade is the expanded portion of the 

 leaf, so variously developed in the 

 different families of plants. Stipules 

 and petioles, in this view, are sec- 

 ondary specializations. Stipules may be 

 considered as segments or subdivisions 

 of the primitive foot element, while 

 petioles apparently are of two or more 

 kinds, of different derivation and hence 

 not strictly homologous. Some petioles 

 represent a narrowed basal portion of 

 the blade element, while others have 

 develoj^ed by narrowing the terminal 

 portion of the foot element. A distinct 

 joint or pulvinus at the end of a 

 petiole may indicate the primitive ar- 

 ticulation of the blade element with 

 the foot element.' 



Stipules on Bud-Scales 



Although stipules are supposed to 

 be absent in the Juglandaceae, stipular 

 elements may be recognized in the 

 bud-scales, in rudimentary organs that 

 correspond directly to the lower pin- 

 nae of the compound leaf. Reasons 

 are found also for viewing the other 

 lateral pinnae as reduplications of the 

 lowest, stipular pair, so that only the 

 terminal unpaired leaflet is left to 

 represent the original blade element, 

 corresponding to the simple leaves of 

 seedlings ancl to simple leaves of other 

 plants. If these homologies are cor- 

 rectlv inferred, the evolution of com- 

 pound leaves may be traced in this 

 family, not through expansion and di- 

 vision of simple leaves, but through 

 development and reduplication of the 

 stipular elements. This interpretation 

 need not l)e refused because in other 

 families the stipules may have been 

 suppressed or retained as merely rudi- 

 mentary organs. ( See Figures 9 to 14, 

 with explanations describing several 

 variations.) 



^General Morphology of Leaves. Journal IVashington Academy of Scirnccs. Some 



leaves may consist of an expanded foot, with the blade element entirely suppressed, or 



possibly never developed. See Arber, Leaf-base Phyllodes among the Liliaceae, Bof. Gas. 

 69: 337- 1920. 



