126 Butler.—A Study on Gummosis of Prunus and Citrus, with 
sickle of pathognomonic tissues in which all stages of gummosis, from the 
initial to the large and extensive gum pockets, are to be found. In such an 
ideal sickle of pathognomonic tissue one would be able to follow, beginning 
at an apex, the malady through its various developmental stages. 
Premonitory to the first symptoms of gummosis or coincident therewith, 
the cambium frequently lays down centripetally cells rich in granular 
protoplasm, though this increased protoplasmic content of the embryonic 
wood cells is not necessary, as Mikosch believed, to gum formation. A 
dense protoplasmic content is not an invariable accompaniment of gummosis, 
and essentially it is only an indicator of active metabolism. The importance 
of starch has also been greatly overestimated. It is invariably absent from 
the young pathognomonic tissues, and I am unable, therefore, to agree with 
the view that it plays an important réle in gum formation. The cell contents 
take no part in the initial stages of gummosis: it will be shown as we pro- 
eeed that they remain passive at all times. The cell-wall, on the other 
hand, is the seat of the malady ad zuitio and throughout its subsequent 
development. 
- Material fixed, preserved, and examined in 95% alcohol will show 
perhaps rather exceptionable activity on the part of the cambium, and 
frequently an increased protoplasmic content of the embryonic wood cells, 
giving them a pseudo-nidulose appearance. The wood in formation, as in 
the case of healthy trees, is more or less distinctly enframed between the 
xylem, the cambium, and the medullary rays. The sections appear normal, 
and it would be impossible to say that they were not (Pl. X, Fig. 5). If, 
however, we replace the 95% alcohol by progressively weaker solutions, 
finally arriving at water, we will notice that while the protoplasm remains 
unaffected, a change takes place in the cell-walls. The walls appear to 
stretch and swell, the swelling taking place in the secondary membrane. 
This change constitutes the initial stage of gummosis and is quite fugacious, 
being rapidly followed by further and more marked swelling and disappear- 
ance of the primary membrane. By further absorption of water the primary 
and secondary membranes, henceforth indifferentiable, increase in bulk, 
become semi-fluid and accumulate, preferably it would appear at the 
corners of the cells, i.e. at the place where a given pressure would be most 
likely to force the cells apart from one another (Pl. X, Figs. 6,7). The 
gum, however, may also accumulate medianly, pushing the cell-walls apart, 
forming small elliptical cavities. Gummous degeneration of the cell-walls 
now continues centripetally, the third lamella rapidly dissolves away, the 
cell contents become part of the gum matrix, which occupies, as a homo- 
geneous mass, the spot but a short time before filled by the degenerating 
cells. The cells bordering the gum pockets become markedly convex 
owing to the release of pressure following the disappearance of the abutting 
cell or cells, and the real period of the growth of the gum pocket may be 
