Observations on Squamosis and Exanthema of the Citrus . 131 
no results, owing probably to the fact that it volatilizes before penetrating 
the tissues. Oxalic acid appears to have been successfully used by Sorauer . 1 
The disease may also be produced by the alkalies, and I have successfully 
used potassium hydrate. Corrosive sublimate was found very effective 
by Beijerinck and Rant . 2 I have tried the hydrocarbon kerosene, but 
apparently it is not sufficiently penetrating. 
7. Nature of Gummosis. 
We may now inquire, What is the nature of gummosis? In recent 
years several hypotheses have been advanced to account for the formation 
of gum within the plant. Beijerinck and Rant 3 supposed that traumatisms, 
chemical substances, or fungous parasites acted in an exactly similar manner. 
The cells of the cambium, according to the view of these authors, contain 
a cytase, which, while they remain alive, is unable to attack the cell-wall, 
owing to the semipermeability of the protoplasm. When, however, any 
cells of the embryonic wood are killed by penetrating hyphae, traumatisms, 
or a toxic agent such as mercuric bichloride, the contained cytase diffuses 
out and is able to attack the walls of the circumambient healthy cells, which 
become gummous, and finally dissolve away. More cytase is thus released, 
attacks other healthy cell-walls, and in this manner the gum pocket is 
formed. The authors further point out that mercuric bichloride yields 
more gum than a traumatism ; and this they interpret as strengthening 
their hypothesis, for mercuric bichloride kills more cells than a simple 
traumatism. As further proof of the existence of a cytase they point out 
that the hyphae of Coryneum Beyerinckii are affected by gummous degenera- 
tion, as well as the cells amidst which they are growing. 
Certain objections can be raised against this hypothesis of Beijerinck 
and Rant. If gummosis is due to the action of a cytolytic enzyme diffusing 
outwardly from necrobiotic cells it seems to me that all sources initiating 
gummosis should distinctly tend to cause ultimately an equal development 
of the disease. 
Let us imagine, for simplicity’s sake, that the embryonic wood cells 
occupy a plane figure instead of a solid, i. e. a rectangle instead of 
a parallelepiped. We will call the base of this rectangle B , and its height 
H. Now if we kill the tissues somewhere within BH so as to have an area 
of necrobiotic cells equal to BG, then the cytase formed will diffuse in two 
directions only. But if we kill the tissues at a point P t the diffusion of 
the cytase will proceed in all directions and gummosis will develop with 
increasing rapidity until an area BG' is affected. When this stage in the 
development of the malady is reached its further progress will be neither 
more nor less rapid than in the first case considered. The ultimate develop- 
ment of all gum pockets should be, therefore, approximately the same. 
1 Loc, cit., ante, 2 Beijerinck, M. W., and Rant, A. : loc. cit. , ante. 3 Loc. cit., ante. 
K 2 
