152 



1. Generally wax yellow. Ps. campestris. 



2. Wax yellow to chrome. Ps. phaseoU. 

 Brightest color <3. Chrome yellow to canary. Ps. hyadnthi. The brightest 



yellow of the four. 

 Buff yellow to chrome. Ps. stewarti. 



N. B.— Old cultures darken and stress must not be laid on slight differences in 

 color at any age, since the yellow color of the same species varies according to the 

 amount of brown pigment produced, and this varies with the medium and sometimes 

 even with slight changes in the medium (see page 142). 



REMARKS ON THE YELLOW PSEUDOMONAS GROUP. 



Characters in Common. 



These bacteria agree in the following particulars: They are yellow 

 rod-shaped organisms of medium size, straight or slightly crooked, 

 with rounded ends. The segments multiply by fission after elongation. 

 They are generally less than 1 ^ in diameter. The segments are of 

 variable length. As taken from the plant or from ordinary culture 

 media, they are seldom more than three times as long as broad, and 

 are often much shorter. The segments occur singly, in pairs or fours 

 joined end to end, or in clumpy masses of variable size (zoogloeae), 

 more rarely the}^ are united into long chains or into filaments in which 

 no septa are visible. Endospores are absent or rare (none have been 

 observed). The segments are motile by means of one polar ilagellum, 

 which is generally several times as long as the rod, and may be wavy 

 or straight when stained. The species grow readily on all of the ordi- 

 nary culture media, but so far as definitely known all require the pres- 

 ence of air- — i. e., are strictly aerobic.^ None are gas producers. All 

 are sensitive to sunlight. All are quite resistant to dry air. They do 

 not reduce nitrates to nitrites. As a rule, they are not easily destroyed 

 by their own decomposition products. The yellow color appears to be 

 a lipochrome. In the different species it varies from deep orange and 

 bufi'-yellow, through pure chrome and canary-yellow, to primrose yel- 

 low and paler tints. In the same species the j^ellow color also varies 

 somewhat, being frequently changed, darkened, or obscured by the 

 production of a soluble brown pigment, the amount of which pigment 

 varies in different species, and in the same species on different media. 

 Organisms parasitic in plants or saprophytic. 



As our knowledge increases it will, of course, be necessary to revise 

 this characterization and probably to subdivide the group. Ps. cam- 

 pestris and Ps. phaseoli are nearly related; Ps. kyacinthl differs from 

 the above very considerably, and Ps. stewarti is still further removed. 



^Note possible exceptions mentioned on pages 66, 67, and 71. 



