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 

 Nucleus, 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-mass ; 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 



