Cell division - Nuclear division

CELL DIVISION 
During telophase spindle fibres remain visible, bridging the space between the daughter nuclei and forming a somewhat barrel-shaped group. Whether they are in fact the same fibres as those attached to the chromosomes, or a newly organized set, is not certain, but the latter is more probable.
Across the equator of this set of fibres a transverse cleavage develops, dividing the cytoplasm and cutting across the fibres. This fissure widens and appears to contain fluid, which soon solidifies as the result of the deposition of pectin, which seems to be the principal substance in the cell plate which is thus formed.
The cell plate first appears in the middle of the cell and extends sideways until it meets the old walls, its extending edge being surrounded by a zone of dense cytoplasm called the phragmoplast. Thus a pectic layer grows across the cell, dividing it into two. This layer becomes the middle lamella of the new cell wall. Its margin is at first separated from the middle lamella of the parent cell wall by the thickness of the cellulose layer on that wall. The margin of the new membrane thickens and vacuolates, the cellulose \yall with which it is in contact breaks down and the vacuolated margin then
joins the middle lamella of the old wall, the vacuole becoming part of the intercellular space system and acquiring a lining of pectic substances. Lastly, cellulose layers are deposited on the new lamella, making up a new primary wall between the two portions of the divided cell
The spindle fibres disappear when the daughter nuclei obtain their nuclear membranes, but it is possible that a part of them may persist through the new cell wall and form the basis of the protoplasmic connections. 
In certain special cases, and especially among lower plants, cell division begins at the periphery of the cell, not at the middle, and a cleavage furrow runs inwards towards the centre, closing in like the iris diaphragm of a lens, until the cell is cut in two. The wall material in this furrow is deposited in a manner similar to the above.
Cell division in the higher plants normally obeys certain principles which haye been called the Laws of Cell Division, though exceptions can be found o each of them in special cases. They are not based upon anything specific to the living cell, but are the expression of physical factors which govern all ftluid films, which is in fact the initial state of the cell plate. They are as follows:

1. Cells normally divide into two equal halves. 
2. The plane of the new wall is perpendicular to the direction of maximum growth. 
3. The new wall tends to occupy the position of minimal area. 

   The new wall intersects the old walls at right angles, and the line of contact is marked by a minute zone of curvature which makes the old and new surfaces continuous. (The" bourrelet " of Plateau's soap-film experiments.)
The time which elapses between successive divisions of a cell may be looked upon as the duration of a cell generation. This period varies roughly in direct ratio to cell size, though exceptions occur. In Bacteria the period may be only twenty minutes ; in larger unicellular organisms such as Diatoms it may be four or five days. The tissue cells of higher plants show corre­spondingly great variations. The cells of the staminal hairs of Tradescantia di"ide about every eighty minutes at summer temperatures. This is one of the shortest cell generations. At the other extreme are the medullary' ray cells of old trees, which can exist ,yithout growth or division for periods of up to 100 years.
It is not known what controls the occurrence of cell division. Various explanations have been suggested, such as the decrease in the ratio of surface .olume as the cell grows, or a corresponding reduction in the ratio of . :: to cell volume. Decreasing viscosity may also be a cause, but it is certain no single theory by itself is sufficient.

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