58 Minnesota Plant Diseases. 



substance which is in a resting state and which approaches 

 most nearly the condition of dead proteid material. More- 

 over, such protoplasm contains much nutrient material. The 

 fungus kills as soon as it comes into contact. It has not acquired 

 any particular choice for specific kinds of hosts but attacks al- 

 most indiscriminately. The damping-off fungus is another 

 amateur parasite, though it has carried a little farther its ability 

 to kill. It is able to attack vigorously growing parts, as seed- 

 lings, but, like the fruit molds, does not exercise any well- 

 marked preferences in its selection of hosts, i. e., it may attack 

 almost any seedling. The beginnings of such a preference are 

 indicated in the great frequency with which it attacks seedlings 

 of the mustard family. The wound parasites of trees show 

 likewise a low degree of parasitic efficiency. They require a 

 mycelium well established by previous growth in the heart- 

 wood before they can successfully attack the living part O'f the 

 tree. 



When, now, we examine the powdery mildews, e. g., the 

 powdery mildew of lilac bushes, w^e find an improved method of 

 parasitism. The fungus does not noticeably injure the host, 

 though it is of course detrimental to it even when the results 

 are not evident, and in some mildews the results are obviously 

 disastrous to the host. Moreover, the parasite requires special 

 hosts and a given mildew is found only on one kind of host, 

 or only on plants which are very close relatives and so furnish 

 very similar materials. In other words, the mildew is more 

 select in its choice of food than the damping-off fungus, and its 

 method of attack is more complicated and exact in its detail. 



Now, if one considers the parasitism of the smut fungus, 

 e. g., the smut of corn, one sees again an improvement in para- 

 sitic methods. In the first place the fungus has refined very 

 much its selective power for food and can now only exist as a 

 parasite on corn, and is unable to live on any other plant even 

 of the same family. But when it has once established itself 

 upon its host it does not immediately destroy the attacked 

 portion of the plant, as would the damping-off fungus; neither 

 is it a passive passenger, as is the mildew; on the contrary, it 

 stimulates the part of the corn plants on which it lives and 

 causes that part to grow abnormally larger at the expense of 



