FIXATION OF FREE NITROGEN 



197 



A transverse section of a nodule {A, Fig. 265) shows a thin 

 layer of cortical tissue, a ring of small vascular bundles, and 

 a large more or less central mass of parenchymatous tissue, 

 each cell of which contains numbers of bacteria belonging to 

 an aerobic species now known as Pseudomonas radicicola Beijk. 

 { = Rhizobium legitminosarum Frank). 



The bacteria, which 



r 



must not be confused 

 ,iy'' with nitrifying and other 



organisms, are present in 

 almost all soils, and are 

 especially abundant in 

 those which have pre- 



fg^ viously borne crops of 



^ I fc,Jv? leguminous plants. From 

 '"fFO %f? ^^ ^^^ they make their 

 Pj,^!^ way through the epid- 

 ermis into the cortex of 



Q 



FlO. :16s.— A, Transverse section oi a nodule from the rOOtS of legUminOUS 

 the bean root, depicted in the previous Fig. 264 ; , 



c cortical parenchyma ; vi small vascular^ bundle ; plants, and thcrC induCC 



t bacteroidal tissue (enlarged about 15 diameters). , _ . _ , 



B, Isolated bacteroids from cell of tissue t (enlarged the lOrmatlOn 01 eXUber- 

 6S0 diameters). .,111 



ant parenchyma, the cells 

 of which they soon fill by their rapid vegetative reproduction. 



At first the bacteria are very minute short rods, but after 

 living for a time in the roots of the infected plants, they in- 

 crease in size and undergo a species of degeneration, most of 

 them becoming somewhat Y'Shaped (B, Fig. 265), or otherwise 

 changed in form ; these degenerate- or involution-forms are 

 usually spoken of as bacteroids. 



Not only do the roots of the bean possess nodules inhabited 

 by these organisms, but the roots of all species of leguminous 

 plants belonging to the sub-order Papilionaceas have them. 



The bacteria from the roots of clover, peas, vetches, sainfoin, 

 and many other species of Leguminosse have been separately 



