74 PLANT PHYSIOLOGY 



in the nitrogen content of the soil. The effectiveness of the 

 micro-organisms, however, may be of one of two sorts; 

 either they may themselves absorb and elaborate the free 

 nitrogen of the air, or they may stimulate the leguminous 

 plants to do so. An acquaintance with the nature of their 

 association with the leguminous plants and with the micro- 

 organisms themselves is a necessary preliminary to an in- 

 telligent discussion of this question. 



On the roots of leguminous plants* develop nodules of 

 various sizes, smooth or convoluted. The roots of plants 

 only a few weeks old begin to lose their even contour and 

 uniform diameter, swellings occur at irregular intervals, and 

 these increase in size very considerably until the plant 

 fruits. These nodules or root-tubercles, white or rose- 

 colored, are composed mainly of thin-walled parenchyma 

 cells, enclosing small intercellular spaces, and directly ad- 

 joining the vascular tissues. Sections of young tubercles 

 show large cells with dense, coarsely granular contents, 

 which, on closer examination, fall under two distinct heads, 

 the protoplasm of the cells, and slender rods which prove 

 to be living bacteria. In older tubercles many of the bac- 

 teria have enlarged and degenerated, are of irregular Y and 

 T forms. In still older ones, the bacteria are dead. In 

 other cells than those composing the tubercles no bacteria 

 are to be found.! When the plant begins to fruit, the tuber- 

 cles lose their plump and even appearance, become emptied 

 and shrivelled. Their cells then contain only fragments of 

 the deformed and dead bacteroids (as the "involution" or 

 degenerate forms are called ) and only a few intact and liv- 

 ing rods. These last, set free in the soil by the complete 

 breaking down of the walls of the tubercles, survive until 



* Also on those of alder (see Hiltner in Versuchsstationen, Bd. 46, 

 1896), Eleagnus (see Nobbe in Versuchsstationen, Bd. 41, 1892, and 

 Bd. 45, 1894) and Podocarpus (see Janse in Annales du Jardin Botan. 

 de Buitenzorg, Bd. 14, 1896). 



f Contrary to Frank's assertion (Uber die Pilzsymbiose der Legumi- 

 nosen. Landwirthschaftliche Jahrbiicher, Bd. 19, 1890), Zinnser (fiber das 

 Verhalten von Bakterien, insbesondere von Knollchenbakterien, in leben- 

 den pflanzlichen Geweben. Jahrb. f. wiss. Bot., Bd. 30, 1897) found bac- 

 teria in the tubercles exclusively. 



