ENTRANCE OF BACTERIA INTO LEGUMINOUS PLANTS 181 



§6. MODE OF ENTEANCE OF BACTEEIA INTO 

 LEGUMINOUS PLANTS. 



As the bacteria are not present in the seeds, they must gain 

 entrance to the roots of leguminous plants from the surrounding soil. 

 This is effected, in some at least of these plants, in the following 

 manner : The end of a root-hair changes its form and becomes more 

 or less sickle-shaped. The next stage consists in the formation on the 

 inside of the wall of the root-hair of a shiny colony of bacteria, which 

 then sends out a tube filled with bacteria. This tube (Fig. Ill) 



h 



Fig. 111. — To illustrate mode of entrance of nitrogen-bacteria into roots of 



leguminous plants. (For explanation see text.) 



a. Bacterial tube consisting of millions of bacteria ; 6, Root-hair. (After Nobbe.) 



branches and works its way into the inside of the root, ultimately 

 causing hypertrophy of the cells, and the formation of the bacteroid 

 tissue. This method of formation, however, has been denied by other 

 writers. It has been stated that it is the plant itself, and not the 

 bacteria, that gives rise to the tube — known as the infection thread — 

 because it is claimed that it has a structure which could be formed 

 only from the protoplasm of higher plants, that is, that its walls are 

 composed of a substance allied to cellulose which is not formed in 

 bacterial cells. This point is not yet settled, but it is highly probable 

 that, as the leguminous plant is ultimately parasitic on the bacteria, 

 and derives such immense benefit from their presence, the latter 

 explanation is the correct one. 



The structure and course of the infection threads can best be 

 ascertained by staining sections with a liquid made up by dissolving 

 equal quantities of fuchsin and methyl-violet in 1 per cent, acetic 

 acid. The protoplasmic contents and the membrane of the bacteroid 

 tissue are coloured blue, the bacteria are coloured red, whilst the wall 

 of the infection thread remains uncoloured. 



The Bacteroids. The bacteria inside the bacteroid tissue undergo 

 strange changes of form, as can be seen in Fig. 112, where some of 

 the more noticeable forms are presented. It must be noted that 



