Minnesota Plant Diseases. 5 i 



lowed by the excessive growth of the stem at the infected 

 points. In many of these unequal partnerships we find again 

 the welding together of two organisms into one individual, a 

 phenomenon comparable with that in lichens, but with a differ- 

 ent final result, at least as far as nutrition is concerned. 



Unequal partnership — fungus dominant. Parasites. An 

 overwhelming majority of the partnerships between fungi and 

 leaf-green plants result in exclusive benefit to the fungus, and 

 this condition is usually designated as parasitism. The host is 

 subservient and the fungus is dominant. It may not be the 

 entire host, though this is so in some cases, that is robbed of 

 nutrition, but special parts only may be attacked and forced to 

 nourish the fungus. From the standpoint of the host plant it 

 may be termed destructive parasitism or simply parasitism in 

 the narrower sense. The destructive effect is the ultimate 

 elTect received by the host plant as a unit. The immediate 

 effect of such parasitism may be very local and may be in the 

 nature of a stimulation. Moreover, the living together of a 

 parasite with special parts of its host often produces again, as 

 in the lichen, an essentially new individual composed of the 

 fungus and the host plant-part. 



Witches'-brooms. Such "individuation" is best shown by 

 the structures known as witches'-brooms. Many of our com- 

 mon trees are attacked by certain fungi, the latter chiefly rusts, 

 whose mycelium becomes confined to a certain well-marked re- 

 gion of the host plant. This part of the host behaves in a 

 peculiar manner. The branches are usually larger than nor- 

 mal, are more numerous, and often, again, profusely branched. 

 The whole mass of branches looks like a httle bush growing 

 parasitically on the host. Moreover, the bush usually arises 

 from one point on the host plant and the main branch of the 

 bush, although it may be a lateral branch of the tree, behaves 

 as though it were the leader and grows straight up in the air. 

 This brings the bush still more into prominence and demon- 

 strates the individualistic character of the broom. Such bushes 

 are known as witches'-brooms. The branches usually bear 

 leaves which fall early and are often yellowish, having lost 

 some or all of their leaf green. This serves to point out an- 

 other important feature of- the broom, viz., that, as an individ- 



