ONION SMUT FUNGUS. 



121 



most advanced was less than | cm. long, and had not even started in nian.y 

 of them, 50 plants were transferred each day to soil which was badly 

 infested and which could be depended on to produce almost 100 per cent 

 of infection. Notes were made on the stage of development of the seed- 

 lings each daj^, and a careful record was kept of all the plants which be- 

 came smutted. After six weeks, when the plants were mostly in the 

 fourth leaf (after which infection never starts), all of them were pulled, 

 and the following table compiled to show the complete results of the 

 experiment : — 



The foUomng conclusion may be drawn from this experiment: Under 

 greenhouse conditions the greater part of the infection occurs within two 

 weeks after planting, and the plants are no longer susceptible after the 

 seventeenth day. Since it seems probable that the period of suscepti- 

 bility is not hmited by the number of days during which the seeds have 

 been in the soil, but by the length of tune required for the seedling to pass 

 through certain stages of development, we may express this first conclu- 

 sion by stating that susceptibiUty begins to diminish from the time that 

 the knees emerge from the ground, and that little if any infection occurs 

 after the first leaf has emerged from the side of the cotyledon. In a large 

 number of experiments in the greenhouse at all times of the year it has 

 been found that the knees begin to appear above ground in seven to twelve 

 days. In one experiment, where the house was very cool, it required over 

 two weeks, and in this case the percentage of infection was 100, and the 

 individual plants were more thoroughly smutted than in any other experi- 

 ment tried. Since, then, the period of susceptibility might be increased 



