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



INFLUENCE OF HUMIDITY ON GROWTH OF THE HOST PLANTS 



The preliminary experiments reported below are indicative of what 

 might be expected. Before placing the plants in the cases, all the pots 

 were wrapped with a double layer of parraffin paper, so that no moisture 

 could escape from the soil. 



EXPERIMENT I 



Two plants each of Poncirus trifoliata, Citrus mitis, and C. grandis 

 were used in each case. For the most part, the plants were dormant or 

 had completed their growth. 



Three cases with humidities of 90 to 95 per cent, 82 to 86 per cent, 

 and 73 to 77 per cent were used. The temperature (dry bulb) in the 

 cases varied between 21 and 23°C. As can be seen in Table XIX, 

 with the exception of two grapefruit plants held at 90 to 95 per cent 

 humidity, none of the plants were pushed into active growth. However, 

 it will be remembered that at no temperature in a saturated atmosphere 

 did the trifoliate oranges produce new growth, and likewise no results 

 were obtained with the calamondin plants at 20 in a saturated 

 atmosphere. The grapefruit plant did make a rapid growth at 20 , 

 in fact much more so than those held at 90 to 95 per cent humidity and 

 at approximately the same temperature. Thus, with dormant plants 

 which have completed their growth, the temperature and humidities 

 used did not stimulate the production of new growth 



EXPERIMENT 2 



In this experiment, three plants each of the Rusk citrange, calamondin, 

 and grapefruit were used. One plant of each species had sufficient new 

 growth for infection, a second had mature leaves, while the third was in 

 a dormant condition. The results of the experiment are reported in 

 Table XX. 



Of the plants used, calamondin appeared to thrive and grow best at 

 the humidities used in this experiment. In the experiment on the 

 influence of temperature in a saturated atmosphere, little or no growth 

 occurred at 2o°C, but here with approximately the same temperature a 

 good vigorous growth was made, even the dormant plants of this species 

 starting. The results with grapefruit and citrange were not so clear-cut. 

 Their behavior was decidedly different from that at 20 in a saturated 

 atmosphere. Growth at the humidities used was faster, and the leaves 

 were much smaller. Apparently, then, low humidities have the same 

 influence as low temperatures on the maturation of the leaves of some 

 of the Citrus plants. The cause for the decided difference in the growth 

 of the calamondin plants is not known. 



