LEGUME TUBERCLES 
The bacterial tubercles which commonly occur on the roots of legumes 
are not generally regarded as evidence of a diseased condition. They are 
nevertheless pathologic in character. That the plant usually’ profits 
from their presence on its roots is no argument against their pathologic 
nature. The roots of all the common leguminous crops usually show these 
galls. 
SYMPTOMS 
Roots of various leguminous plants have been provided. (See also 
the illustration specimens in jars.) Study and compare the tubercles 
on the different hosts. OBSERVE:— 
1. The shape, size and color of the galls; more or less characteris- 
tic for different hosts. 
2. Location on the root-system; deep or shallow; on small 
or large roots; lateral or terminal? 
3. Types of galls; simple and compound (branched). Make 
DRAWINGS of the tubercles on pea, bean, clover and alfalfa. 
ETIOLOGY 
The cause of these tubercles is a bacterial pathogene, Bacterium 
leguminosarum (Frank) E. F. Smith. It is probable that this species 
includes a number of more or less distinct biologic forms. 
Life-history. This pathogene is a highly specialized parasite, develop- 
ing normally in living plant-tissues which it stimulates to overgrowth. 
The galls which it induces are usually annual growths. It is probable 
that in most cases only primary cycles’ occur. 
The Primary Cycles are initiated in the spring and summer on the 
developing rootlets of the host. 
Pathogenesis. Active individuals in the soil-water constitute 
the inoculum. Study a mount from an actively growing pure culture 
provided. OBSERVE :— 
4. The form and size of the bacteria. They are actively motile, 
having one polar flagellum (which can be seen only in specially stained 
preparations). 
These bacteria in the soil accumulate in small groups on the sides of 
the root-hairs. By means of some secretion, they gradually soften the 
wall of the root-hair and penetrate it. The root-hair is stimulated to 
bend or curl about the mass of penetrating bacteria in a characteristic 
fashion. Study the prepared slides showing root-invasion. OBSERVE :— 
5. The curled tip and the infection-thread extending through 
the length of the root-hair. pDRaw. 
6. Under the demonstration microscope an infection-thread 
within the tissue of the root. Note the funnel-like swellings produced 
just before the host-walls are penetrated. DRaw. 
These bacteria, having penetrated to meristematic cells, multiply 
igi and stimulate abnormal cell-division, resulting in the formation of 
the gall. 
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