166 



PRINCIPLES 



OF PLANT-TERATOLOGY. 



the wych-elm (U. montana). The phenomenon may 

 perhaps be regarded as a reversion to the complete, 

 symmetrical type of leaf which was probably the 

 ancestral form, and which occurs normally in other 

 species of elm and in other genera, such as Planera 

 and p Zelkoiva. In other leaves of U. glabra a leaflet 

 occurred on the opposite complete side of the rachis, 

 its presence being clue to a portion of the lamina 

 be^ng separated off on that side, whereby the leaf -base, 

 apart from this separated leaflet, became quite sym- 

 metrical ; in another leaf the presence of a distinct 

 notch near the leaf-base on the same side showed that a 

 leaflet was in process of being cut off. In another 

 leaf both leaflets occurred on the same side, one below 

 the other, while on the other side was a long stretch 

 of leafless rachis ; hence on this latter side leaflets 

 had been cut off and completely eliminated, on the 

 other the leaflets still remained. The two (rather 

 smaller) leaves occurring side by side, which resulted 

 from the complete bifurcation of a leaf, had often 

 very long petioles owing to the fact that leaflets had 

 been cut off on both sides of the rachis and completely 

 eliminated. The theory that the normal gap in the 

 leaf-blade of the elm is due to the elimination of a 

 portion of the leaf-blade is rendered probably true by 

 two facts : firstly, as stated above, other species of 

 elm and other genera of the tribe, though not all, 

 have symmetrical leaves and short petioles ; secondly, 

 a series was observed in TJ. glabra, on the same shoot 

 beginning with a leaf which had a fairly short petiole 

 and an almost symmetrical base, but in which the first 

 indication of a leaflet being cut off on one side was 

 present in the form of a distinct notch a short way 

 from the base; another leaf showed the usual large 

 leaflet excised on one side ; another leaf showed in 

 the gap a leaflet of very small size, evidently on the 

 road to extinction ; yet another leaf showed the 

 normal asymmetrical base with no leaflet present at 

 all. These abnormalities seem to reproduce for us 



