FORMATION OF CELLS.

195

composed of one or more cells, hence all the organs which afterward make their appearance must be produced by the modification of such cells, or by the formation of new

ones.

Cells can only be formed from the thickened fluid called protoplasm, which is contained in their interior, or has been elaborated by their agency; hence, cells can in no case be formed without the influence of living organisms. The cell-wall takes no part in the formation of cells.

Cells commonly originate in one of two ways: either free in the cavities of older cells, or at least in the protoplasmic fluid elaborated by their agency; or by the division of such cells. The first is called Free Cell-formation; the second, Cell-division or Cell-multiplication.

A. Free Cell-formation.-We may distinguish two modifications of free cell-formation: 1. Free cell-formation from a nucleus; and, 2. Free cell-formation without the previous formation of a nucleus.

a. Free Cell-formation from a Nucleus.-This mode, discovered by Schleiden, he describes as taking place as follows: A portion of the protoplasm collects into a more or less rounded form, with a defined outer border, thus forming the nucleus or cytoblast of the cell; upon this a layer of protoplasm is deposited, which assumes the form of a membrane, and expands so as to form a vesicle; on the outside of this a cellulose membrane is secreted, and the formation of the cell is completed.

b. Free Cell-formation without a Previous Nucleus.—In the process of free cell-formation, as described above, the production of the nucleus is regarded as the first step of the process. But Henfrey does not consider the nucleus of any physiological import in free cell-formation, which process he thus describes: "The essential character of free cell-formation lies in the circumstance that the protoplasm which produces the primary cellulose wall of the new cell previously becomes separated from the wall of

196

CELL DIVISION.

the parent cell, so that the new cell is free (or loose) in the cavity of the parent cell." In some cases, it is certain, no nucleus can be detected in a cell previous to the formation of other cells free in its cavity; hence, the presence of the nucleus can not be regarded as essential, but the portion of protoplasm which in such cases separates from the general mass must be capable of covering itself with a membrane and forming a cell.

In Flowering Plants free cell-formation occurs in the embryo-sac, in which part the germinal vesicles, the antipodal cells, and the cells of the albumen (endosperm), thus originate. In Flowerless Plants it is the mode by which the spores in the asci of Fungi (Fig. 247) and Lichens (Fig. 251) originate.

B. Cell-division.-This mode of cell-formation is also called parietal, and merismatic or fissiparous, cell-formation. Cell-division can only take place in cells in which the contained protoplasm is in an active state, as in the cells of the meristem, a name given to that kind of parenchyma the constituent cells of which are thus capable of multiplying by division (see page 44). It may be treated of under two heads, namely: 1. Cell-division without absorption of the walls of the parent cell; and 2. Cell-division with absorption of the walls of the parent cell, and the setting free of the new cells.

a. Cell-division without absorption of the walls of the parent cell. This mode of cell-formation takes place as follows: The protoplasm of the cell, or, according to Mohl and Henfrey, the primordial utricle, becomes gradually constricted on the sides so as ultimately to form a sort of hour-glass contraction, and thus to divide the original contents into two distinct portions (Fig. 232, a, b, c, d). Each portion of the protoplasm, or of the primordial utricle, then secretes a layer of cellulose over its whole surface; and where this is in contact with the original wall of the primary cell, it forms a new layer interior to it; but where

THE PRIMORDIAL UTRICLE.

197

away from the wall, at the new septum, a distinct cell-wall, so that the partition is double. The original cell thus be

a

d

FIG. 232.-a. Cell of Conferva glomerata, with the cell-contents constricted by the half-completed septum. 6. A half-completed septum in which a considerable deposition of cellulose has already taken place. c. A septum in course of development, after the action of an acid, which has caused contraction both of the primordial utricle (b), and the cell-contents (a). d. Complete septum split into two lamellæ by the action of an acid. (After Mohl and Henfrey.)

comes divided into two, and forms two cells, each of which has the power of growing until it reaches the original size of its parent, and then either, or both, may again divide, and each of the newly-formed cells grows in a similar manner to the size of its parent.

