August, '17] BONCQUET AND STAHL: BEET CURLY -TOP 395 



After remaining on the healthy beets for about two weeks all 

 insects emploj^ed in the foregoing experiment were transferred to 

 other healthy beets and the results were duplicated. Actual dates 

 were not recorded, but all cases developed somewhat more rapidly 

 than usual, due to the fact that the plants were kept in the green- 

 house where the temperature was much higher. It will be noted from 

 these results that the time required for the symptoms to develop 

 was quite long, in fact much longer than was the case during the 

 preceding summer in experiments with other phases of the condition. 

 This unusual length of time was due to the unfavorable weather 

 conditions prevailing later. The days were cold and cloudy and 

 growth of the beets was practically at a standstill. It has been observed 

 in these and other experiments that the symptoms of the condition 

 do not appear unless the plant is growing. Two causes may be 

 responsible for the fact that No. 3 did not show curly -top symptoms. 

 Either the mallow was not affected, or the weather and the conditions 

 in the beet plant were such that the organism was killed before it had a 

 chance to develop. This assumption was more strikingly borne out 

 in the following results, obtained from similar experiments at a 

 later date. December 1, 1915, one of the same mallow plants was 

 selected and six non-virulent insects placed upon it in a lantern globe. 

 After a period of seven days they were removed and placed singly 

 on healthy beets. None of these transfers brought about the disorder, 

 all beets remaining healthy. As this mallow plant had been proven 

 to be infected in the past experiment there is no doubt that unfavorable 

 conditions were responsible for the negative results obtained. There 

 may perhaps be a latent period in the life cycle of the causative agent 

 while in the plant, or the agent may have been unable to withstand 

 the unfavorable temperature. 



The actual production of the condition in a healthy mallow plant 

 by a virulent insect, and its subsequent transfer from the mallow 

 to a healthy beet by a non-virulent insect, is the crucial point in the 

 experiment. Small seedling plants for this test were grown in insect- 

 proof cages and used as soon as they were large enough to be easily 

 handled. First, six insects known to be virulent were placed on each 

 mallow plant and were allowed to remain for a considerable period, 

 after which they were removed. Non-virulent insects were then 

 placed on the same plant and allowed to remain at least one week, 

 wdien they were removed and placed on healthy beets. This experi- 

 ment was conducted in a room in the laboratory where conditions 

 were more favorable for the development of the condition. The 

 results obtained are given in Table II. 



