Observations on Squamosis and Exanthema of the Citrus . 1 5 3 
Upper right-hand figure. Citrus Aurantium. Cultivated variety affected by squamosis, environs 
of Whittier, Los Angeles County, California. 
Lower figures. Citrus Aurantium. Cultivated variety. Shoots from tree affected by exan- 
thema. A, nodal gum postules ; b, pseudo witch’s-broom growth following bud proliferation ; 
c, erumpent pustules ; d, bud proliferation (enlarged). Fig. B from California Agr. Expt. Sta., 
Bull. cc. 
PLATE IX. 
Fig. 1. Citrus Aurantium. Cultivated variety. Cross-section of shoot affected by exanthema 
and showing development of gum pockets as in gummosis. 
Fig. 2. Prunus avium. Cultivated variety. Cross-section of a shoot showing various stages 
in the development of gum pockets. Gummosis was induced by Plowrightia morbosa. 
Fig. 3. Prunus sp. Cross-section of shoot of plum showing first stage of disease (fusoid 
development of pathognomonic tissues). Shaded area shows part of bark killed by burning with 
sulphuric acid diluted one-third with water. 
Fig. 4. Citrus Aurantium. Cultivated variety. Cross-section of shoot affected by exanthema 
showing typical fusoid development of gumogenetic tissues and cortical proliferation. 
Fig. 5. Prunus avium. Cultivated variety. Cross-section through a shoot burned with a drop 
of sulphuric acid diluted one-half with water, and showing development of gum pockets and 
re- formation (in progress) of normal xylem after the lapse of fourteen days. The shoot was sectioned 
somewhat above the spot burned. 
B, bark ; c, cambium ; e, epidermis ; g, gum pockets ; M, medulla ; P, pericycle bundles ; 
s, suber; X, xylem; x', imperfect xylem. 
PLATE X. 
Fig. 1. Prunus avium. Cultivated variety. Cross-section through part of a gum pocket in 
branch affected by Plowrightia morbosa some little distance above the ‘ black knot ’. Normal 
xylem is being laid down over the gum pocket. 
Fig. 2. Citrus Aurantium var. Valencia. Cross-section through gum pocket showing dis- 
appearance of cells upon the addition of water. Compare with Fig. 6, which represents the same 
section in alcohol before the addition of water. 
Fig. 3. Prunus domestica var. Robe de sargent. Cross-section through gum pocket showing 
pseudo-cellular structure of gum. 
Fig. 4. Prumis domestica var. Robe de sargent. Cross-section through young gum pocket. 
Fig. 5. Citrus Limonum. Cultivated variety. Cross-section through portion of a fibro-vascular 
bundle in rind of fruit showing in the dense granular protoplasmic area the first stage in gummosis, 
degeneration of cell-walls. Change in cell- walls only becomes manifest when sections are placed 
in water. 
Fig. 6. Citrus Aurantium var. Valencia. Cross-section of a gum pocket. Section in alcohol 
to compare with same section (Fig. 2) in water. Figs. 2 and 6 illustrate the disappearance of cells 
upon the addition of water. 
Fig. 7. Citrus Limonum. Cultivated variety. Cross-section through young gum pockets in 
various stages of development. 
Fig. 8. Citrus Aurantium. Cultivated variety. Cross-section through a gum pocket showing 
a number of cells floating free in the gum that do not vanish when alcohol is replaced by water. 
