488 Journal of Agricultural Research vol. xx, No. 6 



there can be no doubt that the organism entered the tissues of the host 

 shortly after inoculation and remained quiescent until a higher tem- 

 perature was available. This fact may explain the many cases of 

 inactivity of the disease met with under field conditions. 



(9) Plants held at 30 C. for 24 hours after inoculation and then trans- 

 ferred to a lower temperature failed to produce infection except on one 

 grapefruit plant. However, when returned to a higher temperature, 

 most of the plants showed 100 per cent infection. 



(10) At a temperature of 35 C. infection took place only on the plants 

 which made a normal growth, while little or no disease occurred on plants 

 of the Citrus grandis type. However, all successful inoculations even 

 on the Poncirus trijoliata type of plants were made with cultures of the 

 organism grown at temperatures below 35 . 



INFLUENCE OF HUMIDITY ON THE ORGANISM 



The influence of humidity on bacteria resolves itself principally into a 

 question of drying or desiccation. Bacterial growth takes place only in 

 the presence of free moisture. Thus, in a study of the influence of 

 humidity on bacteria, one must consider the viability of the organism 

 and not the growth. 



The common methods used heretofore have been the drying of the 

 organisms on silk threads, glass beads, or glass slides. Some few investi- 

 gators have used seeds. The method ordinarily followed by the pathol- 

 ogist is to smear with a sterile platinum needle on sterile microscopic 

 slides bacteria from vigorous pure cultures and to set these slides away 

 in the dark in a dry-air room. After a few days they are tested for 

 viability, either by pouring nutrient agar over the slides in Petri dishes 

 or by dropping cover glasses, which are sometimes used, into a suitable 

 culture medium. 



In the work on the resistance to drying of bacteria, no one has deter- 

 mined the temperature or the humidity at which the prepared slides have 

 been kept. Again, no attention has been paid to making a uniform 

 smear of the organism on the slides. The only factor which has been 

 considered necessary has been that the smear be taken from young, 

 vigorous cultures. 



A brief review of the literature reveals the fact that organisms dried 

 on seeds or on silk threads remain alive much longer than those dried on 

 glass slides, cover glasses, or beads. However, since conditions varied 

 with each experiment, no comparisons can be drawn. 



Using the prescribed method for testing resistance to drying, Stevens 

 (12) found that — 



bacteria (P. citri) from young and old cultures exposed for two weeks on glass slips 

 to dry in the air of the laboratory failed to germinate. 



