Dec. is, 1920 Effect of Temperature and Humidity on Citrus-Canker 465 



In no instance were the small, undersized leaves, which were pushed to 

 maturity at the low temperatures, affected when transferred to a higher 

 temperature. Thus, a leaf that has once reached its maturity can not 

 be made to increase in size by a change of environment. 



At the temperatures of 20 , 25 , and 30 C, growth responded at a 

 normal rate where the citrange and grapefruit plants were in active 

 condition. In general, no differences were noted in the rate of growth 

 at these temperatures. Thus, the optimum temperature is between these 

 points for the plants named above. With one exception, all dormant 

 grapefruit and citrange plants were forced into active growth. The 

 plants and leaves also made a rapid and large growth and reached 

 maturity rather slowly. Not much difference was noted between the 

 plants kept as controls under greenhouse conditions and those grown at 

 the temperatures named. Apparently, calamondin has a little higher 

 optimum temperature, since little or no growth occurred at 20 . 



EXPERIMENT 3 



This experiment was carried out with a view of determining the rate 

 of growth under a varying day and night temperature. Thus, plants 

 were exposed during the day at 30 C.,and during the night three different 

 sets of plants were placed at temperatures of io°, 15 , and 20 . The 

 bell jars with the plants were shifted from the 30 case at 5 p. m. and 

 replaced at 8 a. m. the next day. 



Two plants each of the trifoliate orange, calamondin, grapefruit, and 

 one of the Rusk citrange were used in each set. The experiment was 

 carried out under the same conditions as the others described above. 



As will be noted in Table VIII, the plants held at 30 C. throughout the 

 experiment produced the most growth. Where a day temperature of 

 30 and a night temperature of 20 were used, there was a very slight 

 slowing down of the growth in all plants except the grapefruit. When 

 night temperatures of 15 and io° were used, there was a decidedly 

 slower growth. However, growth was not checked, especially with the 

 rapidly growing grapefruit plants. The maturation of the leaves was 

 also more rapid at the low night temperatures. Thus, a night temper- 

 ature lower than that at which growth normally occurs merely slows up 

 the growth somewhat so long as a high day temperature prevails; it does 

 not completely stop the growth of the trifoliate orange, citrange, and 

 calamondin plants. Little or no difference could be detected in the rate 

 of growth of the grapefruit plants at the different night temperatures. 

 Leaf maturity was hastened somewhat by low night temperatures. 

 16917°— 20 5 



