132 Butler.—A Study on Gummosis of Prunus and Citrus, with 
It may also be pointed out that the death of a cell is not necessary, as 
we have already seen, for the development of gummosis. Nor could we, 
under the cytase hypothesis, have live cells protected only by the tertiary 
lamella lying unaffected in the gum. Nor does it appear possible to explain 
the greater extension of the gum pockets upwards from the centre of 
initiation if diffusion of cytase from the dead cells is the cause of gummous 
degeneration, for a solution tends to diffuse equally in all directions. The 
lesser development of the gum pockets as one proceeds laterally of the 
inciting centre is also difficult to explain, for the medullary rays are very 
resistant to the disease, and, therefore, arrest lateral diffusion ; but allowing, 
for argument’s sake, the intercommunication of all the gum pockets, the 
asymmetrical oval development of the tissues affected by gummosis is not 
explainable on the hypothesis of a cytase diffusion from necrobiotic cells. 
For these reasons, and for others that will be developed later, I am unable 
to accept Beijerinck and Rant’s explanation. 
Ruhland’s! hypothesis that gum is formed by oxidation of the carbo- 
hydrate substances within the cells that go to the making of the cross-septa 
after division, and of the pectins and pectinates of the cell-walls, is not 
tenable, as a chemical study of the gums clearly shows that they are not 
oxidation products of carbohydrates ; and for the same reason Sorauer’s 2 
notion that the increased gum flow induced by oxalic acid was due to 
oxidation is equally erroneous. 
It being, therefore, impossible to accept the views heretofore held 
regarding the nature of gummosis, it will not be out of place to determine 
whether or not an explanation may be advanced that does not conflict with 
any of the observed facts. Our task then will be, first, to correlate the 
observations recorded in the previous pages, and second, to show that the 
explanation I would substitute for those of Beijerinck and Rant, Ruhland, 
and Sorauer satisfactorily explains this correlation. 
1. For gummosis to develop two conditions must be simultaneously 
fulfilled. The cambium must be actively laying down new tissue elements, 
and a superabundance of water must be present in the soil. 
Though neither growth nor a superabundance of water can of themselves 
induce the disease, their rdle in the development of gummosis may be 
separately discussed if we are careful to continually bear in mind that neither 
is operative without the other. I shall, therefore, consider under separate 
captions the relation between growth and gummosis, and the relation between 
water and gummosis. 
Relation between extent of gummous degeneration and growth. When 
gummosis is artificially produced by means of sulphuric acid, for instance, 
we find that the extent to which the disease develops is very closely related 
to the amount of growth taking place at the time of the wounding. Gum- 
1 Loc, cit., ante, 9 Loc. cit., ante, 
