Oct. 29, 1921 Relation of Soil Temperature to Onion Smut 239 



The foregoing experiment was repeated in a somewhat cooler house, 

 in which the temperature remained close to 15° C. most of the time, 

 rising to about 20° during the middle of the day. Under these conditions, 

 the maximum length of the cotyledon was about 2)4 inches. The data 

 from this experiment are reported in Table II. The plants became im- 

 mune at approximately the same time as noted in the first experiment — 

 between the nineteenth and twenty-fourth days after sowing, when the 

 cotyledon had about attained its full growth and as the first leaf was 

 emerging. It will be recalled that the basal portion of the cotyledon, as 

 with each of the later leaves, forms a collar or sheath inclosing the lower 

 parts of the younger leaves. The question arose as to whether or not 

 immunity to smut infection is directly associated with maturity of the 

 tissues. If so, it would seem that the explanation of this later escape 

 of the onion plant from infection lies in the fact that the maturing basal 

 sheath forms a thin but normally complete barrier of resistant tissue 

 between the potentially infective soil and the deeper-lying embryonic 

 tissues of the younger developing leaves. The removal of this mechan- 

 ical barrier might, therefore, permit of infection at a later stage. In 

 order to determine whether this is the case, the following experiments were 

 undertaken. After the thirty-first day, when the onion seedlings had 

 passed the so-called susceptible period, the cotyledons were carefully 

 removed from the plants in one pot, and infected soil was placed around 

 the base of the exposed first leaf. Sixty per cent of the plants thus 

 treated became infected as shown in Table II, pot 9. This proves that 

 the first leaves are susceptible even after the cotyledon becomes immune. 

 On the fortieth day, a i-inch layer of infested soil was placed on top of 

 pots 10 and 1 1 , so as to surround the first leaves in proximity to the axils. 

 Pot 10 was left at the same temperature (15° to 20°) for 24 days and pot 

 II was removed to a temperature of 25° to 28° for the same period. 

 About 5 per cent of the plants in pot 10 showed infection of the first leaf 

 as compared with 28 per cent in pot 11. The reason for the increased 

 percentage of infection at the higher temperature has not been satisfac- 

 torily explained. It may simply have been consequent upon the stimu- 

 lated growth of the onion foliage. However this may be, it is evident 

 that the basal portion of the first leaf remains susceptible to infection 

 for a short time, at least, after it emerges from the cotyledon. 



From a summary of these results it appears that our own experimental 

 data regarding the duration of tlie period of infection agree in the main 

 with those of previous investigators. The conclusion seems justified 

 that disease resistance is correlated with tissue maturity, and that the 

 onion cotyledon becomes immune to smut infection at about the stage 

 when growth ceases. The rate and nature of growth of the cotyledon 

 will naturally vary with environmental conditions; hence variation in 

 the actual length of the infection period is to be expected. The mature 



