50 EXPERIMENTS IN BLUEBERRY CULTURE. 



cause when she proposed to iiiocuhite mycorrhiza-free seedlings of 

 the host plants with spores from the p^x'nidia that fonned in her 

 cultures she was unable to grow any seedlings that were free from 

 mycorrhiza. 



Notwithstanding the lack of an absolute demonstration that the 

 nitrogen-fixing fungi grown by Miss Ternetz were identical with 

 the mycorrhizal fungi of their hosts, it is regarded as quite possible 

 that the mycorrhizal fungi that occur in perhaps all plants of the 

 iieather and blueberry families, including the swamp blueberry, are 

 nitrogen fixers, and that the host plants absorb this nitrogen, giving 

 in exchange, for the use of the fungus, sugar or some other carbo- 

 hydrate. 



The experiments thus far described in the present paper, and the 

 accompanying discussions, appear to warrant the following theory 

 of the method of nutrition of the swamp Ijlueberrv: 



(a) The sAvamp blueberry grows in peaty soils which contain 

 acid or other substances poisonous to plants. 



(b) As a protection ngainst the absorption of amounts of these 

 poisons great enough to prove fatal. thij< plant, like many other bog 

 and acid-soil plants, is devoid of root hairs and consequently has a 

 restricted capacity for absorbing soil moisture. This low absorptive 

 capacity is correlated with a low rate of transpiration. Many bog 

 shrubs, although living with an abundant supply of moisture at their 

 roots, have been recognized as showing adaptations for retarded 

 transpiration similar to desert plants. 



(c) The special danger to which the swamp blueberry is exposed 

 by reason of its low transpiration and its corresponding reduced 

 capacity for absorption is insufficient nutrition. The danger of 

 nitrogen starvation is particularly great since these soils contain very 

 little nitrates. 



(d) Some bog plants similarly threatened with insufficient nutri- 

 tion, such as the sundews (Drosera), the bladderworts (Utricularia), 

 and the pitcher plants (Sarracenia), possess means of securing the 

 requisite nitrogen by catching insects and digesting and absorbing 

 their nutritive parts. 



(e) In the swamp blueberry the required nitrogen is secured in 

 a different way. The plant associates with itself a mycorrhizal 

 fungus which is able to assimilate nitrogen from the surrounding 

 organic matter, and perhaps from the atmosphere also, and to convey 

 it into the plant without taking along with it a large amount of 

 the poisonous soil moisture. 



Whether this theory of the luitrition of the swamp blueberry is or 

 is not substantiated in all its details by future investigation, it has 

 afforded a useful basis for cultural experimentation, as will be evident 

 from the results about to be described. 



193 



