30 Infection. 



1. Seedling Inb^ection. 



This was at first considered to be the only mode of infection, as it was 

 tlie only one knowai, and even now it is recognised as by far the most common. 

 The spores are generally attached to and sown with the seed, and if both 

 germinate at the same time infection usually occurs. The spore on the moist 

 surface of the grain germinates and produces its germinal tube bearing 

 conidia. The conidium stimulated by the moisture puts forth a deUcate germ- 

 tube Avhich penetrates the young tissues of the seedling if it reaches i: at the 

 right time. The particular spot for infection is at the juiiction of the rootlet 

 just emerged wath the young shoot still wdthin the seed, and this is sufficiently 

 delicate to allow the germ-tube not only to reach the growdng point of the 

 primary shoot, but to branch out into any secondary shoots that may be 

 formed. 



But the question has also been raised as to soil infection, wdiether the 

 spores in the soil and not on the seed wall infect the host-plant. As already 

 stated, Brefeld has shown that this may occur with loose smut of oats 

 {Ustilago avenae), and I have elsewhere proved by experiment that soil 

 infection occurs in flag smut of wdieat {Urocystis tritici). With regard to 

 stinking smut of w^heat, Bolley showed that when the spores winter in the 

 soil they do not infect the second crop, as they have already germinated by 

 that time. If smut-balls are in the soil, however, they may cause infection. 

 Seed was sown close to smut-balls in the ground, and it was found that when 

 the seed was untreated the spores of the smut-balls reached the seed, and 

 infected it, wdiile seed under similar conditions and treated with bluestone 

 Avas unaffected. 



It would appear also that the greater the proportion of sjjores on the seed 

 s,wn the more severe the infection. Thus in inoculating the grain wdth bunt 

 spores, when one ball of smut was applied to a hundred w^heat grains, the 

 percentage of smutted plants in two cases was 56 and 58 respectively, while 

 when the spores were applied at the rate of one bunt ball to five grains, the 

 proportion was 79 and 81 per cent, respectively. 



2. Local Infection. 

 This is seen to perfection in the American corn smut {Ustilago maydis) 

 where any young and growing portion of the host-plant is capable of infection 

 and the action of the fungus is strictly localized to these particular spots. 

 Brefeld^ carried out an extensive series of experiments in which he proved 

 that only those parts of the young plant become smutty which have been 

 directly infected, all the rest remaining perfectly normal, so that the action of 

 the germ is strictly localized. He began by inoculating young seedlings with 

 sprouting conidia produced upon a nutrient solution, and the few plants which 

 developed the smut swellings died completely. In those cases, however, 

 where the axis remained sound he expected to find smut developed in the ears 

 as in the case of grain smuts, but not a single plant v as smutty. This was 

 contrary to the idea then prevalent that infection could only take place 

 through the young seedling, and he came to the conclusion that the germ- 

 tube might penetrate into other parts which were in a young condition similar 

 to the teedling. Accordingly he infected the heart of the plant still growing, 

 and the result was that " the entire leaves were covered with a complete crust 

 of pustule, Avhich,in part, made them almost unrecognisable." He next in- 

 fected the fertile inflorescences in the bud condition, and none escaped. 

 Where the lower flower buds were infected, and not the upper, the latter always 

 remained sound, and even where the exposed ovaries at the tip received the 

 germs, they were swollen and smutted, while the ovaries lower down on 

 the same spike produced normal grains. The adventitious roots which 



