Observations on Squamosis and Exanthema of the Citrus. 109 
dissolved away. The gum pockets enlarged by continued destruction of 
the peripheral cells, When occurring within the young wood, the gum 
pockets were also produced by resorption of the cells, and were even 
observed originating in the vessels. When gummosis was not severe, and 
conditions favourable to normal growth recurred, sound wood was formed 
again by the unaffected cambium, which de zovo laid down gum-forming 
tissue when conditions favourable for its production were brought about. 
Trécul mentioned one causal condition of gummosis: rain. 
In 1863 Wigand! published an important memoir on gummosis. He 
studied gum formation in Prunus avium, and, as a result of his investiga- 
tions, concluded that: (1) the gum found within the vessels was due to 
a change in the cell-walls, the evidence in favour of this view being derived 
from the presence and absence, even within the same vessel, of the internal 
surface sculpturing ; (2) except when occurring in the vessels, gum forma- 
tion in the wood was always preceded by the development of an abnormal 
wood parenchyma. This abnormal parenchyma was laid down between 
the medullary rays at any time during the year’s growth, and within it the 
gum pockets were produced lysigenously : one pocket only was usually 
formed, though at times radial rows of three were produced, each one 
separated from the other by normal xylem ; (3) the gum pockets could be 
laid down year after year in the successive annual rings; (4) gum formation 
in the cortex was also preceded by an abnormal parenchymatous growth, 
the extension of individual bast rays ; (5) the formation of gum pockets was 
due to a centripetal dissolution of the cell-walls, the gum filling the cavities 
thus formed being derived from the dissolved walls and from the starch 
within the affected cells which had suffered a synchronous gummous 
degeneration ; (6) gummosis could begin in the wood and proceed over into 
the bark, and ¢ converso; (7) cherry and plum gums were a mixture of 
arabin and of a gum resembling bassorin, i.e. cerasin, which, unlike it, 
however, was soluble in boiling water. 
Wigand did not consider gummosis a malady fer se, but rather 
a symptom of weakness of the affected tissues. 
In his study on vegetable mucilages, Frank? devoted a section to 
gummosis of the cherry. He found that gum is formed from the secondary 
membranes in normally formed wood, or through the disorganization of an 
abnormally formed wood parenchyma and certain cortical tissues. These 
different areas of gummosis were not necessarily interdependent as regards 
formation, though whenever the seat of the disease was in the wood all three 
forms were observed. 
1 Wigand : Ueber die Desorganisation der Pflanzenzelle, insbesondere tiber die physiol. Bedeu- 
tung von Gummi und Harz. Pringsheim’s Jahrb., iii, 1863, pp. 115-1 82. 
2 Frank, A. B.: Ueber die anatomische Bedeutung und die Entstehung der vegetabilischen 
Schleime. VI: Kirschgummi, Pringsheim’s Jahrb., v, 1866-67, pp. 184-198. 
