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ATOMS, MOLECULES, AND CELLS AS FACTORS 145
While it is difficult to conceive of a cell minus its cell wall and nucleus, there are good grounds for believing
that the intra -cellular substance is really the germinal matter or elementary Ufe-stuff, in which all active growth
proceeds, and from which the various tissues are manufactured. Schwann requires a blastema even for the pro-
duction of cells. The intra-cellular substance is now commonly known as protoplasm (Remak, Von Mohl, and Klihne).
It is, however, variously designated. Thus, as already pointed out, it is spoken of as sarcode (Dujardin), blastema
(Schleiden and Schwann), sohdescible nutritive fluid (WolfE), organised concrete (Haller), germinal matter (Beale),
embryo plastic matter (Robin), primordial protogenes (Haeckel), &c.
A very important part of the cell unquestionably is the cell wall or envelope. This is, strictly speaking, an
osmotic membrane, through which, during Ufe, two opposite currents are continually passing : an
ingoing or
endosmotic current, which carries nutrient materials to the cell, and an outgoing or exosmotic current, which
carries away effete matter and waste products.
On the integrity of the cell wall, as an osmotic medium, the health of the cell, and the parts formed by it,
largely depends.
If the cell wall in plants and animals becomes thickened, and ceases to be porous, abnormal conditions are at
once estabhshed. In such cases growth and natural movement are interfered with, and function more or less impaired.
Cells are divided into :
(a) Permanent healthy cells.
(h) Transition cells.
(c) Unhealthy or morbid cells.
The permanent cells embrace such as do not become tissues : the
into tissue : the unhealthy or morbid cells, those which are abnormal.
Cells reproduce themselves in four different ways :
1. Endogenously or from within, where a cell is generated within a cell.
transition cells include all those converted
2. Exogenously or from without, where a young cell formed within the parent cell is extruded.
3. Gemmiparously, where a young cell is produced by a process of budding.
4. Fissiparously, where the original cell sphts into two.
The rapidity with which cells multiply is, in some cases, extraordinary. Cell production and cell life is, as
a rule, most vigorous at fairly high temperatures. Tropical plants grow more luxuriantly than those in temperate
climes. There is, however, a Umit as regards temperature. Cell hfe will not continue below zero, or above 145° F.
Cold keeps back the growth of young plants and animals, while heat brings them forward.
In no department of physiology is the division of labour more thoroughly carried out than in cell structures.
This is well seen in the case of plants. In the lowest plants, cells resemble each other and have equal values
—that is, they take part equally in the vegetative and reproductive processes. In the higher plants, cells are
specialised and have different values as regards their physical and chemical properties ; some
nutrition, others in secretion, others in reproduction, and so on.
take part in
In common mould, the cells forming the mycelium or spawn absorb nourishment and produce stalks which
bear reproductive cells or spores. In vascular plants, certain cells secrete starch, gum, sugar, oils, milky juices, &c.
The contents of cells are various. Thus, in the grain of the cereals some cells contain nitrogenous compounds,
and others starch ;
in the Equisetaceae, and the steins of some grasses, a large proportion of the cells contain sihca.
In the case of animals, one set (the blood corpuscles) assist in elaborating the blood ; another, in building
up the various tissues ; another in secreting various products useful to the economy ; another in getting rid of ei!ete
products ;
another, in storing up fat, pigment, &c.
Cells, there can be no doubt, are endowed originally with special properties, for while they all drink out of the
same blastema, some take in and elaborate what others reject. This discriminating power of cells is at the root of all
structure and all function.
Cells are highly complex, both as regards their ultimate composition and the nature and variety of the work
discharged by them.
They exercise a most important influence in the economy of plants and animals. In the higher animals they
contribute to locomotion, sensation, and even mental acts. Through their instrumentality absorption, nutrition,
assimilation, growth, development, secretion, excretion, reproduction, &c., are mainly effected.
Many plants and animals consist of an agglomeration of cells ; and whatever plants and animals may be in the
mature or adult state, they are, at the outset, composed of cells.
The life of the cell is frequently manifested by movements in the cell wall, in the nucleus, and in the proto-
plasm or cell contents, These movements are due sometimes to cilia or hair-like processes on the cell wall ; some-
times to independent contractions and expansions in the cell wall itself ; sometinaes to movements conflned to the
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