6o Relations bet-a.'ccn Host and Parasite. 



are free from the fungus, altliougli tliis is only discernible by the 

 mioroscope, and that they produce plants likewise free ; but the poisonous 

 property, due to the alkaloid Temulin, is only developed when the fungais is 

 present. This has been shown byHannig,^ and the production of this toxin 

 by the fungus may prevent its complete development just as the production of 

 alcohol will ultimately overpower the yeast. This raises the further cjuestion 

 whether immunity may not be due to the production of toxins and anti- 

 toxins by both the parasite and its host, Avhich neutralize the effects of the 

 parasite in the host or may actually destroy it. 



The parasite as well as its host feeds and excretes, and the excreta are 

 probably toxic, so that when the nature of the toxins and anti-toxins in th-^ 

 plant body is better understood and when they have been isolated and studied, 

 then some light will be thrown on the cause of immunity itself. 



Potter^ has shown how parasitic diseases in plants may be checked by 

 taking advantage of this property of the waste products of metabolism, when 

 allowed to accumulate beyond a certain stage, checking the growth and 

 finally proving fatal to the parasite. He prepared a toxic solution in pm-e 

 cultures of the parasite which was not only fatal to it, but as might have been 

 expected, exercised also an injurious influence on the healthy cells of the host. 

 This influence, however, only extended to a limited area, and the wound soon 

 healed over. By this means he was able to arrest the decay caused by a 

 Bacterium and a Penicillium, so that the principle is probably applicable to 

 parasites in general. 



4. This absence of spore formation in the seed may be due to hereditary 

 tendencies, as in the Lolium, or it may be brought about by external agencies 

 which affect the growth, and are intermittent in their character. Thus the 

 host-plant may not have every ear attacked, so that in the case of " tillering " 

 of wheat, some ears of the same individual plant may be completely free 

 from the parasite. Or only some of the grains in an ear of wheat may be 

 attacked, as happened particularly during season 1907, when there was only 

 a short growth of straw, but the weather at blooming time favoured the 

 production of large and vigorous ears. 



If we take the concrete instance of bunt in wheat it will be seen how host- 

 plant and parasite get along together. As soon as the smut has infected 

 the seedling and the germinal hyph?e have reached the growing tissue, the 

 curious result is that they both grow together, and there is an inducement 

 for the host-plant to provide for both, as it is constantly, stimulated thereto 

 by the hyphfe growing between the cells. This concurrent gro^vth of host 

 and parasite goes along smoothly and without any external sign of disorder 

 until the grain begins to be formed. Then there is a struggle for existence 

 as to whether the host or the parasite is to be the winner in life's race, for 

 from the very nature of the case, unless the parasite produces its spores it is 

 likely to succumb, and, on the other hand, if the embryo is not formed, there 

 will be no future host-plant. It is a struggle in which it is apparently Avar to 

 the death, and the more successful the parasite is the more injurious it is to 

 the host. But what decides whether the host or parasite will win ? Well, 

 there may be a temporary truce when some ears, or it may be only some 

 grains in the ear, escape destruction by the parasite. But, in the long run, 

 among the combination of causes which favour the preservation of one or 

 other organism, it will be found that a compromise has been arrived at, and 

 the tendency to become immune is the important factor in enabling both 

 the smut and its victim to survive in the struggle. I have only noticed 

 the more evident factors in the struggle ; but it is really between 

 the tissues of the host-plant on the one hand and the germinal hyphae and 

 mycelium of the parasite on the other, and whatever external influences 



