128 Butler.—A Study on Gummosis of Prunus and Citrus, with 
gum. These cells immediately vanish upon the addition of water, but without 
any perceptible change taking place in their protoplasm (PI. x, Fig. 6, 
section through gum pocket observed in alcohol ; Fig. 2, same section after 
being placed in water: three cells have vanished). 
If now we examine a gum pocket which is no longer developing 
actively, we will observe at its periphery, or scattered promiscuously within 
it, a certain number of free cells that no longer vanish upon the addition of 
water (Pl. X, Fig. 8). In studying these cells we find, as hitherto, that 
the protoplasm plays no réle in gummosis. The Citrus are best suited for 
the study of this stage as the gum in the pockets does not dissolve very 
rapidly, and staining may be resorted to. In sections stained with Bohmer’s 
haematoxylin, the gum colours more rapidly than the cell-walls, and the 
contents of the cells remain unaffected. Sections stained in Béhmer’s 
haematoxylin may be counterstained with Congo red; the gum then 
appears blue, as before, but the protoplasm is always coloured red, as like- 
wise the healthy cell-walls. If on the other hand we stain sections in aniline 
blue 2v. the protoplasm only will be coloured. With Bohmer’s haema- 
toxylin on the one hand, and aniline blue 2v. on the other, we are able to 
determine exactly when we are dealing with gum and when with protoplasm. 
We, therefore, arrive at the conclusion that the cell contents are free from 
gum and take no part in its formation from the initial stages of gummosis 
until they finally vanish upon the gelatinization of the tertiary membrane, 
With the dissolution of this membrane the protoplasm is added to the gum. 
We have seen that when a gum pocket is to be formed the following 
changes take place: (1) a susceptible tissue, in reality embryonic wood 
cells, is laid down by the cambium; (2) at no very determinate position 
the secondary and primary lamellae of certain of these cells gelatinize and 
form a nucleus, as it were, for the future gum pocket; (3) the gelatinization 
of these lamellae detaches the cells one from the other; (4) the third 
lamella gelatinizes and the cells vanish in the gum mass; (5) the gum is 
composed of gelatinized cell-walls and of the contents of the dissolved cells 
(i.e. protoplasm), together forming a homogeneous mass. We have still to 
describe the changes that take place in this susceptible tissue when the 
growth of the gum pocket is arrested before it has attained its ultimate 
development, that is to say, before all the cells predisposed to gummous 
degeneration have become affected. 
Following the arrestation of growth of the gum pocket, and this may 
occur at any time during its development, the cells of the susceptible tissue 
lignify. The cells growing into the gum, to use figuratively a phrase that 
has been erroneously employed in the absolute sense, as well as those 
bordering the gum pocket, together with the remaining cells of the sus- 
ceptible tissue, gradually thicken their walls. The lignification of the 
susceptible tissue cells, however, is never as complete as in normal xylem, 
