100 



SYMBIOSIS. 



which may increase to very large tuhers, with surfaces resemb- 

 ling a bunch of grapes (Fig. 21). In the large cells of the 

 middle layers of the primary root-cortex of these growths, 

 coils of very fine fungus-threads are sheltered ; these extend 

 year after year into the younger parts of the enlarging tuber- 

 cles, and gradually disappear in the older part^. What may 

 be the significance of these structures for plants possessing 



Fio. 21. — Frankia alni. Root-tubercles on the Alder, (v. Tiibeuf phot.) 



ohlorophyll and furnished with normal roots is as yet unknown. 

 Plants which have grown well for years in water-cultures do not 

 show them. On account of the cork -covering with which these 

 tubercles are furnished, it would seem that they are not adapted 

 for taking nourishment out of the soil. 



Woronin described them first on the alder. Warming on 

 Elaeagnaceae, while MoUer proved their fungal origin. 



The species of fungi which produce these tubercles have been 



