49 



LEAF CHARACTER IN REVERTED 

 BLACK CURRANTS 



By a. H. lees, M.A., 



Plant Pathologist, Agric. and Hort. Research Station, 

 Long Ashton, Bristol. 



(With 46 Text-figures and 11 Graphs.) 



It is well known that in marked cases of reversion in black currants 

 the leaf undergoes considerable modification. It becomes relatively long 

 in proportion to its breadth though generally of smaller surface than 

 normal leaves. In general effect it bears a considerable resemblance to 

 the leaf of the stinging nettle, and the disease for this reason is frequently 

 known as "nettle leaf." Further, when beyond the youngest stages such 

 leaves acquire a thicker texture and a darker colour and the serrations 

 of the margin become fewer and coarser. There is indeed no great diffi- 

 culty in identifying the disease in its advanced state. At the beginning 

 of an attack, however, it is by no means so easy to identify. Growers 

 usually put down such cases as "suspicious" or "going" and when 

 pressed for their reasons say that they judge from the general appearance 

 and not from any definite signs. 



Such an absence of definite data is not only unsatisfactory in the 

 field in view of the importance of propagation from absolutely sound 

 stocks, but it limits the experimenter very much in attempting any ex- 

 periments under controlled conditions. It is obviously important for him 

 to be able to identify with certainty the initial stages and to have some 

 means of marking the extent of the disease on a bush from year to year. 

 Further, unless some such means be at hand he cannot follow the course 

 of the disease during the season so as to trace any possible seasonal 

 variations. The method to be described was arrived at by careful com- 

 parison of a normal with a reverted leaf. It has the advantage of being 

 a numerical one and therefore independent of the general opinion of 

 the observer as well as of the size and shape of the leaf. 



If any leaf be looked at from the undersurface, it will be noticed that 

 there are five main veins arising from one point at the extreme base of 



Ann. Biol, ix 4 



