598 Hill, — The Histology of the 
deposition from the protoplasm ; but no doubt in some cases 
it may only arise in the one way, as for instance appears to be 
the case in Laminaria \ 
The callus-rods have been shown to be due to the action 
of ferments on the cell-wall, by means of which the cellulose 
is changed into callus, whilst the large callus-cushions found 
on old sieve-plates appear to have been deposited by the 
protoplasm, since, however large the cushion may be, there is 
no diminution in size of the pit-closing membrane (Fig. 22, 
PL XXXIII). 
A certain amount of evidence is also afforded in support of 
the wall-degeneration theory by an anomalous case of callus- 
formation, which was noticed in a transverse section of the 
phloem of Pinns (Fig. 25, PL XXXIII). 
In two adjoining sieve-tubes, next to a medullary ray, the 
cellulose layers of part of the tangential and radial walls 
(next the ray) had undergone a change into callus. The 
callus, which stained in the usual way with water-blue, merged 
insensibly into the cellulose of the rest of the wall, and had no 
relation to any connecting threads. The effect produced, 
which is precisely similar to some figures given by Oliver 2 , 
appears to be due to the action of a ferment, which for some 
unknown reason has brought about the mucilaginous degenera- 
tion of the cellulose 3 . 
With regard to the protoplasmic origin of the callus, there 
seems no a priori reason why the protoplasm should not form 
callus just as easily as it can form cellulose, and indeed 
Gardiner 4 has shown in the case of the mucilage-hairs of 
Blechnum that mucilage is formed in a similar manner to 
1 Oliver, loc. cit. The callus of the ‘ trumpet hyphae ’ of Laminaria is formed 
by the alteration of the cell-wall. 
2 Ibid., Ann. Bot., vol. i, PI. VIII, Figs. 7, 8, 9, &c. 
3 Mangin, Compt. rendus, cxv, p. 260. Callus is described as occurring in the 
membrane of cystoliths and other places, which seems to indicate that in these 
cases also callus is formed from the cell-wall. 
4 Gardiner and Ito, ‘ Mucilage-secreting Hairs in Blechnum and Osmunda ’ ; Ann. 
Bot., vol. i, pp. 33 et seq. (Figs. 40 and 43). Also on p. 39 a formation of callus 
on the transverse walls of the mucilage-cells is also described, which appears to be 
of protoplasmic origin. Cf. Figs. 34, 36, 41, and 43. 
