38 
MORPHOLOGY OF THE CELL. 
The protoplasm of plants, when in a state of vital activity, is generally very 
watery; on one hand it exhibits an internal differentiation of its substance into 
layers and portions differing in their consistence and chemical nature ; on the other 
hand it assumes definite outlines, and becomes bounded by surfaces of determinate, 
and mostly very variable, form. 
The internal differentiation of protoplasm usually manifests itself by the 
formation of an external hyaline, firmer, but very thin layer, surrounding the inner 
mass, with which it remains in the most intimate contact. Every portion of a 
protoplasm-mass immediately surrounds itself, when it becomes isolated, with 
such a skin. In the interior a quantity of fluid sap, which permeates its 
substance throughout, becomes separated in the form of drops ( Vacuoles) ; when 
the protoplasm is contained in a growing cell these vacuoles increase as the 
cell grows, and the protoplasm-mass becomes a sac filled with watery sap. 
One of the most common internal differentiations of the young protoplasm- 
mass, while constituting itself a separate individual, is the formation of the 
NiLcleus. The substance of the nucleus is at first indistinguishable from the 
rest of the protoplasm, and its formation is essentially the accumulation of 
certain particles of protoplasm round a centre, which is also usually the centre 
of the whole protoplasm-mass. Once formed, the nucleus — whose chemical 
nature, as far as observation goes, is altogether similar to that of the protoplasm 
— becomes more sharply defined; it may itself become enveloped in a skin, 
and vacuoles and granules (the Nucleoli) may become separated in it. But the 
nucleus always remains a part of the protoplasm-rnass ; it is always imbedded in 
it; very commonly it becomes again absorbed, after a short existence, in the 
protoplasm, e.g. in cells which divide frequently, see p. 14; in the elongated cells 
of the Characese the nucleus disappears altogether when the circulating motion of 
the protoplasm begins. Another very common differentiation of the substance 
of the protoplasm consists in portions of it becoming separated in a definite 
form, and assuming a green colour, forming the Chlorophyll-bodies, which, like the 
nucleus, not only arise out of the protoplasm, but always remain portions of the 
protoplasm-mass. But since these require more minute investigation, the next 
section will be devoted to them. 
The external configuration of the protoplasm as a mass of definite form can 
be reduced to two cases : — either all its most minute particles group themselves 
concentrically round a common centre; or an internal motion takes place, which 
causes the protoplasm-mass to become elongated in some one direction, and 
disturbs the centripetal arrangement. The former occurs commonly in the for- 
mation of new cells, the latter during their growth. 
The movements of the minute particles of protoplasm which determine its 
grouping and configuration during the formation and growth of cells, are generally 
so slow as not to be visible even when subjected to a very high magnifying power. 
Much quicker movements, even appearing rapid under a very high magnifying 
power, occur in cells either before their growth, as in swarm-cells, or when it 
is nearly completed. Merely having regard to external appearance, the following 
kinds of movements of this nature may be distinguished : — 
(A) Movements of naked protoplasm-masses, (i) Swimming of swarm-cells 
