ROOT TUBERCLES 113 



trees, and some stem- and leaf-galls occurring on certain members 

 of the family Ericaceae. 



The gall-causing organisms, whether plants or animals, are para- 

 sitic on the host plants, and the galls, although composed of plant 

 tissue, are of no benefit to the plants on which they are formed. The 

 food that is stored in the galls can never be used by the host plant, 

 but only by the parasite. These galls then are comparable to ordi- 

 nary diseases and represent antagonistic nutritive conjunctive 

 symbiosis. Usually they are not very harmful but when they be- 

 come excessively thick on a plant they are seriously detrimental. 



68. Root Tubercles. —The best known bacterial tubercles are those 

 found on the roots of many plants of the legume family. These 

 tubercles are small galls, usually only a few millimeters in diameter, 

 and are composed largely of parenchyma cells, like most other galls. 

 Within the cells of the tubercle are found bacteria. The bacteria 

 are facultative parasites which, in a saprophytic condition, are 

 found rather commonly in soils. They enter the roots of leguminous 

 plants through root hairs and become parasitic on the cells of the 

 roots, and the roots are stimulated to produce the gall-tissues that 

 form tubercles. At first the legume plants suffer somewhat from the 

 presence of these parasitic bacteria but eventually many of the 

 bacteria are digested and absorbed by the root cells. 



Root tubercles are also found in the families Cycadacese, Podo- 

 carpacese, Eleagnacese and Myricaceae and in the genera Alnus and 

 CeonotJms. In all of these cases the tubercles are produced through 

 the modification of lateral rootlets. In this respect they differ from 

 those of the Leguminosee which are produced as out-growths of the 

 cortex and not as modified lateral rootlets. 



The bacteria that are concerned in the formation of these tubercles 

 are spoken of as nitrogen-fixing bacteria because, unlike nearly all 

 other organisms, they are able to use the uncombined nitrogen of the 

 atmosphere, combining it with other elements to form nitrates. 

 Nitrogen in the form of nitrate salts is readily utilized by legume 

 plants and by other green plants. For these reasons the symbiotic 

 relation between nitrogen-fixing bacteria and legume plants is 

 extremely important to agriculture since it increases in the soil the 

 available nitrates for other crop plants. As much as 250 pounds of 

 nitrogen per acre may, under favorable conditions, be added to the 

 soil through the activities of these bacteria. It is primarily because 

 of this symbiotic phenomenon that a legume crop is nearly always 

 included in any rotation of crops. 

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