A FURTHER STUDY OF LEAF DEVEr.OP:MENT 



FREDERIC T. LEWIS 



In a previous paper {Amer. Nat, 1907, vol. 41, p. 431-441) 

 the writer discussed whether certain forms of adult leaves could 

 be regarded as due to arrested development, so that by compar- 

 ing the mature leaves of a given plant something of their embryo- 

 logical history could be learned. It was found that where leaflets 

 are formed embryologically from the base toward the apex, as 

 in most pinnate leaves, the terminal leaflet of the mature leaf is 

 often lobed. Where leaflets are formed from the apex toward 

 the base, as in most palmate leaves, the basal leaflets are often 

 lobed. In the rose, in which the leaflets are also formed from 

 the apex toward the base, neither apical nor basal leaflets are 

 lobed, but new leaflets appear near the stipules to which they are 

 often joined. In the previous paper the sumac and honey locust 

 were described as basifugal forms, and the blackberry and rose 

 as basipetal, the latter being of the stipular type. In the follow- 

 ing pages it will be shown that the basipetal and basifugal direc- 

 tions of growth may both occur in a single leaf; and that, although 

 one becomes predominant, evidences of the other are apparent. 

 In some cases a single species presents both pinnate and palmate 



The simplest form of compound leaf is three-parted or ternate, 

 and is produced by the lateral lobation of a simple leaf. The 

 stages in this process as seen in the mature leaves of Clematis 

 virginiana are shown in Figs. la-Id. A ternate leaf may be 

 basipetal in character and pass on to digitate forms with four, five, 

 or more leaflets, or it may be basifugal and produce pinnate leaves. 

 The leaf of Clematis, Fig. Id, exhibits both tendencies. Basi- 

 petal growth is manifest in the coarser teeth on the lower margins 

 of the lateral leaflets and in the fact tluit the part ..f each lateral 

 leaflet below i\w inidiil) \vi.l(>r than the part xihnxv. Basifugal 

 growth is sliown in the coarse tooth on either side of the apical 

 leaflet. Although Clemaiis viraimana stops ordinarilv at this. 



