50 Minnesota Plant Diseases. 



undiminished quantity. In other cases, where the orchid plant 

 has learned to derive more of its nutrition from the fungus 

 partner, it may economize in its own structure and labor and 

 dispense with some or all of its leaf-green. As a matter of 

 course, the leaf-green organ would also be reduced. Such 

 plants have a yellowish appearance with little or no green color 

 and the leaves are mere bracts. The well-known coral-root 

 orchid is an excellent example of such a plant. The little plant 

 known as Indian-pipe is an even more conspicuous example. 

 It is a little forest-floor plant, whose relatives are members of 

 the blueberry family, and has formed a very effective fungus 

 partnership. So great have its profits become that it has entire- 

 ly dispensed with leaf-green and derives all its nutrition through 

 the fungus in its roots. The leaves consequently are reduced 

 to mere colorless scales and the whole plant has a pure white 

 appearance. There are many other plants, however, which 

 have fungus root-hairs but which have not yet abandoned their 

 leaf-green apparatus for starch-making. They may also pos- 

 sess ordinary root hairs in addition to the fungus. For in- 

 stance, many oak trees, and perhaps most plants of the heath 

 family, possess a subservient fungus root partner. Further in- 

 vestigation will probably show many more plants with this 

 same device, as the list is constantly increasing. 



Bacteria and bacteria-like plants are also met with as sub- 

 servient root partners of the green plants. Such are the or- 

 ganisms of the root tubercles which are so commonly found on 

 the roots of plants of the pea family. These bacterioids are 

 capable of converting nitrogen, one of the unavailable gaseous 

 constituents of the air, into an available compound, and thus 

 prove of great benefit to the host plant. Often special struc- 

 tures are formed upon the roots by the stimulation of the fun- 

 gus or bacteria and in them these organisms are found. Such 

 structures are usually tubercle-like bodies, as in the clover 

 roots; or they may be dense, grape-like clusters of tubercles, 

 as 'are found in the roots of alder trees. 



In recent years it has been asserted by a French botanist 

 that the well-known potato tubers, which are swellings of the 

 underground stem of our common potato plant, are due to an 

 infection bv a certain fungus. This infection is said to be fol- 



