THE CELL-WALL. 
19 
Sect. 4. The Cell- Wall ^ — The substance of the cell-wall is secreted from 
the protoplasm. In what form it is previously contained in the protoplasm is not 
yet certainly known. The substance capable of forming cell-wall always consists 
of a combination of water, cellulose, and incombustible materials (ash-constituents), 
but may afterwards undergo further chemical changes. 
By the continual secretion from the protoplasm of the substance out of which 
the cell-wall is formed, and its intercalation between the micellae (see Bk. III. 
Sect, i) of the cell-wall already in existence, this grows in both surface and thickness. 
The mode of both processes of growth is dependent on the specific nature of the 
cell, and on the function which it has to fulfil in the life of the plant; it therefore 
varies almost infinitely. Generally the surface-growth first preponderates, afterwards 
that in thickness. Neither the one nor the other is uniform over all points of a 
cell-wall ; hence each cell during its growth also changes its form. The growth of 
a cell-wall continues only so long as it is in immediate contact on its inner side 
with protoplasm. 
The want of uniformity in the surface-growth at different points causes cells 
which are at first, for example, spherical, ovoid, or polyhedral, to become subse- 
quently cyHndrical, conical, tubular, tabular, bounded by waved surfaces, &c. 
The want of uniformity in the growth in thickness usually brings about a sculpture 
of the surface which is very characteristic. The thickened parts may project either 
outwardly or inwardly. The former occurs commonly in the surface of the cell-wall 
which is exposed, the latter in the partition-walls of adjoining cells. The thickenings 
which project outwards may take the form of knots, humps, spines, or ridges ; but 
those which project on the inside are much more various. In this case conical 
protuberances occur but seldom ; annular ridges or spiral bands are much more 
common ; these latter may be united in a reticulate manner, so that the 
thin interstices are polygonal; or the thickened part of the wall may be broader, 
and the thin parts then appear in the thick wall as fissures or roundish pits. If 
the wall is very thick, the latter become channels, which pass entirely or partially 
through the wall. Not unfrequently the thin portion of the wall which at first 
closes such a channel on the outside becomes absorbed, and the cell-wall is then 
perforated. But as, when contiguous cells are united into a tissue, the partition- 
wall usually becomes thickened in the same manner on both sides, the pits and 
pit-canals on the two sides meet, and the intermediate thin portion of membrane 
(sooner or later) becomes absorbed; a channel thus arises uniting two cell-cavities 
(Bordered Pits, perforated septa of vessels). 
During the increase of the wall both in surface and thickness by the inter- 
calation of new substance between the micellae already in existence, a further 
internal structure usually becomes visible, which is termed Stratificaiion and Stria- 
Hon. Both are the result of a different regularly alternating distribution of water 
and solid material in the cell-wall ; at every point water is combined with cellulose, 
^ H. von Mohl, Vermischte Schriften bot. Inhalts, Tübingen 1845 (numerous memoirs). — 
Schacht, Lehrbuch der Anat. und Phys. der Gewächse, 1856. — Nägeli, Sitzungsberichte der Münch. 
Akademie, 1864, May and July. — Hofmeister, Die Lehre von der Pflanzenzelle, Leipzig 1867. 
Also numerous memoirs in the Botanische Zeitung. 
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