396 BACTERIAL ORIGIN OF GUMS OF ARABIX GROUP, 



The bacterial origin of that variety of gum acacia exuding from 

 Acacia binervata having been proved, it is a just assumption that 

 all other gums of the arabin group are likewise bacterial products, 

 and not substances formed by the plants in a pathological con- 

 dition. 



The formation of gum is, therefore, a bacterial disease, for the 

 parasitic bacteria obtain their nourishment from the plant juices 

 which they elaborate into gum, ^vhich is not required by the host 

 plant. It is still undecided what constituents they may alter. 

 Gum can be formed from saccharose and from dextrose, but I 

 will deal with this side of the subject in a future paper. 



The bacteria that are parasitic in plants are usually found in 

 the feebly acid or neutral juices of the vessels, and probably 

 the gum bacteria are located in the sieve tubes of the soft bast, 

 as Kraus * has indicated, although the gum is not a true cell 

 content as he supposed. 



Bacterium acacia, n.sp. 



Shape, etc. — The bacteria appear as short rods with rounded 

 ends, and occur singly, in pairs, and in groups, sometimes within 

 a well-defined slime case (ascus). On nutrient agar the rods 

 measure 0'5 : 1 /x ; in bouillon 0*5 : 1-5-2 /x, pairs commonly 

 occur simulating a long rod ; on glycerine-saccharose agar the 

 bacteria vary from 0-5 : 0'5-l'5 fx, appearing as cocci and as short 

 rods; on saccharose-potato agar they measure 0-6:l-2'5/x and 

 average 0*6 : 1'5 /z. The cells stain well with the ordinary stains; 



* "Herr G. Kraus has determined by observations on the exudation of 

 gum from Acacia melanoxylon that it is formed only in the bark and not in 

 the wood, and only in the bast layer, never in the parenchyma nor in any 

 more external portion that the bast fibres have no share in its formation ; 

 that it flows from the cells of the soft bast, and especially from the sieve 

 tubes, and that it is not a product of degradation of the cellulose but is a true 

 cell content flowing out unchanged through the unchanged cell walls." 

 Abstract in Journ. Roy. Micr. Soc. [ii.] vi. (1886), 90, from Ber. Sitz. 

 Naturf. Gesell. Halle, 1884, pp. 19-20. 



