302 SYMBIOTIC NITROGEN FIXATION 



laginous (in appearance, though not always to the touch), gray- 

 white to pearly white in color, glistening, and semitranslucent 

 to opaque. The edges are smooth and even. Under the low power 

 the interior is granular. They frequently attain considerable size, 

 a centimeter or more in diameter. 



" Plates made direct from the nodule lack uniformity to a marked 

 degree. The undiluted plate (first plate) begins to show a few 

 colonies in two to four days. These colonies become extremely 

 large in a very short time, their rapid growth being due to small 

 pieces of nodule tissue or to clumps of bacteria carried over into the 

 agar. In five or six days numerous colonies begin to make their 

 appearance, most of them as submerged colonies, which later grow 

 to the surface. 



' The dilution-plate (second plate) colonies are always extremely 

 slow in growth. Generally colonies are large enough for transfer 

 in six to fourteen days, the plates should not be discarded for two or 

 even three weeks. 



' The rate of growth of colonies also varies with the organisms of 

 different nodules. Among the fast growers are the organisms from 

 pea (Pisum), vetch (Vicid), lentil (Lens), sweet pea (Lathy rus), 

 bean (Phaseolus), lupine (Lupinus), wild bean (Strophostyles) , 

 clover (Trifoliwri), sweet clover (Melilotus), alfalfa (Medicago), 

 and fenugreek (Trigonella). The organisms appreciably slower in 

 growth are those from the cowpea (Vigna), Japan clover (Lespedeza), 

 tick trefoil (Desmodium), acacia (Acacia), partridge pea (Cassia), 

 false indigo ( Baptism), dyer's greenweed ( Genista), peanut (Arachis), 

 soybean (Glycine), and hog peanut (Amphiearpa)." 



Morphology of the Bacteria. They are bacilli and when full-grown 

 vary in length from 1 to 4 or 5ju- It is not uncommon to find them 

 from 0.5 to 0.6yu wide and from 2 to 3/z long and some have been 

 found to measure only 0.18/j, wide and 0.9^ long. The bacilli prevail 

 in the young nodule, whereas the branched forms or bacteroids 

 predominate in the older structure. In the cowpea nodules Burrill 

 frequently found large club-shaped bacteroids, though the branched 

 forms were not so numerous. The bacteroids are best' demon- 

 strated when the young nodule is just beginning to show a reddish 

 interior. At this stage the characteristic x and y forms 'occur in 

 great number and show considerable vacuolation and unevenness in 

 staining, especially when stained with carbol-fuchsin. 



"In the old, decomposing nodule the bacteroids are extremely 

 vacuolated and ghost-like, showing small, oval, deep-staining bodies 

 within. The inference is that these bodies are motile swarmers 

 which later free themselves from the ghost-like capsules, rather than 

 bud off, as has been described by some writers. Frequently the 

 swollen rods have a beaded appearance with unstained bands or 

 areas. A few motile rods may sometimes be seen in hanging drops, 



