THE LEAF. 



237 



type of phyllotaxis in the Urticacege. Vuillemin 

 supposes that the alternate arrangement came about 

 by suppression of one of the two leaves of each pair, 

 for this actually occurs sometimes in the seedling stage. 

 This view seems, however, less likely than the view that 

 the alternate arrangement arose as a result of the fusion 

 of the two leaves of each pair ; this would certainly 

 best explain the dividing leaves, for each is merely 

 retracing the steps taken in the past, and undoing 

 the act of fusion in order to reproduce the original 

 pair of leaves. This might well be caused by the 

 pendulous habit of the tree causing the branches to 

 grow in a more vertical direction than is normally the 

 case; the distichous arrangement is probably the 

 direct result of the oblique or horizontal direction of 

 growth of the shoots ; in the erect-growing first year's 

 seedling the opposite-decussate type prevails. Meehan 

 found that in U. americana some individuals exhibited 

 opposite-decussate, others alternate phyllotaxis, and 

 this from the earliest stage. 



The phyllotaxis presented by many of the abnormal 

 shoots of this elm suggests that after the production 

 of two leaves by fission of a single leaf, one leaf of the 

 pair becoming pushed over into an anterior or posterior 

 position on the shoot, an internode becomes developed 

 between the two leaves, all of which gives the impres- 

 sion that a J- phyllotaxis is being incompletely induced, 

 and this may indeed be the case. 



Loesener observed a plant of maize in which the 

 leaves were in whorls of three instead of being 

 distichously arranged. 



There are thus at least three distinct types of forked 

 or double leaves : 



(1) Those which have no direct morphological sig- 

 nificance, being comparable to a forked stem, as in ferns. 



(2) Those which are due to fusion of two leaves, as 

 in the case of Lonicera thibetica and Klein's case of 

 Weigelia. 



(3) Those which are due to fission and concerned 



