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(days = days after flowering)

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Cell layers inside a grain

Many of the developmental changes, here described for wheat, are common to all seed bearing plants. They enable the seed, which may be genetically different from its parents, to move away and exploit new environments or simply survive under adverse conditions. Viable wheat grains have been found, during archaeological excavations, that have survived many hundreds of years in the desiccated and protected state achieved through the changes described in this section. However, it is also important for us to understand the structural properties of the wheat grain if we are to optimise its behaviour during threshing, storage and milling.

The diagram in the first section shows the theoretical changes in the cell layers surrounding the embryo sac from flowering until harvest ripe. Time is represented along the x- axis. Sixteen separate cell types, cell layers and cuticles are involved. With care you can trace the presence or absence of each one from flowering to ripeness. There is some explanation with the diagram and, in the next section, we present micrographs of the actual cell layers in the wheat grain at specific times during grain growth, grain filling and dry down with annotation and some more explanation.

Here is a table of the names of the cell layers, including the terms used by farmers and millers, which may help to unravel the story. The names change as the tissues change, which can be confusing.

Pericarp	Number in diagram	Name of tissue	Becomes later	Collectively called / Mill Fractions
	1A	Cuticle of Outer epidermis		Beeswing	Bran
	1	Outer epidermis	Outer pericarp
	2	Hypodermis
	3	Parenchyma (thin-walled)
	4	Intermediate cells	Inner pericarp
	5	Cross cells
	6 becomes7	Inner epidermis/ Tube cells
Embryo sac wall	8A and 8B	Outer integument	Pigmented seed coat (testa)
	9A and 9B	Inner integument
	10	Nucellar epidermis
	11	Nucellus
	12	Aleurone
	13	Starchy endosperm		White flour
		Embryo and scutellum		Germ