342 Ever-sporting Varieties 



the mountain-ash {Sorbus Aucuparia) has six 

 pairs of leaflets in addition to the terminal one. 

 But this number varies slightly, the weaker 

 leaves having less, the stronger more pairs than 

 the average. Such however, is not the case 

 with ternate leaves, which seem to he quite con- 

 stant. Four leaflets occur so very rarely that 

 one seems justified in regarding them rather as 

 an anomaly than as a fluctuation. And this is 

 confirmed by the almost universal absence of 

 two-bladed clover-leaves. 



Considering the deviation as an anomaly, we 

 may look into its nature. Such an inquiry 

 shows that the supernumerary leaflets owe their 

 origin to a splitting of one or more of the nor- 

 mal ones. This splitting is not terminal, as is 

 often the case with other species, and as it may 

 be seen sometimes in the clover. It is for 

 the most part lateral. One of the lateral nerves 

 grows out becoming a median nerve of the new 

 leaflet. Intermediate steps are not wanting, 

 though rare, and they show a gradual separa- 

 tion of some lateral part of a leaflet, until this 

 division reaches the base and divides the leaflet 

 into two almost equal parts. If this splitting 

 occurs in one leaflet we get the ' ' four-leaved ' ' 

 clover, if it occurs in two there will be five leaf- 

 lets. And if, besides this, the terminal leaflet 

 produces a derivative on one or both of its sides, 



