234 



PRINCIPLES OF PLANT-TERATOLOGY. 



shows us another method by which reversion to the 

 primitive spiral arrangement is attained ; in this case 

 the transition is complete and perfect from the earliest 

 stage onwards. 



Consistently with the view as to the origin of the 

 Dicotyledonous character of the seedling, and having 

 regard to the great influence which the cotyledonary 

 has npon the subsequent nodes, we are constrained 

 to believe that the first one or few pairs of opposed 

 foliage-leaves each arose by fission of a single leaf in 

 the same way as did on this theory the cotyledons, or 

 at least that in many plants this may have been their 

 mode of origin. Higher up on the stem, however,, 

 away from the influence of the cotyledonary node, the 

 leaf- pairs probably arose from the extreme shortening 

 and eventual abortion of alternate internodes* ; and 

 in cases where the influence of the cotyledonary node 

 is nil (and such might occur) internode-suppression 

 would be the sole method of formation of opposite 

 leaves. The cases of fusion of a pair of opposite 

 leaves observed by Braun, Jannicke, Steinheil, Clos, 

 and the present writer support the above view ; if these 

 are not reversions how else can they be explained ? 

 But the phenomena of displacement are certainly 

 commoner than those of fusion. The alternate phyllo- 

 taxis of the stem, immediately succeeding and caused 

 by the single cotyledon resulting from the fusion of 

 two in the abnormal seedlings mentioned, springs into 

 being congenitally and complete, so that we cannot 

 trace the steps by which it arose. 



During the displacement which occurs from the 

 opposite to the alternate arrangement it may happen 

 that two leaves, from pairs above and below, may 

 fuse together to form a double-leaf, with distinct 

 apices and two separate midribs ; such a fusion would 

 seem to occur for the purpose of economising space on 



* Cf. the normal formation of false whorls in Lilium, Fritillaria, Paris, 

 etc. However, even in the higher region of the stem the opposite-decussate 

 arrangement may often have arisen by division of the alternating leaves. 

 This also appears to be the view of Velenovsky (see his figure of Vincu). 



