A. H. Lees 67 



from mite. In 4, provisionally included in the non-mite infected, the 

 recovery of margin was not complete even though the mite infection 

 was very small. Similarly in 10-11, also mite infected cases, the leaf 

 margin never completely recovered. Evidence pointing in the same 

 direction may be found on consideration of the relation of "oak leaf" 

 to mite infection. In all such cases so far examined mite has always 

 been found though not by any means always in the terminal bud. It 

 seems quite clear that the effect of a massive infection of mite is con- 

 veyed by some means to the terminal growing point when this is itself 

 quite free from direct infection. The oak leaf effect has never been 

 observed by the writer so far in bushes where no mite has been found, 

 though reverted leaves of the usual type are common on mite free shoots 

 in certain cases. Now before oak leaves are produced the shoot always 

 makes ordinary reverted leaves as in the series of figures in Figs. 40-46. 

 One may therefore hazard the opinion that the same cause which pro- 

 duces oak leaf will, when acting in greater dilution, simply produce 

 reverted leaves. So conversely, when one finds reverted foliage un- 

 associated with mite (other than temporary reversion) there is at least 

 an indication that either the same cause, namely mite, had been at work 

 and the mite has for some reason disappeared, or the plant is still 

 suffering from a residuum of previous infection. If these speculations 

 have any basis in fact, and the writer does not put them forth except 

 as speculations, then it would appear that reversion is largely quanti- 

 tative in action. That is to say, the effect will depend more on the dose 

 than anything else. This seems to be borne out in the cases examined. 

 Where, as in 7 and 8, isolated mite infected buds occur a depression 

 shortly follows in the graph, even when this should be normally ascending. 

 The secondary shoot in case 10 appears to be suffering in the same way 

 from the load of mite in all the buds of the primary shoot. 



The conclusions above arrived at are emphatically only preliminary. 

 The evidence supporting them is largely circumstantial and the reasoning 

 is largely in the backward direction. This was for the moment impossible 

 to avoid as no selected material was at hand, and no material could be 

 safely selected vmtil an effective method of diagnosing slight cases of 

 the disease had been found. This, the writer maintains, is furnished by 

 the leaf vein and margin method. It is now possible to select absolutely 

 healthy material during summer for experimental purposes for the 

 following season. It is hoped therefore that it will be possible to do 

 direct experimental work under controlled conditions and so avoid the 

 pitfalls hitherto unavoidable. 



5—2 



