134 Butler. — A Study on Gummosis of Prunus and Citrus , with 
susceptible tissue produced below the wound will be less extensive than 
that which is laid down above. An elliptical development of xylem tissue 
round a wound is, therefore, a perfectly normal phenomenon, but whenever 
the tissues are at the same time gumogenetic, as in Prunus and Citrus , we 
must consider this character as an acquired one. Growth and gummosis 
are nevertheless directly correlated. 
Relation between extent of gummous degeneration and water . The 
relation between gummosis and growth is direct, the relation between 
gummosis and sapidity of the tissues indirect. Sapidity of the tissues 
is insufficient in itself for the development of the disease, but as soon 
as it is combined with the necessary factor, growth, we may say that its 
role changes : it will then govern the extent of gummous degeneration. 
The effect of a high water-content of the soil on increasing the 
susceptibility of Prunus and Citrus to gummous degeneration has been 
mentioned by most writers on gummosis, its significance, peculiarly enough, 
only being overlooked by those who have studied the histology of the 
disease. Amongst this latter class, however, we find an exception in 
Trecul , 1 who states that the disease arises from a too abundant accumulation 
of sap at any point, this accumulation being due to a variety of causes, 
rain being the only one that he specifically mentions. 
In one of Aderhold’s 2 infection experiments with Clasterosporium 
carpophilum , we find it mentioned that the gum produced in infected 
wounds largely increased after watering. The trees experimented on were 
peach, apricot, and cherry. 
In the case of Primus the horticultural writers agree that excessive 
soil moisture favours the development of the disease. Gummosis occurs 
in retentive, poorly drained soils ; in non-clay soils underlaid by an im- 
permeable subsoil and unfavourably situated as regards drainage ; under 
abnormal climatic conditions, such as a very wet spring, or long sequence 
of cloudy weather inducing a rupture of equilibrium between transpiration 
and absorption. 
In the case of the Citrus the opinions of the writers who have studied 
the disease in this genus are no less emphatic. Excessive humidity of the 
soil, they all agree, is very favourable to the development of gummosis. 
Whether the trees are grown in poorly drained retentive clays, or upon 
an impermeable subsoil, or are subject to excessive irrigation, the result is 
the same — the trees succumb to the disease in a comparatively short time. 
The climatic conditions favourable to the development of gummosis in 
Prunus are also conducive to its outbreak in Citrus . 
It is, therefore, clear that growth and a high water-content of the 
tissues are necessary for gummous degeneration. We have now to determine 
to what degree our studies on the nature and origin of the resulting 
1 Loc. cit., ante. 2 Loc. cit., ante. 
