62 Leaf Character in Reverted Black Cirrrants 



The behaviour of leaf 19 calls for an explanation. It stands as an 

 island of greater reversion in a sea of lesser reversion in the same way 

 that bud 13 stands as an island of mite infection in a sea of mite freedom. 

 The hypothesis immediately suggests itself that the two facts are con- 

 nected and that the isolated mite infection has caused an isolated drop 

 back into the more reverted state. In this case the two spots are six 

 buds away, not four. It is of course impossible to lay down any fixed 

 number of buds which would separate a mited bud from its possible 

 effect on the growing point. In practice this has been found to vary 

 between three and six, and four has been selected as an average to apply 

 to the graphs. Referring to Graph 7 the crosses (which indicate mite 

 infection) are generally followed by an increase of reversion. At first 

 sight one might argue that as a drop is always experienced in mite free 

 reverted twigs at this period (c/. Graph 3) so the drop in Graph 7 

 was due to this cause alone. On the other hand the drop is far more 

 marked than in mite free cases, the vein figure reaching one, while three 

 is usually the lowest reached in a mite free specimen. 



However this may be, the isolated mite infection occurring in bud 13 

 stands in a portion of the curve that should be ascending and neverthe- 

 less an increase of reversion follows shortly afterwards. It distinctly 

 suggests a close connection between mite infection and reversion. 



These changes are illustrated in Figs. 24-39. Fig. 24 is the third leaf 

 from the base. It is quite normal in every way. Fig. 25 is the sixth leaf 

 where reversion is just appearing in the margin though the venation 

 number is still five. Figs. 26-31 illustrate leaves 7-12 and show the 

 gradual reduction of the veins to one and the gradual coarsening of the 

 lobes of the margin up to Fig. 30; the nettle-leaf appearance of the whole 

 leaf has also been gradually increasing. Fig. 31 shows the most reverted 

 leaf of the whole series, and as has already been shown in certain cases 

 of temporary reversion such leaves tend to be deformed. It suggests 

 a very strong interference with the normal physiology of the plant. 

 From this point to the 17th leaf (Fig. 35) reversion became less. Figs. 31- 

 35 represent this initial improvement (leaf 14 missing). In Fig. 36, 

 representing leaf 19, the leaf vein number has dropped to three again 

 and the whole margin appeared more reverted. The differences in the 

 margin may be more clearly shown by analysing the figures for the leaf 

 on each side. Thus leaves 17 and 20 (18 was practically identical with 17) 

 have each seven points between the apex and the sinus, four of which 

 receive submain veins, while leaf 19 (Fig. 36) has only six, of which 

 three receive submain veins. There is therefore a distinct numerical 



