THE CELL-WALL. 43 
the cell-body, is one quite suitable for the purpose. But this earliest covering under- 
goes many modifications. The protoplast is able to store up in it suberin, lignin, 
silica, and water in greater or smaller quantities, and by this means it either makes 
the envelope more flexible than it was in the first instance, or else hard and 
stiff, converting it into a shell-like case. Even the shape is seldom preserved as it 
was originally. The solitary protoplast surrounded by its cell-membrane is gener- 
ally in the form of a roundish ball, and its envelope, which is closely adherent, 
exhibits a corresponding configuration. Young cells, aggregated together, have 
outlines too which remind one of crystalline forms, such as dodecahedra, cubes, 
and short six-sided prisms. But when a protoplast has produced its first delicate 
covering it does not come to rest, but goes on working at the membrane, distending 
and thickening it, transforming a cavity which was originally spherical or cubical 
into one of cylindrical, fibrous, or tabular shape, and strengthening its walls with 
pilasters, borders, ridges, hooks, bands, and panels of various kinds. Where a 
number of protoplasts work gregariously at one many-chambered edifice, cells of 
most diverse forms are produced in close proximity to one another. These 
varieties are, however, never without method and design, but are invariably such 
as to adequately equip each cell for the position it holds and for the particular 
task allotted to it in the general domestic economy. 
The volume attained by cell-cavities in consequence of the expansion of their 
walls varies within very wide limits. The smallest cells have a diameter of only 
one micro-millimeter, ü.e. the thousandth part of a millimeter; others, as for example 
yeast-cells, measure perhaps two or three hundredths of a millimeter; and yet 
others have outlines perceptible to the naked eye and have a volume amounting 
to one cubic millimeter. Tubular and fibrous cells often stretch longitudinally 
to such an extraordinary extent that some with a diameter of scarcely the hun- 
dredth part of a millimeter reach a length of one, two, or even as many as five 
centimeters. An instance may be seen in the filaments of Vaucheria clavata 
(Pl. I, figs. «-d), and again in the fibrous cells from which our linen and cotton 
fabrics are manufactured. 
The enlargement of a cell-cavity, or, in other words, the growth in area of 
its walls, ensues in consequence of the intercalation of fresh particles between 
those which, by their mutual coherence, form the delicate skin of the protoplast 
—the earliest stage of the cell-wall. When these intercalated particles are situ- 
ated in the same plane as are those already deposited, the cell-wall resulting 
from this method of construction will increase in area without adding to its 
thickness. But when once the cells are full-sized, the constructive activity of 
the protoplasts has to be directed in many cases to the strengthening and thick- 
ening of their walls, so that later on they may be able to perform special duties. 
From the appearance of this thickening one would judge that a number of layers 
were deposited on the thin original wall according to requirement, and in many 
instances no doubt the process corresponds to this appearance; but, as a rule, the 
thickness of the wall is increased by intercalation, on the part of the protoplasts, of 
