1921.] 



Reversion of Blackcurrants. 



1123 



(1) Leaf Venation. — The blackcurrant leaf venation is 

 palmate in type. That is to say, the chief veins spread out like 

 the palm of the hand, and, in this case, include five members. 

 They all originate at the same point, which is situated at the 

 extreme base, and run to the main points of the leaf, which, for 

 convenience, have been labelled A, B, C, D and E. These may 

 therefore be called main veins. The next point to notice is the 

 number of subsidiary veins running from the median vein or mid- 

 rib (that running to C) to points on the leaf margin. It should be 

 observed that the vein running to D, being a main vein, is 

 not counted as a subsidiary vein. The first one to count is 

 that immediately above it, running from the midrib to the 

 margin. All the veins can be seen best from the underside. 

 In Fig. 1, there are five of them, numbered 1-5, each of which 

 may be called a submain vein, and each ends in a point in the 

 margin. It will be noticed that not all the points in the margin 

 receive submain veins. As a matter of fact, they are innervated 

 by veins of a lower order, which branch off from the submain 

 veins. These need not be considered, as they are of no use in 

 identifying the disease. The first character, therefore, in 

 identifying the disease is that a normal leaf has at least five 

 submain veins running from the midrib to a point in the 

 margin. Sometimes the number may run up to seven, but 

 never descends below five. Usually, the submain veins on each 

 side of the midrib are the same in number, though slightly 

 different in position i it is immaterial which side is counted. 



(2) The Leaf Margin. — The second point to note is the 

 character of the leaf margin. In normal leaves, such as those 

 shown in Fig. 1, the margin is finely serrated, and there are a 

 number of pointlets which do not receive a submain vein. This 

 may, if necessary, be reduced to a numerical basis, though in 

 practice it is not usually necessary. Thus, on counting the num- 

 ber of pointlets not receiving submain veins between C and the 

 sinus between C and D (marked X), it is found that there are eight. 

 To sum up: the characters that make a perfectly normal leaf 

 are (a) at least five submain veins, and (b) a finely serrated 

 margin with many pointlets not receiving submain veins. 

 To bring out the points of difference, it is best to compare 

 a typical reverted leaf, as shown in Fig. 4. For convenience of 

 reproduction, only half is showm. This type is a very common 

 one in reverted bushes and will be readily recognised by 

 growers. Here the number of submain veins has been reduced 

 to three, and the margin is noticeably coarse in outline. If 



