Cell layers inside a grain

Cell layer micrographs


The tube cells (7) are an incomplete layer that is stretched and torn apart by the growth of the endosperm; they are only a narrow band on the dorsal side, but provide more complete coverage over the embryo and the brush end of the grain.
The nucellar epidermis (10) continues as a living tissue but its cell contents become disorganized, the remaining cytoplasm collects as a thin layer across the middle of the cell, and the radial walls are absorbed. The seed coat or testa develops from the integuments (8 and 9). By six days after flowering both the layers of the outer integument (8A and B) had degenerated.
The cell contents of the outer layer of the inner integument (9A) disappear at about day 11; this layer develops a semi-permeable cuticle on the outside and its cells collapse.
The cell cavities of the inner layer of the inner integument (9B) are recognizable for some time and a yellow or reddish-brown oily resinous substance forms inside, visible through the translucent pericarp, giving the grain its characteristic colour.
Transverse section at eleven days after flowering.
The embryo sac wall in ts at high magnification two days after flowering.
The thin-walled parenchyma (3) , a protective and supportive structure, is 15-20 cells thick at anthesis. The cross cells (5), with their axis at right angles to the long axis of the grain, show a large cross section. The tube cells (inner epidermis) (7 was 6) have their long axis parallel to the long axis of the grain so, in TS they show a small cross section. The outer and inner integuments (8 and 9) are two cells thick. The nucellar epidermis (10) is a single cell layer.
The nucellus (11) is still a bulky multi-layer tissue cushioning the newly fertilized embryo and providing nutrients for its first divisions and those of the coenocytic endosperm.
About the time the embryo commences division the thin-walled parenchyma (3) starts to degenerate. At the same time the outer integument (8), which at first consists of two cell layers, begins to degenerate; cells lose turgidity, cytoplasm disappears and elongated nuclei become thinner. The cross cells (5) persist. They have elongated nuclei and vacuolated cytoplasm and contain chloroplasts (4µ), which give the green colour to the young grain, and minute starch grains 3µ by 1.5µ.The nucellus (11), which is at first several cells thick, gradually becomes disorganized and is absorbed. It disappears most rapidly near the embryo and on the dorsal side of the grain.
TS at four days after flowering.
This high power ts at 16 days after flowering shows the layers of the seed coat starting to compress. Although little of the maternal pericarp can be seen in this section its parenchyma cells, next to the cross cells (5), have degenerated. This happens at a faster rate than the endosperm enlargement, resulting in a space between the remaining inner and outer pericarp, allowing the endosperm (13), which is packing with starch, to force the remaining layers of the embryo sac together.
This transverse section at maximum fresh weight, (day 21) shows, on the outside, the remains of the outer layers of the maternal pericarp (the outer pericarp; beeswing), the cross cells (5) and the integuments which constitute the seed coat, the nucellar epidermis (10) , which is still being compressed, and mature aleurone (12) cells surrounding the starch filled endosperm (13) . These layers will be consolidated during the remaining grain fill time.
Same magnification as above, five days later. More compression of the outer pericarp, but especially of the cross cells (5), seed coat and nucellar epidermis (10). At maturity the nucellus will be a single crushed layer of empty epidermal cells between the endosperm and seed coat.
The aleurone (12) cells are embedded in a firm matrix; they will be the only endosperm tissue still alive at maturity.
TS at six days after flowering.
The starch grains in the parenchyma (3) disappear starting at the top of the grain. They remain longest where they cover the embryo and in the ventral groove. The cross cells (5) and tube cells (7) are the tissues that give strength to the embryo sac at this time. The two layers of the outer integument (8A and 8B) have disappeared, as has the outer layer of the inner integument (9A). The inner layer of the inner integument (9B) persists and acquires resinous substances. The nucellus remains only as the nucellar epidermis (10). Internally, cell walls grow enclosing the nuclei of the coenocytic endosperm. Completed cells form at the edge as a result of periclinal divisions and become the aleurone (12) layer. The aleurone (12) cells are first distinguishable at about 6 days after flowering and are later characterized by a large number of small vacuoles. The rounded vacuoles become filled with reserves to form aleurone grains, (a form of protein body) lipid droplets form a ring around the aleurone grains and a single layer of droplets line the plasmalemma.