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.
Below 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 rather 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 |