Summary.The first lateral leaflet on the left of the mid‐rib of each leaf is measured, and its index breadth/length X 100 calculated.The Leaf Index of a variety must be ascertained from adult leaves on a healthy plant.The younger leaves where the long axis is less than two‐thirds its final length will have a somewhat higher index than the normal.Leaves of secondary growth have, at first, a higher index.Leaves affected with slight mosaic mottling have a reduced index.The variation of the index within any given variety is a normal one and represented by a normal frequency curve.The mean value of the p.e. for one observation when 36 varieties are examined of each of which a minimum of 20 leaves have been measured = 2·36.The p.e. of the difference of two means of 20 each is 74. A difference of two units in the Index may be considered as indicating odds of about 18: 1 against the two varieties being the same, and a difference of 3 units as being almost certain evidence of such distinction, the odds being over 140: 1.Of 65 varieties of which the index was measured, the value of the latter varies between 50 and 72.Neither the place of origin of the seed tubers, nor the locality where the plants are raised have any effect on the Leaf Index, always providing that the plants are healthy.The Leaf Index is a constant for each variety.There is some correlation between early varieties and a low index, and late varieties and a high index.The Leaf Index has given valuable evidence as to the identity of suspected synonymous stocks growing for trial at Orms