*

It is by this process of cell-division that all the growing or vegetating parts of plants, whether Flowering or Flowerless, are produced and increased.

b. Cell-division with absorption of the walls of the parent cell, and the setting free of the new cells.-The pollen cells of all Flowering Plants, and the spores of the higher Flowerless Plants, are formed by this process, which only occurs in connection with the organs of reproduction. The manner in which it commonly takes place in the production of pollen cells has already been described at pages 114 and 115 of this volume. The manner in which spores are

* It should be noticed that the primordial utricle of Mohl here referred to differs from that defined by us at page 8 of this volume. Thus, according to the views adopted by us, the primordial utricle is characterized as the thin layer of protoplasm which lines the cell-wall after the cell has grown too large to be filled by protoplasm alone; while Mohl regards it as a more or less thickened layer of protoplasm, having the appearance of a membrane lining the cellulose wall, and inclosing the ordinary protoplasm of the cell.

198

FIG. 233-a. Cylindrical cell from which are formed the parent cells

CELL DIVISION.

formed in the higher Flowerless Plants is substantially the same in most cases. It sometimes happens, however, that, in the development of pollen and spores, the special parent cells are not formed, as has been shown by Henfrey in the spores of Marchantia polymorpha (Fig. 233).

In other cases, instead of the development of only four secondary cells in the cavity of the parent cell, the whole mass of the protoplasm may break up into a great number of small particles, as in the production of the swarm-spores of many Alga (Fig. 228) and Fungi. In this case the new cells (primordial) are only clothed by a cellulose wall after cells isolated. d. The their separation from the parent or moth

of the spores of Marchantia polymorpha. 2. Protoplasm of the

parent cells. b. The same cell converted into a string of cells. c. One of the parent

er-cell.

Some of these modifications of

four spores free. (After Henfrey.) the process of cell-division are closely analogous to the ordinary process of free cell-formation, to which by many authors they are referred.

A

c. Another method of cell-
formation is that which is termed
rejuvenescence or renewal of a cell,
where the whole contents of a
cell contract and expel a portion
of the water of the cell-sap, the
arrangement of the differentiated
protoplasmic body is changed
and its whole form alters as it
escapes from the cell-wall and
eventually forms a fresh cell-wall
(Fig. 235, E). This process may
be well seen in the swarm-spores of Edogonium.

FIG. 234.-Zcosporangia of Achlya
prolifera. A. Still closed. B.
The zoospores escaping. (After
Carpenter.)

ZYGOSPORE—OSMOSE.

The production of a zygospore, which occurs in the process of conjugation, as already noticed in Spirogyra (Fig. 230), is also another method of cellformation. It occurs frequently in Algae and various groups of Fungi. (2) Absorption Transmission of Fluids. -The cell-wall of all young and vitally active cells is capable of readily imbibing fluids, and we find, accordingly, that liquid matters are constantly being absorbed

and

E

D

B

199

A

FIG. 235.-A, B. Escape of the swarmspores of an Edogonium. c. One in free motion. D. The same after it has become fixed, and has formed the attaching disc. E. Escape of the whole protoplasm of a germ-plant of Edogonium in the form of a swarmpore. (After Pringsheim.)

transmit fluids is called osmose. It is, moreover, by a somewhat analogous process (diffusion of gases) that the cells on the surface of the plants are enabled to absorb and transmit gaseous matters.

Osmose may be explained as follows: Whenever two fluids of different densities are separated by a permeable membrane FIG. 236.-Appara- which is capable of imbibing them, there is

tus to show os

consists of a blad

motic action. It always a tendency to equalization of density der filled with sy- between the two, from the formation of a

rup, to the open

end of which a current in both directions, which will be

tube is attached

and the whole modified by the action of the membrane, as placed in a vessel

containing water. Well as by their own rates of diffusion. This