482 
AMERICAN JOURNAL OF BOTANY 
[Vol. 10, 
(Roberts, 1920). While tree performance during the bearing and “off- 
years” indicates marked variability in the physiology of the tree as a 
whole, spur analyses and defoliation experiments show a similar difference 
more locally (Heinicke, 1917; Wiggans, 1918; Hooker, 1921, and others). 
Control Measures. It may be of interest to investigators in other fields to 
note briefly some of the methods of controlling sterility which have been 
worked out to date. The remedy is of course suggested largely by the cause. 
Means of counteracting the influence of unfavorable weather have been 
given less attention than nutritional genetic causes. Orchard heating has 
been used more in the West than in the East and is effective only within 
a limited range. There may be some advantage in some seasons in extend¬ 
ing the period of bloom either by a succession of varieties or by an extended 
period of bloom with the variety, but this is not always effective in avoiding 
the effects of prolonged adverse weather. There may be some promise of 
control within narrow limits from the use of pollenizers whose pollen will 
develop tubes at lower temperatures than others. This suggestion comes 
from some of the hardier plums from northern sources. There is a still 
more readily available remedy in the early application of nitrate of soda, 
which has given some indication of effectiveness in recent preliminary tests. 
All told, however, weather will still prevent the set in spite of any of the 
remedies yet tried when extreme conditions are encountered. 
Given favorable weather and growth conditions, a remedy for the genetic 
causes of sterility is found in mixed planting. This phase of sterility has 
been given considerable attention, and control measures are well known as 
a result of the studies made of the self- and cross-relationship of varieties. 
The effect of aborted pollen, the dioecious condition in the grape and the • 
strawberry, as well as incompatibility, are all counteracted by properly 
mixing varieties. In fact, the problem can be eliminated in the grape and 
strawberry by the selection of varieties with perfect flowers. Two methods 
have come into common use in mixing varieties for pollination purposes— 
top working and inter-planting. The former is usually adopted in older 
plantings to correct mistakes and the latter in new plantings. Serious 
errors have been made in some orchards by planting trees or vines alternately 
in the rows instead of in narrow blocks or strips. When varieties are 
planted alternately in this way, both spraying and harvesting are made 
more difficult. In making mixed plantings careful attention should be 
given to the relative time of dehiscence of the anthers and to the receptive¬ 
ness of the stigma. 
The details of bringing about the optimum nutritional conditions for 
fruit-bud initiation and formation, fruit-setting, and fruit-development 
have as yet only been approached. The control of the carbohydrate- 
nitrogen-moisture ratio, appears to be a workable point of attack. The 
investigations to date are sufficiently definite to indicate that each of the 
fruits presents a problem different in many respects. On account of the 
