599 
Sieve-Tubes of Pinus. 
cellulose by deposition from the protoplasm, and judging 
from its reactions callus only appears to be a special type 
of mucilage or hydrated cellulose. 
The protoplasmic origin of the callus was suggested by 
Russow in order to account for the vast amount of this 
substance, which is found in the older sieve-tubes of various 
plants, without any apparent diminution in size of the original 
pit-closing membrane taking place ; and although there is no 
doubt that a small callus-cushion can swell enormously with- 
out any addition to its substance 1 , swelling in itself does not 
offer a sufficient explanation of the observed facts, and it 
seems likely that the protoplasm found in the older sieve- 
tubes adds to the callus-mass which has been formed in the 
first place from the cell-wall (Fig. 7, PI. XXXI; Fig. 14, 
PL XXXII ; Fig. 22, PI. XXXIII). 
An interesting case of the protoplasmic origin of the callus 
was also noticed in a section of the phloem of Pinus excelsa , 
where callus had been formed against the tangential wall of 
a medullary-ray cell ; the wall appeared quite normal and 
was traversed by threads in the usual manner, but a convex 
pad of callus, equal in thickness to the wall, had been 
developed on the phloem-side of the membrane ; as the 
original wall was unaltered the protoplasmic origin of the 
callus-pad seemed to be the only possible explanation 2 
(Fig. 27, PL XXXIII). 
In cells of this same medullary ray nearer the cambium- 
region, a peculiar appearance of the starch-grains was noticed, 
after staining the section with water-blue ; for here and there 
in the cell-protoplasm fairly large crescentic bodies were 
seen, which stained a pale sky-blue, the colour being just like 
that assumed by the callus under the same conditions. These 
bodies appeared to be starch-grains undergoing mucilaginous 
degeneration (Fig. 26, PL XXXIII). 
1 By the action of orange G., the callus-cushions and the free ends of the callus- 
rods swell to nearly twice their former size. 
2 The section, which was a thin one, was shaken up several times in water with 
fine sand in order to see if the callus was not some foreign inclusion ; but it could 
not be dislodged, and appeared definitely to belong to the wall in question. 
