Dec. i 5 . 1920 Effect of Temperature and Humidity on Citrus-Canker 459 



INFLUENCE OF TEMPERATURE ON GROWTH OF THE HOST 



PLANTS 



The literature on the influence of the environmental conditions on the 

 growth and development of Citrus plants is very meager. What litera- 

 ture is available concerns itself chiefly with the injury to Citrus orchards 

 caused by low temperatures, with an occasional reference to the maxi- 

 mum temperatures at which the Citrus plants can thrive. 



The most complex factor entering into the study of the temperature 

 relations of Citrus plants is the fact that they have rest and growth 

 periods which vary to some extent with each group, although they are 

 more or less definite within the group itself. Under greenhouse condi- 

 tions, the rest and growth periods are variable. However, as a general 

 rule, most Citrus plants can be forced into active growth within short 

 periods of time. An exception to this statement must be made for 

 deciduous plants like Poncirus trifoliata. With plants of this type, 

 external conditions in the greenhouse have no influence on the rest 

 period, within certain limits. 



Three types of plants were used — Poncirus trifoliata (L.) Raf. and 

 Rusk citrange (a hybrid between P. trifoliata and Citrus sinensis Osbeck, 

 Florida sweet orange), plants which are deciduous, hardy, susceptible to 

 citrus-canker, and having a very definite dormant period; C. grandis 

 (L.) Osbeck, grapefruit, an evergreen and nonhardy plant, extremely 

 susceptible to citrus-canker and having a dormant period of variable 

 nature; and C mitis Blanco, calamondin, an evergreen and nonhardy 

 plant, somewhat resistant to citrus-canker, and native of the Philippine 

 Islands. 



The plants were grown from seed in the Crop Physiology greenhouses 

 at Washington, D. C. The seedlings ranged from 6 to 14 inches in 

 height and were shipped from Washington from time to time, both in 

 pots and balled. Several shipments of Poncirus trifoliata were made of 

 seedlings growing outside, from Auburn, Ala., during the month of Jan- 

 uary. The plants were kept under greenhouse conditions until needed. 



In the experiments reported below, the plants were placed under large 

 bell jars in the various temperature cases. During the course of the 

 experiments, a saturated atmosphere was maintained in the bell jars. 

 Observations and readings were made of the condition of the plants 

 from time to time. 



Experiment 1 



Two plants of each species were placed in the cases at the various 

 temperatures, while one set was kept under greenhouse conditions where 

 the temperature range was considerable, varying from 20 to 30 C. 

 All plants, with one or two exceptions, were either in a dormant state or 

 had completed their growth. In Table VI are given the observations 

 made on the plants at intervals for a period of six weeks. 



