1889.] Tubercles on the Roots of Leguminous Plants, 441 



No decision is arrived at as to whether the nitrogen is got from 

 nitrogen compounds or from the free nitrogen of the air, nor as to 

 what advantage accrues to the bacteria and the host-plant respec- 

 tively. 



As regards the plants' mode of utilising the presence of the bacteria, 

 cultivated bacteria (from pea tubercles) in nutritive media divide 

 indefinitely, and are found there as moving rodlets. In tubercles 

 they are only rodlets while enclosed in tubes ; they change their forms 

 in the substance of the protoplasm, becoming forked and developing 

 into bacteroids. As bacteroids they can long go on multiplying by 

 continually developing lateral branches, even in proper nutritive solu- 

 tion (" In diesem Zustande der Bakteroiden konnen sie sich noch eine 

 Zeit lang vermehren unter fortwdhrender Bildung von Seitenzweigen 

 selbst dann, wenn sie aus dem Knollchen heraus,in geeignete Nahrldsung 

 versetzt iverden"). With the further development of the tubercle 

 they become hyaline, cease to multiply, and at length dissolve. The 

 contents of the bacteroid cells are resorbed as the bacteroids dissolve, 

 certain substances being left behind. In other words, the plant 

 utilises the substance of the bacteria. 



When emptying begins, and with what energy it proceeds, depend 

 especially on the quantity of nitrogenous compounds at the disposal 

 of the roots. In a soil rich in nitrogen the tubercles go on developing 

 unhindered, become large and typical, and rosy inside, and are not 

 exhausted till late ; in poorer soils they attain no great size, are soon 

 emptied, and are green-gray inside. 



In both cases the exhaustion proceeds acropetally, from the base 

 onwards. At the apex remains a zone which is not emptied, and its 

 cells are full of bacteria. Moreover, some bacteria in and out of 

 tubes remain in all the cells, and escape during decay into the soil ; 

 also animals eat the tubercles and disperse the bacteria. In such 

 injured tubercles the bacteroid masses often envelop themselves anew 

 with membranes, and form smaller and smaller colonies ; these the 

 author previously mistook for spores (see p. 438). 



From the preceding, we see that the tubercles depend on a 

 symbiosis which is advantageous to both the plant and the bacteria. 

 The latter feed on the sap and cell -contents, and multiply through 

 innumerable generations, and, both during the life of the host and 

 afterwards, become redistributed in the soil. The plant derives 

 advantage in that it obtains nitrogen by means of the bacteria. 



Though the symbiosis is useful to both, the plant gains most, for 

 it is the more powerful, and sooner or later overcomes the bacteria, to 

 the multiplication of which it sets limits and finally absorbs the 

 substance of the latter. Being the stronger, the plant directs the 

 symbiosis. It encloses the bacteria in the " Bahteroidengewebe" by 

 means of cork, and also protects them. By an apical meristem the 



