9 o 



BACTERIA AND THE NITROGEN CYCLE 



plant (Fig. 19,^). If a transverse section of a young and firm nodule, which 

 exudes a turbid milky juice on crushing, be examined, masses of large cells 

 are seen, distinguished from those that surround them by being filled with 

 a finely striate or punctate mass. Sometimes there are numerous small 

 nests of cells, sometimes they run together to form larger masses. These 

 cells, which constitute what was formerly known as the ^bacteroidal tiss e' 

 (Fig. 19,$ and c], are nothing more or less than the enlarged cells of the root 

 itself crammed full of fine, slender, rod-shaped bodies (Fig. 19 d). The nature 



of these has been variously inter- 

 preted. Woronin, the first observer 

 (1866), took them to be bacteria-like 

 parasites ; other investigators since 

 then have looked upon them as pro- 

 teid crystals. If this view were correct 

 we should have to regard the root- 

 nodules not as pathological structures, 

 but rather as a kind of nitrogenous 

 potato with the albumen in the form 

 of bacteroids. Recent investigations 

 have, however, placed it beyond dis- 

 pute that these rod-shaped bodies are 

 bacteria, living and growing in the 

 cells of the root (Fig. 19, c-f). The 

 normal straight, rod-shaped bacteria, 

 well tingible with anilin colours, are 

 only found in the young nodules. As 

 these grow older the bacteria take 

 on all manner of distorted shapes ; 

 spindle-shaped, branched, bifurcated 

 or inflated forms are common. It is to 

 these deformed bacteria only (Fig. 19, 

 e and/) that the term bacteroids is still 

 applied. They are so-called 'involu- 

 tion forms,' similar to those found among 

 many other species of bacteria when 

 the conditions of growth are unfavourable (the acetic ferment, for instance, 

 or the bacilli in old tubercle or diphtheria cultures). The cell contents as 

 well as the shape are affected. They stain badly, and large numbers of the 

 bacteroids seem to be mere empty shells. The conversion of the bacteria 

 into bacteroids is, in short, nothing but a sign that the micro-organisms are 

 dying and yielding up their protoplasmic contents to the plant, which begins 

 to grow more vigorously as soon as the bacteroids appear. By the time the 

 seed of the plant is ripe the shrunken empty nodules contain only fragments 



FlG. :Q. Root-nodules of Leguminosae. a, root 

 nodule of the lupine, nat. size (from Woronin) ; 6, 

 longitudinal section through a lupine root and nodule ; 

 jf, fibro-vascular bundle of root giving off fine branches 

 to every part of the nodule, and its bacteroidal tissue 

 (w), (low power, from Woronin) ; c, a cell from a lupine 

 nodule filled with bacteria (black) between which the 

 finer stroma of the protoplasm is visible ; at the angles 

 of the cells intercellular spaces ; from a section (fixed 

 with Fleming's solution, stained by Gram's method). 

 d, bacteria from root nodule of lupine, normal un- 

 degenerate form ; e andyj bacteroids from Vicia villosa 

 and Lupinus albits (from Morck). Magn. c 600, d-f 

 about 1500. 



