Meristems: Sites of cell division
Cell division in plants is concentrated in meristems, although plants are unusual in that mature cells can return to cell division under the right conditions. Shoot and root apical meristems are already organized in the embryo of the seed.

The root apical meristem (RAM) is the simpler of the two apical meristems. The dividing cells are just a little way behind the tip and they bud off cells in two directions:

• towards the tip to form the root cap
• away from the tip to form the body of the root.

Onion (Allium) root tip in longitudinal section

The shoot apical meristem is more complex. In addition to producing the stem it produces leaf and bud initials. The outer layers of cells tend to divide perpendicular to the surface and form the tunica whereas the inner cells of the corpus can divide in any direction.

Apical meristems give rise to the primary meristems that form the tissues of the plant:

• The protoderm (equivalent to the outer layer of the tunica in the stem) forms the epidermis.
• The procambium forms the xylem and phloem
• The ground meristem forms the cortex in both stem and root and the pith in stems

Coleus shoot apex

Other meristems arise later in plant growth, particularly in woody plants. The vascular cambium generates new xylem and phloem every year and the cork cambium produces the outer layers of bark.

Mitosis
Mitosis is a process through which the cell ensures that two new cells each receive a complete set of genetic information:

The typical state for most cells is interphase. At the beginning of interphase (G1 phase) there is a single copy of the genetic material. If a cell is going to divide the DNA is replicated during the S phase. Then the cell is in the G2 phase with two copies of its DNA and ready to divide.

In prophase the nuclear membrane breaks down, nucleoli disperse and chromosomes go from the extended configuration, typical of interphase to the condensed form. Each chromosome can be seen to be made up of two chromatids

In metaphase the chromosomes line up on a"plate" about where the new cell wall will form; a spindle is organized from microtubules

In anaphase the centromere of each chromosome splits and is drawn along the spindle fibers, taking a chromatid to opposite ends of the cell.

In telophase the chromosomes cluster at each pole and the DNA becomes diffuse. The nuclear membrane and nucleoli reform; a cell plate begins to form between the new cells.

Stages of mitosis in an onion root tip. It is easiest to find nuclei in prophase, metaphase and telophase. You can see Virtual Mitosis online.

Cell wall development
The cell plate is rich in the acidic polysaccharide, pectin. The primary cell wall is laid down on each side of this and contains more pectin with cellulose microfibrils running through it. The cell plate becomes the middle lamella which holds adjacent cells together. Holes, called plasmodesmata are often left in the walls between daughter cells so that they retain cytoplasmic continuity.

Part of the primary cell wall in a young strawberry fruit showing plasmodesmata (3). The relatively clear zone (4) in the middle of the wall is the middle lamella. The electron dense dots in the wall are cellulose microfibrils in section

The epidermis and much of the ground tissue (pith and cortex), particularly in herbaceous plants, consists of parenchyma cells that have enlarged but undergone little differentiation. Parenchyma cells typically have thin, primary cell walls and most of their cell volume is occupied by vacuole. Leaf petioles and some stems are strengthened by collenchyma, parenchyma cells that develop thicker primary walls

Parenchyma cells in Helianthus pith

Sclereids or fibers in Helianthus

In other, more specialized cell types cell wall development continues with the deposition of the secondary wall. This has a higher proportion of cellulose and less pectin than the primary wall; secondary walls contain hemicelluloses (xyloglucan, xylan, mannan) in place of pectin and are often lignified. Thus when parenchyma cells have developed a lignified secondary wall they become sclerenchyma. Sclerenchyma cells can be globular sclereids as in the"stone cells" of pear fruits or elongated fibers which occur in many plant stems. Traditionally, fibers from flax, jute, hemp and sisal have been important for string, rope and woven fabrics, but these have been replaced to some extent by synthetic materials.

A transverse section of a vascular bundle in a Helianthus stem. The bundle is embedded in thin walled parenchyma cells. The thick red stained cell walls belong to fiber cells. The rows of large cells are vessel elements and between these and the fibers are phloem cells

Copyright © Michael Knee,
The Ohio State University
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