In the cytoplasm occurs one or more cavities, vacuoles, filled with cell sap. Embedded in the cytoplasm are numerous chromatophores, which vary in color in the different cells, from colorless to yellow, to red, and to green. The nucleus is the seat of the vital activity of the cell, and the seat of heredity. The whole life and activity of the cell centre, therefore, in and about the nucleus.
The outer portion of the nucleus consists of a thin membrane or wall. The membrane encloses numerous granular particles—chromatin—which are highly susceptible to organic stains. Among the granules are thread-like particles or linin. Near the centre of the nucleus are one or more small rounded nucleoli. The liquid portion of the nucleus, filling the membranes and surrounding the chromatin, linin, and nucleoli, is the nuclear sap.
Other cell contents characteristic of certain cells are crystals, starch, aleurone, oil, and alkaloids. The detailed discussion of these substances will be deferred until a later chapter.
The cell wall which surrounds the protoplast is a product of its activity. The structure and composition of the wall of any given cell vary according to the ultimate function of the cell. The walls may be thin or thick, porous or non-porous, and colored or colorless. The composition of cell walls varies greatly. The majority of cell walls are composed of cellulose, in other cells of linin, in others of cutin, and in still others of suberin, etc. In the majority of cells the walls are laid down in a series of layers one over the other by apposition, similar to the manner of building a pile of paper from separate sheets. The first layer is deposited over the primary wall, formed during cell division; to this is added another layer, etc. A modification of this manner of growth is that in which the layers are built up one over the other, but the building is gradually done by the deposit of minute particles of cell-wall substance over the older deposits. Such walls are never striated, as is likely to be the case in cell walls formed by the first method. In other cells the walls are increased in thickness by the deposition of new wall material in the older membrane. The cell walls will be discussed more fully when the different tissues are studied in detail.
INDIRECT CELL DIVISION (KARYOKINESIS)
The purpose of cell division is to increase the number of cells of a tissue, an organ, an organism, or to increase the number of organisms, etc. Such cell divisions involve, first, an equal division of the protoplast and, secondly, the formation of a wall between the divided protoplasts. The first changes in structure of a cell undergoing division occur in the nucleus.
CHANGES IN A CELL UNDERGOING DIVISION
The linin threads become thicker and shorter. The chromatin granules increase in size and amount; the threads and chromatin granules separate into a definite number of segments or chromosomes (Plate 1, Fig. 2). The nuclear membrane becomes invested with a fibrous protoplasmic layer which later separates and passes into either end of the cell, there forming the polar caps (Plate 1, Fig. 3).
The nuclear membrane and the nucleoli disappear at about this time. Two fibres, one from each polar cap, become attached to opposite sides of the individual chromosomes. Other fibres from the two polar caps unite to form the spindle fibres, which thus extend from pole to pole. All these spindle fibres form the nuclear spindle (Plate 1, Fig. 5).
The chromosomes now pass toward the division centre of the cell or equatorial plane and form, collectively, the equatorial plate (Plate 1, Fig. 5). At this point of cell division, the chromosomes are U-shaped, and the curved part of the chromosomes faces the equatorial plane. The chromosomes finally split into two equal parts (Plate 1, Fig. 6). The actual separation of the halves of chromosomes is brought about by the attached polar fibres, which contract toward the polar caps (Plate 1, Fig. 7). The chromosomes are finally drawn to the polar caps (Plate 1, Fig. 8). The chromosomes now form a rounded mass. They then separate into linin threads and chromatin granules. Nucleoli reappear, and nuclear sap forms. Finally, a nuclear membrane develops. The spindle fibres, which still extend from pole to pole, become thickened at the equatorial plane (Plate 1, Fig. 8), and finally their edges become united to form the cell-plate (Plate 1, Fig. 9), which extends across the cell, thus completely separating the mother cell into two daughter cells. After the formation of the cell-plate, the spindle fibres disappear. The cell becomes modified to form the middle lamella, on either side of which the daughter protoplast adds a cellulose layer. The ultimate composition of the middle lamella and the composition and structure of the cell wall will differ according to the function which the cell will finally perform.