Observations on Squamosis and Exanthema of the Citrus. 135 
gum are satisfactorily explained on the basis of a growth and water 
relation. 
In our study of the gums it was clearly brought out that they were 
composed of hydrated carbohydrates and nitrogenous matters. From our 
histological studies we are able to assign the origin of the hydrated carbo- 
hydrates to the cell-walls, the nitrogenous matters to the cell contents. 
The power of diseased cell-walls to swell upon the addition of water, to 
contract on dehydration, and of the gum to change from a hemicellulose, 
when first formed, to a xylose-like substance with the lignification and 
dehydration of the surrounding tissues, all these facts point to the con- 
clusion that gummosis is due to a hydration of the embryonic wood cells. 
The importance of the water relation is, therefore, capital. If not enough 
water is present in the tissues to completely hydrate the affected cell-walls, 
we will find cells floating free in the gum, or only partly detached from one 
another, in fact any of the changes described in the section devoted to the 
histology of the disease. 
Growth and high sapidity of the tissues are essential for the develop- 
ment of gummosis. Must we assume also the concomitant action of a 
cytolytic enzyme ? Let us see what the consequences of such an assumption 
would be. 
If we suppose that the gum is formed as a result of enzyme action in 
conjunction with the essential factors, growth and water, we would find 
serious difficulties in explaining the unilateral excretion of the cytase, 
the form that must be assumed if we are not to enter into conflict with 
the conditions described in our histological study of the disease. Again, the 
cytase excreted from the protoplasm would necessarily permeate the third 
lamella, and, while hydrolysing the secondary membranes, would also 
be acting upon it. But microscopic observation shows that gummous 
degeneration proceeds centripetally in the cell-walls, and yet we know that 
the cells, under certain conditions, will vanish instantly when sections are 
placed in water, without the third lamella showing signs of diffusing into the 
cell lumen. If an enzyme does induce, in the presence of water, hydrolysis 
of the cell-walls, for it to act in the manner observation shows that it does 
it would have to attain the power of action only after having diffused out into 
the secondary and primary lamellae, thus only being able to attack the third 
membrane, which it permeates, after having hydrolysed the former, and that 
in a centripetal manner. Such a mode of action, it must be admitted, 
would be indeed very peculiar. Again, if the gum originated through 
enzymatic hydrolysis of the cell-walls, we would have to suppose that, with 
a change in the water or growth relation, it was acted on by another enzyme 
excreted from the cells, and transformed gradually into an insoluble xylose- 
like compound. The cells of the susceptible tissue would first have to 
excrete a hydrolysing enzyme, and then an enzyme capable of bringing 
