88 MANUAL OF TREE DISEASES 



poplar, willow, maple, butternut, walnut, oak and hickory are 

 the most commonly affected by this wood-rot. Specific men- 

 tion of this disease in alder is less frequent in this country be- 

 cause of the slight economic importance of the species of alder. 

 For a description of the symptoms of the common white wood- 

 rot, see under poplar diseases, page 305. 



Root-Tubercles 

 Caused by Bacillus radidcola Beijerinek 

 The roots of alder conoanonly show large clusters of short 

 stubby roots. These abnormal roots represent a diseased con- 

 dition by which the alder benefits. The dwarfed roots are in- 

 habited by the same bacteriiun which causes the root-tubercles 

 of clover, bean, cowpea, locust and other leguminous plants. 

 The bacteria gain entrance to the young lateral rootlets by way 

 of the root-hairs. They midtiply within the cells of the cortex 

 of the root and stimulate this tissue to over-groAvth. They 

 live parasitically and obtain their food materials from the proto- 

 plasm and cell sap of the alder roots, but they do not kill the 

 cells they inhabit. These bacteria take the free nitrogen gas 

 from the air and combine it with other substances. After this 

 is accomplished, the alder roots eventually receive this combined 

 nitrogen and the tree uses it in its metabolic processes. In this 

 way large quantities of nitrogen are obtained indirectly from 

 the air by the alder. The higher plants cannot utilize nitrogen 

 gas from the air and the plants which are parasitized by the 

 nitrogen-fixing bacteria are thus greatly benefited. Such a 

 mutual-benefit relation between the alder and the bacteria is 

 known as symbiosis, although strictly speaking the bacteria are 

 parasitic even though they do not cause the death of the root- 

 tissues. 



Refehencb 



Spratt, Ethel R. The morphology of the root tubercles of Alnus and 

 Ekeagnus, and the poljrmorphism of the organism causing their 

 formation. Ann. Bot. 26 : 11&-128, pis. 13-14. 1912. 



