Glossary
A)
- after-ripening A period of dry storage of a seed needed to overcome embryo dormancy
- aleurone (Aleurone layer). The outermost cell layer of the endosperm, usually only one cell thick in wheat and the only endosperm tissue alive at maturity. The cells of this layer are responsible for the de-novo synthesis of enzymes needed at germination.
- anther The terminal part of the stamen containing four pollen sacs.
- anthesis Flowering. The moment when the anthers release their pollen
- antipodal cells A group of cells, situated at the opposite end to the micropyle, in the mature embryo sac of flowering plants. They are very large and have highly endopolyploid nuclei.
- apex The tip of an organ (adj. see apical).
- apical meristem Growing point. A zone of cell division at the tip of the stem or the root. Behind the meristem the cells continue to divide and differentiation begins.
- awn A projection from the tip of the lemma Less prominent in wheat than in barley.
B)
- brush A collection of hairs at the distal end of the wheat grain.
C)
- carpel The female part of the flower. A carpel consists of three parts: 1) the ovary, a swollen basal portion containing the ovule, which becomes the seed after fertilisation, 2) styles, extensions of the ovary and 3) stigmas, specialised filaments on which the pollen falls and germinates.
- caryopsis The fruit, of grasses, in which the ripened ovary wall, the pericarp, is fused to the seed coats, the testa, at maturity.
- cellular endosperm The endosperm formed at the base of the embryo sac from divisions of the fused polar nucleus. This region of endosperm nurtures the embryo during its first divisions
- chalaza The basal region of the ovule where the stalk is attached.
- chloroplast A chlorophyll-containing organelle (plastid) that gives the green colour to leaves and stems. Plastids harvest light energy that is used to fix carbon dioxide in the process called photosynthesis.
- chromosomes Thread-shaped structures that occur in pairs in the nucleus of a cell. Chromosomes contain the genetic information of living organisms.
- coenocytic endosperm A multinucleate condition in which nuclei divide within a common cytoplasm without being separated from each other by a plasmalemma or cell wall.
- coleoptile A protective cylindrical sheath-like structure that surrounds the shoot apex in cereal and grass embryos.
- coleorhiza The protective layers of tissue surrounding the primordial root (radicle) within the grain.
- cortex In the mature wheat embryo, the cylinder of parenchyma cells that surround the central vascular core in the shoot and root.
- crease The longitudinal furrow on the ventral side of the wheat grain.
- cross cells These cells are only found in grasses. They are formed from cells of the pericarp that elongate transversely, lose their chlorophyll and become lignified. They lie between the tube cells and the parenchyma of the pericarp.
- cuticle An extra-cellular material secreted by plant cells that restricts water movement. It is composed of a fatty substance called cutin.
- cytoplasm The living contents of a cell containing different types of organelles and bounded by a plasma membrane.
D)
- differentiate Change from a simple to a more complex structure or function. Developing tissues and organs become specialized for particular functions. n. differentiation.
- diploid A condition in which the chromosomes in the nucleus of a cell exist as pairs, one set being derived from the female parent and the other from the male. In cereals such as barley and rice all the cells except the reproductive cells are diploid. When gametes are formed the diploid number (2n) is reduced by half to the haploid number (n) by meiosis When gametes fuse at fertilization the diploid condition is restored. In modern wheat the situation is more complex because it contains three genomes derived from different ancient diploid ancestors.
- dormancy A resting period in the life of a plant during which growth slows or appears to stop. In cereals this is between the start of seed dry-down and maturity. A period of 'after-ripening' is often required before dormancy can be successfully broken
.
- dorsal On or near the back. The side of the organ away from the central axis.
- double fertilisation The unique process, in flowering plants, when two male nuclei, from pollen, fuse separately with different female nuclei. The first male nucleus fuses with the egg cell to form the zygote that develops into the embryo The second male nucleus fuses with the two polar nuclei (the second polar event) to form a triploid nucleus that then develops into the endosperm.
- double ridge This is the term used to describe the transition from the 'vegetative' to the 'floral' state in the shoot apex of wheat and barley. Growth of the leaf primordium (ridge) is suppressed and the floral bud primordium (ridge) begins to grow out, giving a 'double ridge' appearance to the dissected shoot apex when view under the microscope. At this time the plant apex no longer produces leaves but is 'florally initiated' and only flower parts will subsequently develop.
- dpa 'Days Post Anthesis' or the number of days after flowering and fertilisation.
E)
- early milk A description the young grain. When punctured the grain contents are starting to look milky.
- embryo The rudimentary plant present in the seed; composed of the embryonic axis (shoot end and root end) and the scutellum.
- embryo sac A large cell that develops in the ovule of flowering plants. It contains a large number of nuclei derived by division of the megaspore nucleus. At the micropyle end is the egg nucleus and two synergids; at the opposite end are nuclei that will become the antipodals and in the centre are two polar nuclei which fuse to become the primary endosperm nucleus.
- embryonic axis The major part of the embryo, consisting of the shoot, mesocotyl and the root.
- endopolyploid when the DNA in the nucleus of a cell is replicated again and again, without any cell division taking place. The DNA C value goes from 4 to 8 to 16 to 32 etc and the chromosome number (n) goes from 2 to 4 to 8 to 16 etc.
- endosperm A nutritive tissue, characteristic of flowering plants, which nourishes the embryo In most plants it is a transient tissue absorbed by the embryo before maturity whereas in cereals and grasses it contains storage reserves in the mature grain and is not absorbed until after germination. (see also coenocytic, cytoplasmic and syncytial endosperm).
- epiblast A small flap of tissue on the side of the grass embryo axis opposite the scutellum.
- epicotyl The upper portion of the embryonic axis, above the cotyledons (in cereals: the scutellar node) and below the first true leaves.
