Ai>r. ax, 1933 
Gummosis of Citrus 
223 
It is the purpose of Part III to discuss more especially the process of 
gum formation itself, the conditions facilitating its formation, and its 
relation to the development of diseases. 
NATURie AND ORIGIN OF THE GUM 
Although gum appears to be formed in many other plants continuously 
as a ''normal'' process, it is usually not formed in Citrus except under the 
influence of stimuli more or less injurious to the tree. Citrus gum is 
similar to cherry gum and gum arabic, the latter of which is known to 
retain the nature of an acid, the molecule being composed of a number of 
sugar residues grouped about an acid nucleus in such a way as to leave 
the acid group exposed. 
These gums differ from resins in being for the most part soluble in 
water and insoluble in alcohol, while the opposite is true of the resins. 
This solubility in water also distinguishes them in a rough, imperfect 
.way, from mucilages, which have a more slimy consistency and merely 
swell up in water. There are, however, all gradations between gums so 
defined and the mucilages. As to origin and chemical cUmposition, the 
distinction between the vegetable gums and some of the mucilages can 
probably not be clearly maintained. These exuded gums, however, 
should probably be distinguished from a hard, vitreous gumlike substance 
known as wound gum, which Higgins {40) refers to as occurring in the 
wood elements in the vicinity of wounds as a general phenomenon in 
woody plants. It differs from the ordinary gums in not swelling in water 
and in giving the lignin test. 
Although there has been considerable difference of opinion as to tjie 
direct origin of gums and mucilages in plants, most investigators, includ¬ 
ing Grafe-Wien (35), Czapek (j<^), and Lloyd (45), have concluded that 
the gums like those represented by gum arabic,^ cherry gum and Citrus 
gum, are derived mainly from the cellulose walls. Greig-Smith (jd), 
however, has reported the formation of gums of this nature by the direct 
action of bacteria on certain culture media containing no Cellulose. 
Oum! related to the dextrins also has been obtained by the action of 
Bacillus rddicicola on culture media containing sacchrose by Buchanan 
(12), who concludes that this gum arises from the diffluent wall of the bac¬ 
terial cell. MacDougal, Richards, and Spoehr {47) have pointed out that 
in cacti the formation of mucilagelike gum results from the dehydration 
of sugars and condensation of their products as a normal process in this 
plant. 
In Citrus, however, the gum appears to arise mainly from the hydroly¬ 
sis of the cellulose walls. The initial gum originates, according to Butler 
(j3) and Floyd (51), between the medullary rays usually in thin-walled 
xylem cells newly laid down. The protoplasm becomes more granular, 
the cells round out, separate from each other, and dissolve from the 
outside inward, and the space is finally occupied by a mass of gum. It 
may also be derived apparently from the breaking down of other and 
older tissues in connection with some of the severe types of Citrus 
gummosis. 
PHYSICAL EFFECTS AND GUM FORMATION 
MECHANICAL INJURIES 
The writer has not been able to induce gum exudation on healthy 
Citrus trees by mechanical injuries alone, provided these injuries are 
30617—23 - ^^3 
