IRREGULAR NITRITION 



20^ 



same spots as roots, and vascular tissue extends from liic stele into 

 them as they mature. But their structure is different from tliat of roots 

 (Fig. 156). They consist chiefly of parenchyma, whicli when young 

 is found to be traversed by thread-Hke bacterial masses, extending from 

 cell to cell. From these masses numerous minute bodies are derived, 

 which crowd the cells of the tubercle, but do 

 not destroy the protoplasts, while the nuclei 

 of the cells remain well nourished and of large 

 size (Fig. 156,2). In annual Leguminosae 

 the greatest development of these bodies 

 in the cells is reached about the time of 

 flowering. But from this time onwards 

 their numbers diminish. Most of them 

 become dissolved, and the evidence indi- 

 cates that their organic materials arc- 

 drafted off into the conducting system of 

 the host. When the seeds of the Legu- 

 minous plant are ripe the nodules have 

 shrivelled, and relatively few of the small 

 bodies remain. These are set free into the 

 soil as the nodules rot. The bodies 'thus 

 set free are rod-like and motile, and are 

 called Bacterium radicicola. These Bacteria 

 have been cultivated apart from the plant, 

 and it appears that in presence of proteid 

 and sugar they are able to bring the free 

 nitrogen of the air into combination. 



The infection of the roots has been traced 

 to the root-hairs, and doubtless the infec- 

 tive germs are derived from the former 

 tubercles rotted in the soil. A bright spot 

 appears close to the tip of the root-hair. 

 This is a small mass of that gelatinous state of Bacteria known 

 as the zooglaea stage. It works its way through the cell-wall into tlic 

 cell, and extends downwards as the thread-like body already noted 

 (Fig. 156, 3). It traverses the cells of the cortex to the point outside 

 the stele, where the tubercle arises. Cell-division is stimulated, and 

 the zooglaea disintegrates, while the Bacteria multiply greatly, filhng 

 the cells. Most of them assume a branched and turgid form, and arc 

 distinguished as " Bacterioids.'' These are digested by the cells in 

 which they developed, and their materials are used bv the host-plant. 



ii... .^.>. 



Root of Viiia Faba, with nu- 

 merous root-tiit>crrlcs. Rr<1nrr<1. 

 (After Strasburgcr.) 



