UNFRUITFULNESS ASSOCIATED WITH INTERNAL FACTORS 505 



a preventive. Dorsey^^ has observed the same occurrence in the plum 

 group in Minnesota. He mentions two cases 'in particular: "One 

 variety, Wickson, bore two heavy crops of crossed plums in the greenhouse 

 and the following year all pistils were aborted. In the second instance, 

 Wolf under orchard conditions bore heavily in 1914, and for three con- 

 secutive seasons afterward produced less than 1 per cent of normal 

 pistils." Hendrickson^^ mentions two French prune trees in California, 

 one of which bore a heavy and the other a hght crop in 1916. In 1917 the 

 conditions of these two trees were reversed. Paralleling these alter- 

 nations in crop yields were differences in the actual percentage of blossoms 

 setting and maturing fruit. In each case the light crop was due partly 

 to a poorer setting of the blossoms through exhaustion from heavy bearing 

 the previous season. 



Fruit Setting of Flowers in Different Positions. — Some fruits, like the 

 plum and cherry, bear on both shoots and spurs and it is to be 

 expected that slightly different nutritive conditions obtain in these diff- 

 erent tissues. Dorsey" studied fruit setting of the plum in these positions 

 and found a distinctly heavier June drop in the shoot-borne fruits. Some 

 of his observations are particularly interesting: 



"In the varieties available in this investigation^"" there was a pronounced 

 June drop in the plums borne on the terminal wood. In fact, on the older trees 

 fruit seldom matured in this position. The dropping of fruit from the terminal 

 growths can be partly accounted for on the basis of the competition from a 

 thorn or branch which is developed between the lateral fruit buds on the terminal 

 twigs the second season. This condition occurs over the entire outer area of the 

 tree. . . . Under favorable conditions fruit matures on the terminal shoots, 

 but the percentage to set is small considering the mass of bloom, and even the 

 small setting noted above is far in excess of the usual condition when there is a full 

 crop on the remainder of the tree. It is apparent that in this position competi- 

 tion takes place between fruit and branch as well as between different fruits."^' 



Strong and Weak Spurs. — A number of important correlations have 

 been reported between fruit setting in the apple and nutritive conditions 

 in the spurs or limb upon which the blossoms are borne. ^^ As between 

 limbs from the same trees, on those with a light bloom 73.8 per cent of 

 the spurs set fruit, while on those with a heavy bloom only 14.1 per cent 

 set fruit. Of the spurs on vigorous limbs with large leaves 41.6 per cent 

 set fruit; 15.7 per cent set on weak limbs with small leaves. Spurs that 

 lost all their flowers and fruit at the time of the first drop had the smallest 

 average number of flowers (4.45) and those that flnally set had the largest 

 average (5.74). Furthermore, a slightly higher percentage of the flowers 

 borne on spurs with many flowers actually developed into fruits than of 

 those borne on spurs with few flowers. Of 2066 spurs making more than 

 1 centimeter growth in length in 1915, 791, or 38.3 per cent, set fruit 

 in 1916; of 3,171 spurs making less than 1 centimeter of growth in length 



