Observations on Squamosis and Exanthema of the Citrus. 115 
for Mikosch tells us that owing to the activity of the protoplast gum 
was continually excreted. This excretion continued until the protoplast 
was gradually pushed to the base of the cell, where a few remnants of 
it might be found. The cell-wall was scarcely if at all affected and remained 
distinct. It may be noted that in some cases, evidently when gum secretion 
was only active periodically, the protoplast would secrete a layer of gum, 
then a layer of cellulose, then a layer of gum and a layer of cellulose. 
Mikosch figured a cell with three distinct walls enclosing two gum layers. 
Mikosch did not make it very clear how the gum finally became ex- 
tracellular. The cell-walls persisted for a long time unaltered, and it was 
only towards the centre of a gum pocket that they became indistinguishable 
from the gum matrix. These cell-walls furnished the cerasin component 
of the gum. 
Gum pockets were not necessarily confined to the wood in formation: 
they also occurred in the cortex. 
When the attack of gummosis was not severe, and conditions favourable 
for its development were soon followed by conditions unfavourable, then 
normal wood would be laid down on the outside of the gum-forming tissues, 
and the cells lying within the pathognomonic zone gradually lignified. 
Mikosch concluded from his study that gummosis was a pathological 
phenomenon due to the response of the cambium to wound stimuli. 
Ruhland! in 1907 advanced the opinion that gum formation was not 
due ger se to traumatisms, but to the fact that they allow, when deep 
enough, air to penetrate to the cambium, or more properly to the young 
wood in formation. Atmospheric oxygen, he believed, was the active 
agent in gum formation. This gas acted upon the pectins and pectinates 
of the cell-walls and upon the carbohydrates within the cells that should 
have gone to the building of the new septa following cell-division, oxidizing 
them into gum. 
The hypothesis regarding the intracellular formation of gum was 
arrived at through a study of diseased tissues. Ruhland found that a 
number of the large swollen cells bordering the gum pockets contained two 
well-developed nuclei but no cross-walls, and he concluded that intracellular 
gum was formed through a process of oxydation homologous to that which 
was effective in producing intercellular gum. 
Ruhland performed a few experiments with the view of determining 
the effect of oxygen exclusion upon gum formation. He used cuttings of 
peach, plum, and cherry, the bottom ends of which were placed in water, 
while the distal cut surfaces were capped with a mantle of hydrogen or 
nitrogen, or some such impervious substance as cocoa-butter, paraffin, &c. 
The experiments gave very marked results: no gum formed in the cuttings 
1 Ruhland, W.: Zur Physiologie der Gummibildung bei den Amygdalaceen. Ber. d. Deutsch. 
Bot, Ges., xxv, 1907, pp. 302-15. 
12 
