Minnesota Plant Diseases. 



(b) Many rusts have remarkable powers of stimulation, not 

 only in their influence on size but also in the age of parts. Al- 

 though a host plant may bear the load of such a parasite the 

 fungus may still stimulate it sufficiently to enable it to maintain 

 its normal age relationships so that the fungus and host may 

 live together for years. In the darnel grass lives a smut-like 

 fungus which is parasitic and which infects the embryo in the 

 seed before the latter is ripe, and thus this fungus lives on from 



year to year in- 

 fecting its host 

 without the need 

 of spores. Such 

 a partnership has 

 become almost, if 

 not altogether, an 

 equal partnership, 

 and approaches 

 the degree of uni- 

 fication attained 

 among the 

 lichens. 



(c) Among 

 the most remark- 

 able effects of 

 stimulation are 

 the changes in 

 the floral parts of 

 host plants. It is 

 well known that 

 in some plants there are two kinds of flowers, one bearing 

 stamens and the other pistils. This is the case in certain mem- 

 bers of the pink family. In these plants, however, this so-called 

 (and incorrectly) "unisexual" condition has been brought 

 about by the failure of the beginnings of one of the floral parts 

 to develope. Thus, we find in such flowers either the stamens 

 alone fully developed, with the undeveloped beginnings of the 

 pistil, or vice versa. Very often such flowers are attacked by 

 certain smut fungi and the parasite often exerts a stimulating 

 effect upon the undeveloped beginnings of the floral parts and 



FIG. 37. Fungus gall on leaves of Labrador tea. The fungus 

 (Exobasidium) is one of the gall-forming basidium-bear- 

 ing fungi and causes a stimulation in the leaf which 

 thereby furnishes additional nourishment for the fungus. 

 The latter is an accomplished parasite. Original. 



