Cell layers inside a grain

Diagram of layer changes


This model shows the theoretical changes in the constituent layers of the grain from flowering until dry down; sixteen separate tissue types, as cell layers or cuticles, are involved. For simplicity we first consider the tissue layers of the pericarp (on the outside) and then the tissue layers intimately surrounding the embryo sac.
Time is represented along the x- axis. You will see how some tissues are present at flowering but degenerate and disappear, or metamorphose into something else, during the grain growth and grain filling stages. New tissues arise; they ensure that the developing embryo and its food reserves are prepared for the next stages. During dry down the cell layers that have persisted are compressed and form a protective coat to ensure survival through seed dormancy until seed re-generation.
From the outside going in: The maternal pericarp.
At the time of fertilization the pericarp consist of about 25 layers of thin-walled parenchyma cells (3) bounded by, on the outside the outer epidermis (1), the outer epidermis cuticle (1A), which limits water movements, and the hypodermis (2) and on the inside by a layer of chloroplast containing cells, the cross cells (5), and an inner epidermis (6). The outer epidermis and hypodermis (and later the remnants of the thin-walled parenchyma) form a loose wrapper that some authors call the outer pericarp. The outer pericarp is easily detached at harvest and, because of its pale membranous appearance, is called 'beeswing' by grain millers.
Intermediate cells (4) lie just below the thin-walled parenchyma cells but do not form a complete layer. They occur principally at both ends of the grain. The cross cells (5) are a one cell thick layer inside the parenchyma, interior to the intermediate cells.
The cells of the inner epidermis (6) do not keep pace with the growth of the tissues of the inner embryo sac and are torn asunder to become sinuous 'tube cells' (7), appearing as an incomplete layer or randomly sited individuals.

Going further in: The cell layers surrounding the embryo sac.
At the time of fertilization the embryo sac is enclosed in two integuments, outer (8) and inner (9) surrounding the nucellus (11) . Each integument consists of two layers of cells (A and B). About the time the zygote commences division the outer integument begins to degenerate and a cuticle develops on the outer surface of the inner integument. Later the cell-contents of the outer integument disappear entirely, the delicate walls shrivel and finally become completely disorganized. The semi-permeable membrane cuticle on the inner integument (9B) persists until the grain reaches its maximum size.
The nucellar tissue (11) surrounding the developing embryo is at first several cells thick but it, too, gradually becomes disorganized and absorbed. It is widely assumed that degeneration of the nucellar cells provides nutrients as well as space for the developing endosperm and embryo. The nucellar epidermis (10) continues as a living tissue, its cells dividing and expanding as the ovule grows until, later, its cell contents also become disorganized, the remaining cytoplasm collects as a thin layer across the middle of the cell and the radial walls are absorbed. The single-celled tissue collapses during ripening, upper and lower walls are crushed together, and finally it becomes a delicate covering on the outer surface of the aleurone cells (12).