THE CELL- WALL. 
37 
the mass immediately turns black, and a violent formation of gas follows; the heat 
must be continued until only the pure white ash remains. This is soon eifected by this 
means, whereas otherwise the reduction to ash is generally very tedious, and often 
does not afford an entirely colouiless skeleton ^ 
Sect. 5. Protoplasm and Nucleus ^ — Now that the significance of proto- 
plasm as the peculiar living essence of the cell has been sufficiently brought out, 
we need only add what is absolutely essential, both as respects its chemical and 
physical nature and its structure and movements. Protoplasm consists of a com- 
bination of (apparently different) albuminous substances (proteids) with water and 
small quantities of incombustible materials (ash). In most cases it also contains, 
as may be concluded on physiological grounds, considerable quantities of other 
organic compounds, belonging probably to the series of carbo-hydrates and oils. 
These admixtures are distributed through its mass in an invisible form ; but it 
not unfrequently includes granules of starch and drops of oil, which at a 
subsequent period may either entirely disappear or may increase in bulk. Very 
commonly the rapidly increasing protoplasm, in itself colourless and hyaline, is 
rendered turbid by numerous small granules, consisting, probably, of minute drops 
of oil. The protoplasm, as it is generally met with, ought therefore to be con- 
sidered as true protoplasm with varying admixtures of different formative materials 
(Metaplasm of Hanstein). The consistence of protoplasm varies greatly at different 
times and under different circumstances, even in the same protoplasm-mass. It 
commonly appears soft, plastic, tough, inelastic, and very extensible; in other 
cases it is more gelatinous, sometimes stiff, brittle (in the embryos of seeds before 
germination); but very commonly it gives the impression of being a fluid. All 
these properties depend essentially on the quantity of water it has absorbed. But, 
however great may be the quantity of water, and its consequent similarity to a 
fluid, the p7'otoplasm is nevertheless never a fluid ; even the mucilaginous or gelatinous 
conditions of other bodies can only be very superficially compared with it. For 
the living and life-giving protoplasm is endowed with internal forces, and, as the 
result of this, with an internal and external variability which is wpmting in every 
other known structure ; its active molecular forces cannot, in short, be compared 
with those of any other substance^. The capacity which protoplasm has, in 
consequence of the forces which become manifested in it, of assuming varying 
definite external forms, as well as its capacity of secreting substances of different 
chemical and physical properties according to definite laws, is the immediate cause 
of cell-formation and of every process of organic life. 
^ On the crystals sometimes deposited in the cell-wall, see Sect. 11. 
2 H. von Mohl, Bot. Zeitg. 1844, p. 273, and 1855, p. 689 ; [Ann. des Sei. Nat. 1857, vol. VII. 
p. 253]. — Unger, Anatomie und Physiologie der Pflanzen, p. 274, 1855. — Nägeli, Pflanzenphysiol. 
Untersuchungen, Heft I. Zürich. — Brücke, Wiener akad. Berichte, p. 408 et seq., i86i. — Max 
Schultze, lieber das Protoplasma der Rhizopoden und Pflanzenzellen, Leipzig 1863. — De Bary, 
Die Mycetozoen, Leipzig 1864. — Hofmeister, Die Lehre von der Pflanzenzelle, Leipzig 1867. — 
Hanstein, Sitzungsberichte der niederrheinischen Gesellschaft in Bonn, Dec. 19, 1870. 
^ For further details on this point, see Book III ; also my Handbuch der Experimental- 
Physiologic der Pflanzen, § 116, Leipzig 1865. 
