Minnesota Plant Diseases. 49 



two plants ma)'' work as one individual, one unit, though really 

 composed of two plants. Such are the organisms known as 

 lichens, which occur in abundance as flattened crusts on rock 

 surfaces or on the trunks of trees. Each lichen is composed 

 of a fungus and an alga. The algae are for the most part rela- 

 tives of the flower-pot algae, while the fungi are almost all rela- 

 tives of the cup fungi. The little green algal spheres are en- 

 closed in dense wefts of the fungus threads. To the former is 

 assigned the task of starch-making on account of the leaf 

 green, to the latter the task of protection and also of the ab- 

 sorption of mineral salts from the soil. Together the two 

 plants thrive, while, if separated, the fungus at least would 

 perish and the alga would probably not thrive so well. This 

 partnership is therefore of benefit to both plants and the result 

 has been a unifying of two organisms into one. A somewhat 

 similar living together is also encountered among certain 

 plants and animals, e. g., the little wheel animalcule which is 

 always found in the small cup-shaped portions of certain leaves 

 of one of our common liverworts. The cup furnishes a pro- 

 tected harbor for the wheel animal and the plant probably de- 

 rives nitrogenous food from the animal, and thus a living to- 

 gether on equal terms is efifected. 



Unequal partnership — host dominant. The number of 

 equal partnerships among plants is of comparatively rare oc- 

 currence. In a vast majority of cases one partner becomes 

 dominant and the benefits are shared unequally, if not entirely 

 appropriated by the dominant party. In a very few of those 

 very numerous cases when a fungus and a leaf-green plant en- 

 ter an unequal partnership the leaf-green plant is dominant, 

 and what might be termed nutrient parasitism arises. In such 

 cases the fungus derives nourishment from the soil and trans- 

 mits it to the host plant, getting, as far as one can see, no bene- 

 fit in return. In other words, these fungus partners of leaf- 

 green plants behave much as do the tiny absorbent root hairs 

 which are commonly found on the roots of leaf-green plants. 

 Most of our native orchid plants as well as many foreign mem- 

 bers of the same family possess such a fungus partner. Some 

 of these orchid plants show no external evidence of the pos- 

 session of fungus partners, still retaining their leaf-green in 



