Dec. is, 1920 Effect of Temperature and Humidity on Citrus-Canker 499 



organism, while at high temperatures it is the limiting factor. It is 

 extremely interesting to note that at the medium humidities the organ- 

 ism is alive at all temperatures for the period of the experiment. Even 

 at the extremely low humidities the organism is viable for varying lengths 

 of time, depending somewhat on the temperature. 



No attempt will be made at this time to explain the results obtained, 

 nor to compare them with those showing that in ordinary distilled water 

 the organism is alive at the end of eight days at temperatures between 

 io° and 35 C. It is sufficient to state here that the death rate of the 

 organism on the silk threads is not due to the rapidity with which drying 

 takes place, since at the low humidities where drying is most rapid, the 

 death rate is slow, while at high humidities where the rate of drying 

 is slowest the death rate is most rapid. At the medium humidities, 

 where the organism is alive at all temperatures for the duration of the 

 experiment, some other factor or factors must enter in other than the 

 rapidity of drying. It should be noted that the medium humidities 

 used in these experiments are the ones most generally prevalent under 

 field conditions in Alabama during the greater part of the year. 



The life of a plant-disease bacterium in culture in the laboratory and 

 in the field outside of the host plant is ruled by entirely different factors 

 from those which govern when it is parasitically active in the host tissues. 

 Thus, a sharp distinction must be drawn between these conditions. 



It is extremely difficult to compare the results obtained in the green- 

 house experiments with observations in the field, because of the widely 

 divergent conditions which exist. In the greenhouse work constant 

 temperatures and humidity controls were used, while in the field all sorts 

 of conditions are met. After the problem has been studied from all 

 angles, it appears that only general statements can be made at this time. 



For the purpose of this discussion, two types of rest periods can be 

 distinguished without entering into a long explanation of the probable 

 causes of rest periods in horticultural plants — namely, winter dormancy 

 brought about by either the approach of cold weather or freezing tem- 

 peratures and the short rest periods which occur during the growing 

 season. During winter dormancy the cell activities cease to a great 

 extent, while during the short rest periods which occur in the growing 

 season some of the cell functions merely slow up. 



In Alabama, as a rule, grapefruit and allied plants usually grow on 

 into the winter, until temperatures of 5 C. or lower are reached. At 

 this time, the plant is thrown into a state of dormancy, which persists 

 until a period of higher temperatures occurs and active growth is resumed. 

 This may happen several times during the winter. With Satsuma 

 {Citrus nobilis var. unshiu, Swingle) and other mandarin oranges growth 

 proceeds until low temperatures occur and after that no growth takes 

 place until suitable temperatures prevail. Kumquats (Fortunella marga- 

 rita (Lowr.) Swingle) go into dormancy and cease their growth with the 



