Loose Smut of Oats. 105 



practicallv no infection — to 1 per cent, diseased. These experiments con- 

 clusively prove that the younger the seedling the more susceptible it is to 

 infection. 



The course of the infection was closely followed. The germ-tube pene- 

 trated the cuticle and grew obliquely through the cells. In the earliest stages 

 of the seedling the hyphae were numerous and distinct ; but as the tissues 

 lengthened and strengthened it was more difficult to trace the mycelium. In 

 the fully-formed plant the mycelium could only be detected in the tissue of 

 the nodes, in a more or less ruptured condition. The fungus is unable to 

 keep up with the rapid longitudinal growth of the plant, and its mycelium is 

 torn up into fragments, so that it becomes isolated and does not reach a 

 suitable spot for the production of spores. In the rapid elongation of the 

 internodes the oat therefore exercises a kind of check upon the parasite, but, 

 on the other hand, if the mycelium has once reached the growing point then 

 it is always present there, and finally produces its spores. It is evident, 

 therefore, that not every infection of the host-plant will necessarily attain to 

 the production of spores, but only in those cases where the fungus has been 

 able to reach the tissue of the growing point and continue to keep up with it 

 until the flowering stage. This supplies an explanation of the fact that in- 

 fection is much more successful in younger than in older seedlings, for in that 

 case the mycelium can much more easily reach the growing apex and remain 

 there than when the elongation of the internodes has taken place. 



There are several conditions which render infection more or less certain, 

 and also affect its virulence. If the " yeast-conidia " used for spraying the 

 seedlings were cultivated in a solution obtained from fresh horse-dung, then 

 there might be as much as 46 per cent, of diseased plants, and if the spores 

 were cultivated for a number of generations the virulence decreased towards 

 the end. Spores that have wintered in the soil will, under suitable conditions, 

 germinate in the spring and form conidia : but whether these conidia will infect 

 young plants directly or sprout in a yeast-hke manner or die, will depend upon 

 the weather, particularly in regard to the heat and the moisture. Since our 

 sowing time is generally in the autumn and before the winter, the problem 

 we have to solve is what happens to the spores in the soil during the summer, 

 as, by the time winter is reached the spores have either germinated or have 

 missed infecting the seedling stage. 



This smut produces its spores in the ovary of the flower ; then they are 

 dispersed by the wind or rain, and some of them fall on the soil or are carried 

 to healthy grain. Infection takes place when the seed begins to germinate, 

 and the young mycelium grows alon^ with the seedling, showing no external 

 signs of its presence until the flowers are produced, when it again forms its 

 spores in the ovary. Jensen^ proved that oat smut will not infect wheat or 

 barley, and that it is, therefore, biologically distinct from the smuts attacking 

 either of these cereals. 



Between the loose smut of wheat and of oats there are marked differences 

 in the behaviour of the two. To begin with, the spores in the case of wheat 

 only retain their germinating power for a few months, while in oats thev have 

 been germinated at the end of seven years. 



Then again, on germination the wheat loose smut only picduces a j^erm- 

 tube, which may be variously branched, while in oats not only are conidia 

 formed, but these in turn may give rise to sprouting, conidia. 



These differences are correlated with the mode of infection. In wheat it 

 is flower infection, which must be done within a limited period, and after the 

 manner of a pollen-grain, and in oats it is seedling iiifection, which may be 

 prevented by treatment similar to that of stinking smut of wheat. 



