SAPROPHYTISM AND SYMBIOSIS 



791 



devoid of nitrogenous compounds; moreover, tubercles do not develop 

 and the plants are depauperate if the soil is sterilized. 



There is now ample direct evidence that Bacillus radicicola can fix 

 free atmospheric nitrogen to some extent when isolated in pure cultures, 

 and much more abundantly when growing in the roots of leguminous 

 plants, the resulting compounds accumulating within the bacterial body. 

 Such nitrogen fixation has been shown to be facilitated by the presence 

 of an abundance of sugar, especially maltose, and of free nitrogen, and 

 to be retarded by the presence of an abundance of nitrates or of albu- 

 minous substances. It is scarcely to be doubted that Bacillus radicicola 

 and the leguminous plants live in a state of reciprocal parasitism, the 

 bacteria deriving carbohydrates from the legumes, while the latter derive 

 nitrogenous compounds from the bacteria. The parasitism of the two 

 symbionts appears in part alternative. The bacteria soon after entrance 

 into the root exhibit great vigor and activity, probably through their 

 bettered food relations, while the legume is injured rather than benefited 

 by their presence, since in the cells which they occupy, the bacteria utilize 

 the starch and much of the cytoplasm and cause the nuclei to become 

 partly disorganized. After a time the leguminous plant appears to 

 overcome the bacteria, and it enters upon a state of vigor because of the 

 appropriation of nitrogenous compounds, while 

 the bacteria enter the bacteroid or Rhizobium 

 state of relative inertness, after which there is 

 no recovery of the power to fix nitrogen or to 

 infect other leguminous roots; probably some 

 nitrogenous compounds become available for the 

 legume during the later phases of bacterial activity, 

 though it is through the final dissolution of the 

 bacteroids that the main supply appears to be 

 derived. 



Mycosymbiosis. Ectotrophic and endotrophic 

 mycorhiza. Fungi are associated habitually with 

 the roots of many plants, such as the oaks, pines, 

 orchids, and ericads (Ericaceae). The root in 

 combination with its fungus is known as a myco- 

 rhiza (i.e. fungus root), and the phenomenon may 

 be termed mycosymbiosis. If the fungal hyphae 

 invest the roots, as in the beech (fig. 1103), the 

 mycorhiza is called ectotrophic (i.e. nourished 



FIG. 1103. A root 

 tip of the European- 

 beech (Fagus sylvatica), 

 illustrating ectotrophic 

 mycorhiza; the fungal 

 hyphae (h) form a dense 

 layer which ensheathes 

 the root; considerably 

 magnified. After 

 FR'ANK. 