- epidermis The outermost protective cell layer of tissue, usually covered by a cuticle.
F)
G)
- gamete A reproductive cell that fuses with another gamete to form a zygote Gametes are haploid, (they contain half the normal (diploid) number of chromosomes); thus when two fuse, the diploid number is restored. Gametes are formed by meiosis.
- germ The term used by millers to denote the embryo of cereals.
- glume A membranous bract at the base of a cereal or grass spike. Chaff.
H)
- haploid Describes a nucleus or cell with a single set of unpaired chromosomes The haploid number is designated as n. Reproductive cells, formed as a result of meiosis, are haploid. Fusion of two such cells restores the normal (diploid) number.
- hard dough A description the grain at maximum dry weight. The grain is hard to puncture and has started to dry off.
- harvest ripe The grain has dried off and is ready to be harvested.
- hypodermis The cell layer beneath the epidermis of the pericarp.
I)
- imbibe Absorb water: re-hydrate. n.imbibition.
- integuments Two distinct tissue layers that surround the nucellus of the ovule When mature they become the testa or seed coat.
- intercalary meristem A meristem found between already differentiated tissues. Occurs in the internodes of grasses (just above leaf nodes) and enables longitudinal growth of the stem.
- internode The part of a plant stem between two nodes.
- lemma The lower of two bracts that protect the floret in grasses.
- lodging Term used when the crop canopy becomes flattened by heavy rains. Often triggered by severe weather at the soft dough stage. The problem is a combination of heavy heads and weak stems. Modern breeding increases stem strength.
- lodicules Two small structures below the ovary which, at flowering, swell up and force open the enclosing bracts, exposing the stamens and carpel.
- ls Longitudinal Section. The tissue sections were cut along the long axis of the grain. The central sections go lengthwise through the embryo and along the length of the ventral groove.
J)
K)
L)
M)
- medium milk A description the half-grown grain. When punctured the grain contents are milky.
- meiosis The process of cell division that results in the formation of haploid cells from diploid cells to produce gametes.
- meristem The region of active cell division in plants, from which permanent tissue is derived. adj. meristematic.
- mesocotyl The region where vascular tissues of the root join the epicotyl at the scutellar node.
- micropyle The small opening in a plant ovule through which the pollen tube passes in order to effect fertilisation.
N)
O)
- osmoprotection A survival strategy brought about by the synthesis of groups of proteins (termed dehydrins) in response to water stress. Most tissues produce these proteins which protect cellular organelles and membranes from the damaging effects of water loss (desiccation). A notable exception being the cells of the starchy endosperm which undergo programmed cell death.
- ovule The part of the plant that, after fertilisation, develops into the seed. It is composed of the embryo sac, the nucellus, the integuments and the funiculus.
- ovum Unfertilised egg-cell containing a haploid nucleus.
P)
- palea The upper of two bracts enclosing the flower in cereals and grasses.
- parenchyma A primary tissue with polyhedral cells and thin cell walls.
- peduncle The final internode of the plant stem, that bears the flower.
- pericarp The ovary wall; later the fruit/seed/grain wall.
- pigment strand A cylinder of cells that lies between the vascular tissues and the nucellar projection in the crease region.
- plasmalemma The membrane that surrounds a protoplast.
- polar nuclei Two haploid nuclei in the embryo sac. These nuclei fuse with a pollen nucleus to form a triploid endosperm nucleus, which subsequently divides to form the endosperm.
- polarized light A technique used in light microscopy. A beam of polarized light is passed through thin slices of preserved tissue. The light is diffracted by some parts of the plant tissue and not others. Starch grains diffract the light in a very characteristic manner producing an image of the Maltese Cross. This image is diagnostic of the presence of starch grains.
- pollen The male gametes, which are produced in the stamens of flowering seed plants.
- pollination The transfer of pollen from an anther (the male reproductive organ) to a stigma (the receptive part of the female reproductive organ).
- primordia A group of cells that represents the initial stages in development of a plant organ. s primordium.
- protein body A membrane-bound granule of storage proteins.
Q)
R)
- rachilla The axis in the centre of a spikelet The secondary axis of the flower spike.
- rachis The primary axis of the wheat ear, it bears the spikelets.
- radicle The part of a plant embryo that develops into the primary root.
S)
T)
- terminal spikelet This marks the completion of the spikelet initiation phase in wheat. The terminal spikelet has its plane of symmetry at right angles to the other spikelets.
- testa The outer coat of the seed, formed by the hardened integuments.
- tiller A lateral shoot that develops from the axillary bud of leaves at the base of a cereal stem.
- transfer cells Specialized cells with intricate cell walls and large membrane surface thought to function in the transport of materials.
- triploid Describing a nucleus or cell that has three times (3n) the haploid number (n) of chromosomes.
- triploid fusion nucleus The result of the fusion of one male gamete (pollen) with two haploid female nuclei (polar cells) in the 'second polar event'. This nucleus divides to form the cells of the endosperm.
- ts Transverse Section. The tissue sections were cut across the grain. The images presented are of sections from the middle of the grain (median).
- tube cells Cells formed from the inner epidermis of the pericarp in which the cells elongate parallel to the grain's long axis.
U)
V)
- vascular tissue A tissue system which transports water and nutrients (called xylem) and photosynthates (called phloem) around the plant.
- ventral The side of the grain towards the central axis.
- ventral groove Also called the crease The furrow on the ventral side of the grain. The tissues of the ventral groove are adapted for efficient transfer of nutrients.
W)
- water ripe (also called Pre-milk). A description the newly pollinated grain. When punctured the grain contents are very watery.
X)
Y)
Z)
- zygote The cell resulting from the fusion of male and female gametes during sexual reproduction. Subsequent cell divisions and differentiation result in the production of the embryo.