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THE AMERICAN NATURALIST 



\Yo-L. XLI 



basal leaflets are often joined to the thin sheath-like stipules. In 

 fact the relation of the leaflets to the stipules is strikingly like that 

 in the rose, as shown in Figs. 15a-15c. In the first there is a well 

 developed leaflet proceeding from the stipule on one side, and there 

 is no corresponding opposite leaflet. In the second the stipules 

 are prolonged into small green leaf-like appendages, and in the 

 third the small pair of leaflets above and separate from the stipules 

 suggests a stipular origin. If this is true, leaflets in Slum are added 

 from both ends, and the basal pairs of leaflets are not always homol- 

 ogous as stated by Shull.^ 



Shull's study of Sium supplies an admirably complete record 

 of the leaf-forms presented by a single species. They are, however, 

 considered from the biometric rather than the embryological point 

 of view. Thus the early leaves are divided arbitrarily into six 

 groups or categories. One of these contains the ternate leaves 

 with basally lobed lateral leaflets and three lobed terminal leaflets, 

 — that is, leaves like those of the poison ivy (Fig. 2a) and Negundo 

 (Fig. 5a). This fundamental class which exliibits symmetrically 

 the basipetal and basifugal directions of growth, is described as 

 simply a special case of variously notched three-parted leaves 

 " which was separated from the others only because it could be so 

 definitely characterized." Although Shull includes only 20% of 

 the first lea^'es of Sium in this category, a large proportion of the 

 forms placed in the remaining five groups are but variations of this 

 type, — the terminal lobes may be suppressed on one or both sides, 

 the basal lobes may be secondarily notched, etc. The study of 

 Sium shows that the leaves at first exhibit both basipetal and basi- 

 fugal tendencies and that the latter becomes predominant. 



The leaves of very diverse species show a common method of 

 leaf development in which the basipetal and basifugal directions 

 of growth arc com}>ined. Tliis is shown by the widespread occur- 

 reiuc of \]w ternate leaf witli the three lobed apical leaflet and 



