5<x> Journal of Agricultural Research vol. xx, no. 6 



approach of low temperatures and remain dormant for a longer period 

 in the spring than any other of the Citrus plants. The trifoliate orange, 

 being deciduous, has a very fixed dormant period. The plants become 

 dormant in the fall with the approach of cold weather and do not start 

 growing until a period of favorable temperature is reached in the spring. 

 To summarize, the Citrus plants in Alabama become dormant in the fol- 

 lowing order, trifoliate orange, kumquat, Satsuma, and grapefruit. New 

 growth starts out in almost the reverse order, grapefruit, Satsuma, tri- 

 foliate orange, and kumquat. 



Thus, with other factors eliminated, grapefruit plants develop at the 

 lowest range of temperatures, both in the fall and spring, in the field. 

 In all cases, the leaves formed late in the fall and early in the spring are 

 much smaller in size and mature in a shorter period than those which are 

 formed later in the season. 



In the greenhouse experiments under control conditions it was found 

 that grapefruit could develop very slowly at 15 C. and also that in no 

 instance could any of the other plants used be pushed into growth at this 

 temperature. However, at 20 all plants became active, although the 

 calamondin, which resembles the kumquat in some respects, did not de- 

 velop rapidly until a temperature of 25 was reached. The differences in 

 the size of the leaves and time required for their maturation in comparison 

 with those obtained at 30 were also noticed at the lower temperatures, 

 grapefruit leaves being one-fourth to one-half the size of those produced 

 at 30 . It was likewise observed that 16 to 20 days were required at 30 

 to complete the maturation of the grapefruit leaves, while at 15 7 to 8 

 days were sufficient. 



Thus, a mean temperature of 15 C. or thereabouts is sufficient for 

 starting active growth of grapefruit plants in the field, while tempera- 

 tures of 20 or slightly less are needed for the trifoliate orange and 

 Satsuma. Kumquat does not start until a slightly higher mean is 

 reached. These figures are borne out by the weather records and obser- 

 vations of the conditions of the plants in the field for the past four years. 



The optimum temperature for the growth of the Citrus plants used in 

 the greenhouse experiment lies between 20 and 30 C. Some differ- 

 ences were noted in the behavior of the different plants at these tempera- 

 tures. There is no question but that at 30 the best development of all 

 the plants occurred. Above 30 grapefruit was inhibited, while plants 

 like the trifoliate orange seemed to make as good a growth as they did 

 at 30 . 



The short rest periods of Citrus plants during the growing season are 

 in all probability a maturation phase, following the period of elongation 

 of the new growth. Field observations have shown that temperature 

 and humidity play an important part in the rate and amount of growth 

 made during these periods; in fact, they determine to some extent the 

 number of growth periods which occur during a season. 



