368 PLANT GROWTH SUBSTANCES 



spring crop exceeded by almost 3 times those of the fall crop may be 

 partially explained in the fact that the average daily solar radiation in 

 gram calories per square centimeter received in the spring crop was 

 354.2 and 437.3 during the periods of fruit setting and fruit harvest, 

 respectively, compared with 183.2 and 102.6, respectively, for the fall 

 crop. Spartan Hybrid, an American Globe type of forcing tomato, 

 responded favorably to supplementary fruit-setting treatments in both 

 the spring and fall, while Improved Bay State, an English forcing tomato, 

 showed no response in the spring but gave variations comparable to 

 Spartan Hybrid in the fall. Differences in floral structure and quality 

 of the pollen of the two types as affected by light intensities and photo- 

 period have been offered as possible explanations of the varied results 

 obtained (5,20). In fruit size both varieties showed a similar response 

 in the two crops. 



The use of growth substances on outdoor tomatoes for improving 

 yields and fruit set has resulted in varied and contradictory data. Both 

 successes (27,29) and failures (31,34) have been reported. The usual 

 controlling factor for early fruit setting in field tomatoes is night 

 temperature. Exacting studies of Went and Cosper (49) under controlled 

 environments, those of Smith and Cochran (42) on pollen germination 

 and pollen tube growth, and our own (56) under field conditions have 

 estabhshed that the optimal range of night temperature for fruit setting 

 in tomatoes is 59° to 68°F. (15° to 20°C.). Temperatures below 55°F. 

 will cause failure of fruit set, even on early varieties, and irrespective 

 of the fact that the vines are making good vegetative growth and ap- 

 parently flowering normally. Little viable pollen is produced and much 

 of that appears incapable of normal germination and of producing tubes 

 of sufficient strength to traverse the style. Limitations on tomato fruit 

 set imposed by cold night temperatures (56,57) or extremely hot tem- 

 peratures (29) can be overcome by using hormone chemicals. 



Rather striking results have been obtained. Typical comparative data 

 on early yields, total yield, and fruit size of hormone-treated versus 

 nontreated plants obtained in the summer of 1948 in East Lansing, 

 Michigan, are given in Table 2. The explanation for the unusual results 

 in early yield is found in the low night temperatures prevaiHng during 

 the month of June (Fig. 3), which delayed fruit set by i to 3 weeks on 

 the nontreated plants. 



The averaged effect of a hormone spray consisting of p-chlorophcnoxy- 



