FACTORS AFFECTING PLANT DISTRIBUTION 29 



of extracting free nitrogen from the air. The bacteria are para- 

 sitic upon the legume which furnishes food in the form of com- 

 pounds of carbon of its own manufacture. As the tubercles pro- 

 duced by the bacteria grow older, the appearance of the bacteria 

 themselves is altered. They become larger, sometimes branching, 

 and in this form are known as "bacteroids." Active nitrogen assimi- 

 lation by the green plant coincides with the appearance of the bac- 

 teroids in the root nodules of the legume. If a large enough portion 

 of the legume is then allowed to remain in the soil to offset nitrate 

 nitrogen absorbed by the plant, there results a corresponding gain 

 in soil nitrogen. 



The reciprocal symbiosis of Phoma an endotrophic mycorhiza 

 should also be mentioned as a relation which nets a gain in the nitro- 

 gen supply for the host plant upon which the fungus lives. The 

 mycorhiza as a class are so important to many of the green plants 

 that the latter are unable to grow except in the presence of the 

 fungi. 



Protozoa are also important, though mainly unfavorable, as 

 they destroy the beneficial bacteria. Brown ('16) has described the 

 importance of mold action and in addition there are other fungi — 

 all the saprophytic ascomycetes and basidiomycetes. 



The white pine blister rust and the chestnut blight are exam- 

 ples of biotic factors which cause certain plants to drop out of the 

 native vegetation as surely as changing climatic or soil conditions 

 cause them to disappear. Chestnut trees in New York and Pennsyl- 

 vania have already been killed in such numbers as to alter entirely 

 the nature of some of our finest forests. If the disease goes on un- 

 checked it would not be strange to have inhabitants of these states 

 inquiring in a hundred years from now if the chestnut actually 

 grew in these portions of Eastern America. And yet the chestnut 

 bark disease was noted as a menace on Long Island for the first 

 time about the year 1905. 



The white pine blister rust, as it has been named, has not yet 

 done any considerable damage and it looks as though this disease 

 will be checked, thanks to the efficient service of our forest patholo- 

 gists. Yet in those small localities which were infected the white 

 pine was completely destroyed. This disease is not only a menace 

 to the white pine of Eastern United States, but likewise attacks the 

 western yellow pine, another of our valuable timber woods. 



