78 PLANT PHYSIOLOGY 



nitrates, which the plant can utilize either before or after the death 

 of the nodule-bearing rootlet. Lineberger (1926) reports a sym- 

 biosis of somewhat similar nature between the hydrophyte, 

 Azolla, and the alga, Anaboena. The latter lives in the leaf cavi- 

 ties of the former and there seems to fix nitrogen, which the 

 Azolla can later use. If there is already plenty of nitrates in the 

 solution, the Anaboena may become a bit of a nuisance but never 

 an actual pest. 



Lichens are plant complexes consisting of a fungus and an alga 

 living in such close conjunction that the resulting structure be- 

 haves as an organic unit. They are found on trees, boards, rocks, 

 and similar places where the supply of food materials would seem 

 to be rather meager. Formerly they were considered to be in- 

 dividual plants but, when it was discovered that the alga and 

 fungus could be grown independently, the dual nature of the or- 

 ganism was recognized. When grown separately the fungus differs 

 more than the alga from the condition found in the dual relation- 

 ship of the lichen, thus indicating that the fungus is more depend- 

 ent upon the alga than the alga upon the fungus. The exact nature 

 of the relationship is somewhat in doubt. It is commonly thought 

 to be one of helot ism, the alga profiting and losing about equally 

 by the association, while the fungus apparently obtains its organic 

 food supply from the alga and thus profits considerably. Others 

 hold to the opinion that it is genuine mutualism in which the 

 alga is protected and enabled to live in drier habitats than it other- 

 wise could. At the other extreme is the opinion that the relation 

 is one of pure parasitism in which the fungus robs the alga and 

 reaps all the benefit, giving nothing in return. 



Parasitic Fungi. — The fungi which are parasitic include the 

 rusts, smuts, mildews, and some of the bacteria. Here are to be 

 found the forms which cause the large majority of plant and an- 

 imal diseases. It is worthy of note that man in his conquest over 

 nature is to-day fighting his most serious enemies in the world 

 of microorganisms. He has conquered many of the inanimate 

 forces of nature and bent them to his will. He has defeated the 

 other animals (except the insects), after having invented weapons 

 which made their subjection comparatively easy, but the micro- 

 organic population still contains some of his worst enemies, and 

 here some of the most important battles are now being waged. 



The measures of microorganic control are two: first, the cure 



