S6 Paras/tis/n and Inununitv. 



\vith buut spores derived from a common source, in the same way at the same 

 time and sown on the same day in similar soil alongside each other, they are 

 not all equally infected. Even in the most favorable cases there are 

 generally a number which escape, and it requires to be explained how some 

 are smutty and others not. There are various reasons which may be assigned 

 and there are a number of conditions which must obtain at the same time 

 for successful infection. It need not be assumed that the failure is due to 

 the non-germination of some of the spores, for the germ-tube may penetrate 

 into the host-plant and yet not necessarily produce infection. 



In the first place, the host-plant must be at the right stage of development 

 when the spore or conidiuni has put forth its germ-tube, and this period in 

 the young seedling does not last very long. Further, when the germ-tube 

 'has penetrated into the seedling, it must reach the growing point, otherwise 

 it would not continue to keep up with the growth of the host-plant and 

 finally produce smutty ears. Now, the slower this stage of growth is the 

 more likely it is that the germ-tube will reach its destination in time, whereas 

 the more rapid the growth the less likely it is to reach the growing point of 

 the young seedling before it begins to elongate. It is well known that 

 individual grains vary in the rapidity of their germination, and this might 

 be so hastened that the penetrated germ-tube would fail to reach the growing 

 point. This suggests another reason for some of the plants escaping 

 infection. Some varieties are very slightly susceptible to infection by bunt, 

 and only some of the plants would under these circumstances become smutty. 

 This applies particularly to Florence and Genoa, which are only slightly 

 susceptible, and in the case of Medeah it was the only one dusted with spores 

 which escaped infection entirely during 1908. In fact, the whole question 

 of Parasitism and Immunity or Susceptibility and Insusceptibility to disease, 

 seems to be bound up with the presence or absence of certain chemical substances 

 in the host-plant. Miyoshi^ of Japan, was the first to show that the germ- 

 tube of a fungus will only enter the tissue of a living plant and develop there 

 when the special substances which attract it are present. This sensitiveness 

 to certain chemical substances possessed by the fungi is known as Chemotaxis, 

 or Chemotropism, and the substance which exercises this attractive influence 

 is spoken of as a positively chemotactic substance. Miyoshi demonstrated 

 that the hyphfe of Penicillium glaucum for instance, pierced the epidermis 

 of living leaves when previously hypodermically injected with a chemotactic 

 substance such as sugar, and Massee^ proved that certain cucumber plants 

 were immune to a disease to which they were normally subject, owing to the 

 absence of the special substance which attracted the parasite. The quantity, 

 as well as the quality, of the dissolved substances and their unequal distribu- 

 tion influence the result ; and some substances are not only negatively 

 chemotactic, but they are positively injurious, and repel the entry of the 

 parasite into the tissues of the plant. It is easy to understand that in the 

 early stages of the seedling there will be a variety of substances dissolved in 

 the cells, and these may vary considerably in the neighbourhood of the 

 growing point. Sugar is the substance which is the great source of attraction 

 to the germ-tubes of fungi, and this will vary considerably in amount, 

 according to the nature of the growth, and may even be neutralized in its 

 action by the presence of other substances. 



For individuals of the same variety we must assume a slightly hastened 

 or retarded development in the seedling stage, and a cjuicker or slower 

 growth afterwards, in order to account for the unequal infection ; but this 

 will necessarily be associated with a variation in the chemical composition 

 of the substances in solution in the plant. 



