THE LEAF. 



167 



the stages which have been gone through in the past 

 in the elimination of a portion of the leaf-blade on 

 one side. It may be mentioned that leaves of U. 

 glabra frequently occurred bearing three basal leaflets 

 (fig. 44 6), two on one side and one on the other, 

 showing that the process of excision of portions of 

 the lamina may occur on both sides of the leaf. Some 

 of these phenomena are therefore probably to be re- 

 garded as retrograde or reversionary, viz, those in 

 which leaflets occur, filling, or partially so, the normal 

 gap ; others as progressive, viz. those in which the 

 normally complete half of the leaf-base becomes split 

 up or eliminated. 



It has been suggested, and probably correctly, that 

 the asymmetrical form of the elm-leaf is due to the 

 dorsiventral character of the shoot and to the econo- 

 mical distribution of the leaf-surfaces to receive as 

 much of the incident light as possible ; on these shoots 

 the gap on the lower* side of each leaf is filled by the 

 complete half of the leaf next below it, whereas if the 

 lamina of the lower half of the leaf were also com- 

 plete it would be shaded and excluded from the light 

 by the leaf next below. Vuillemin suggests that when, 

 as would probably occur in this pendulous and spe- 

 cially vigorous variety, the direction of growth of the 

 shoots became altered, a partial reversion to the sym- 

 metrical leaf might occur ; or if the shoots became in 

 places abnormally shaded the lamina might become, 

 as a result, more dissected and incomplete than is 

 normally the case, and thus, perhaps, all these various 

 forms of abnormal leaves might be accounted for. 



In the specially vigorous, erect shoots of the 

 English elm (U. campestris) produced by hedge- 

 clipping, the leaves are nearly always symmetrical. 



In some plants of the hollyhock (Althaea rosea) the 

 lower axillary buds of the flowering stem had com- 

 pound leaves consisting each of two perfectly sepa- 

 rated, short, roundish leaflets ; others were undivided, 



* I. e. the side nearest the base of the twig. 



