66 Leaf Character in Reverted Black Currants 



had it not been highly mite infected. In Table II, No. 11 a horizontal 

 line is drawn at the spot where the first growth ceased. 



Leaves 1 and 4 of the second growth shoot have the low vein number 

 of two almost certainly owing to temporary reversion. Indeed, the whole 

 shoot is not long enough to be certain that one has completely got rid 

 of temporary reversion. On the whole the leaf vein numbers are on the 

 low side though not so low as the secondary shoot of case 10. They indi- 

 cate that they are still under the influence of the infected buds at the 

 base of the primary shoot, but owing to the infestation being much less 

 than in the case of the primary shoot of 10 the effect is also much less. 



DISCUSSION. 



These eleven cases may be divided into three classes, normal shoots 

 as No. 1, reverted but mite free as Nos. 2-6, and reverted and mite 

 infected as Nos. 7-11. With the exception of 5 and 6 all showed a vein 

 number of about five {i.e. normality) at the beginning of growth. Both 

 Nos. 5 and 6 were hard cut back shoots and these nearly always begin 

 growth at a later period. Therefore it would appear that as a rule re- 

 verted bushes will commence the season with normal leaves. 



Secondly all the shoots showed a drop in leaf vein number at the 

 height of the growing season in May and June. The drop was slight in 

 the normal shoots, well marked in non-infected reverts and still more 

 marked in infected reverts. It would appear therefore that there are 

 three factors at work, a seasonal one tending to reversion of a temporary 

 kind, a reversion factor and a mite factor. One can of course never be 

 sure unless one has carefully selected material that the fall in leaf vein 

 number may not be due to the reversion factor only and not to the mite 

 factor or that it may be due to both. In the mite infected cases above 

 cited care was taken to select as far as possible only those shoots coming 

 from bushes which otherwise appeared normal. Even supposing that all 

 these cases would have proved reverted without mite infection, never- 

 theless the amount of fall is greater than in pure reversion cases and one 

 may therefore presume that the mite infection has increased the eifect. 

 The same conclusion can be drawn from the behaviour of the ends of 

 the graphs. Where the graph is long enough to show complete recovery 

 the final leaf vein number is five or more for reverted cases (Nos. 3-6), 

 but less where mite infection is not confined to the basal series (8). In 

 7 and 9 the recovery of the leaf vein number is practically complete, 

 but the margin does not regain normality in either though it does in 3, 

 the only reverted case with description of leaf margins completely free 



