58 Rdaiions heiivecn Host and Parasite. 



CHAPTER IX. 



Relations between Host and Parasite in Smut Diseases. 



The smut fungi, which are dependent upon certain host-plants for their 

 existence, very frequently convert the seed into a black mass of spores, and 

 when this is done in a wholesale manner, the host-plant is unable to reproduce 

 itself. I had an instance of this in the case of Brome smut in the early part 

 of 1907, when a paddock sown with prairie grass {Bromus unioloides), in which 

 the seed supplied was smutted, showed not a single plant free from the disease. 

 The seed of the host-plant was entirely destroyed, and if this were to happen 

 over a large area, in the case of an annual such as wheat, the fungus itself 

 would run the risk of becoming extinct from the failure of its food supplies. 

 The law of self-preservation comes into play here, and the total extinction 

 of the host-plant may be guarded against by some of the grains becoming 

 more resistant to the entrance of the parasite on germination than others. 

 The less resistant would succumb, while the more resistant would survive, 

 and by a process of selection it might eventually come to pass that a race of 

 plants would be produced immune to the parasite. We have a parallel 

 case in the human race in connexion with pulmonary consumption. 

 As Sternberg 1 points out, there are certain races which may be specially 

 liable to infection by the tubercle bacillus, while there are others such as 

 Russian and Polish Jews, which enjoy comparative immunity, as shown 

 by vital statistics. But the same principle applies generally to infectious 

 diseases, and the above-mentioned author, in his recent work on " Infection 

 and Immunity," clearly expresses it as follows : — " In general, it may be 

 said that when an infectious disease is first introduced among primitive 

 races, who, by reason of their isolation, have been previously exempt from 

 it, it is apt to be exceptionally fatal. This is, no doubt, due to the fact that 

 there has been no opportunity for the operation of the laws of natural 

 selection, by survival of the fittest. But under the operation of these laws, 

 in process of time, a certain degree of race immunity is likely to be established." 



In drawing analogies from human diseases it may not be out of place to 

 remark that the eminent surgeon, the late Sir James Paget, considered that 

 the study of plant diseases might be very helpful to human pathologists. 

 In an address on " Elemental Pcithologv " delivered before the British 

 Medical Association, at Cambridge, in 1880, he pointed out that many of 

 the morbid processes occurring in plants are quite comparable with those 

 occurring in animals, such as hypertrophy or excess of nutrition, atrophy or 

 defect of nutrition, repair of injuries and even inflammation in plants, which 

 has since been elaborated by Dr. Ransom in his work on " The Inflammation 

 idea in General Pathology." It is in the field of parasitism, however, that 

 the likeness is most striking, and which Paget considered the most Avorthy 

 of study, for he says : — " But of all morbid processes in plants, none, I think, 

 are so suggestive as are those produced by parasites, whether vegetable or 

 animal." I can foresee a great advantage in thus regarding plant diseases as 

 part of the general phenomena of disease, not merely as aids to human pathology, 

 Init in turning human pathology to account in the investigation of diseases of 

 plants. It has been too much the custom hitherto to consider plant pathology 

 as a thing apart, as dealing with a class of disease which had nothing in 

 common with the great discoveries in the domain of bacteriology, for instance, 

 From this point of view great possil^ilities are opened up, and investigations 

 are suggested which might otherwise be overlooked. The questions of 



