>> Saturday, August 21, 2010
When the seed germinates in well-drained and well-aerated soil, the coleorhiza, a covering enclosing the radicle or primary root, protrudes first.
Shortly after the coleorhiza appears, the radicle or primary root breaks through the covering.
Two or more sparsely branched seminal roots follow. These roots eventually die and are replaced by many secondary adventitious roots.
If the seed germinates in water, the coleoptile, a covering enclosing the young shoot, emerges ahead of the coleorhiza. The coleoptile emerges as a tapered cylinder.
The mesocotyl or basal portion of the coleoptile elongates when the seed germinates in soil, and in darkness. It pushes the coleoptile above the soil surface.
The first seedling leaf, or primary leaf, emerges from the growing seed. It is green and shaped like a cylinder. It has no blade. The second leaf is a complete leaf. It is differentiated into a leaf blade and a leaf sheath.
The seedling will grow and develop branched tillers. Parts of the rice tiller include the roots, culm and leaves. Mature roots of the rice plant are fibrous and produce smaller roots called rootlets. All roots have root hairs to absorb moisture and nutrients.
There are two kinds of mature roots:
1. secondary adventitious roots
2. adventitious prop roots prop roots.
Secondary adventitious roots are produced from the underground nodes of young tillers.
As the plant grows, coarse adventitious prop roots often form above the soil surface in whorls from the nodes of the culm.
The culm, or jointed stem of the rice, is made up of a series of nodes and internodes.
Young internodes are smooth and solid. Mature internodes are hollow and finely grooved with a smooth outer surface. Generally, internodes increase in length from the lower to the upper portions of the plant. The lower internodes at the plant base are short and thick.
The node is the solid portion of the culm. The node or nodal region bears a leaf and a bud. The bud is attached to the upper portion of the node and is enclosed by the leaf sheath. The bud may give rise to a leaf or a tiller.
Early tillers arise from the main culm in an alternate pattern. Primary tillers originate from the lowermost nodes and give rise to secondary tillers. Secondary tillers produce tertiary tillers.
The node or nodal region of the culm will bear a leaf.
Leaves are borne alternately on the culm in opposite directions. One leaf is produced at each node. Varieties differ in the number of leaves produced.
The topmost leaf below the panicle is the flag leaf. The flag leaf contributes largely to the filling of grains because it supplies photosynthetic products, mainly to the panicle.
The leaf sheath and leaf blade are continuous.
A circular collar joins the leaf blade and the leaf sheath.
The leaf sheath is wrapped around the culm above the node.
The swelling at the base of the leaf sheath, just above the node, is the sheath pulvinus. It is sometimes incorrectly referred to as the node.
Leaf blades are generally flat. Varieties differ in blade length, width, thickness, area, shape, color, angle and pubescence.
With many parallel veins on the upper surface of the leaf, the underside of the leaf blade is smooth with a prominent ridge in the middle; the midrib.
Most leaves possess small, paired ear-like appendages on either side of the base of the blade. These appendages are called auricles. Auricles may not be present on older leaves. Another leaf appendage is the ligule, a papery membrane at the inside juncture between the leaf sheath and the blade. It can have either a smooth or hair-like surface. The length, color, and shape of the ligule differ according to variety.
Although similar, rice seedlings are different from common grasses. While rice plants have both auricles and ligules, common grassy weeds found in rice fields normally do not have these features. These characteristics are often helpful in identifying weeds in rice fields when the plants are young.
Panicle and Spikelets
The terminal component of the rice tiller is an inflorescence call the panicle. The inflorescence or panicle is borne on the uppermost internode of the culm. The panicle bears rice spikelets, which develop into grains.
The panicle base often appears as a hairlike ring and is used as a dividing point in measuring culm and panicle length. The panicle base is often called the neck.
The panicle axis is continuous and hollow except at the nodes where branches are borne.
The swellings at the panicle axis where the branches are borne are referred to as the panicle pulvinus.
Each node on the main panicle axis gives rise to primary branches which in turn bears secondary branches. Primary branches may be arranged singly or in pairs.
The panicles bear spikelets, most of which develop into grains. These spikelets are borne on the primary and secondary branches. The spikelet is the basic unit of the inflorescence and panicle. It consists of the pedicel and the floret.
The floret is borne on the pedicel.
The rudimentary glumes are the laterally enlarged, cuplike apex of the pedicel. The rudimentary glumes are the lowermost parts of the spikelet. During threshing, the rudimentary glumes are separated from the rest of the spikelet.
The sterile lemmas are small, bractlike projections attached to the floret. The rachilla is a small axis that bears the single floret. It is between the sterile lemmas and the floret.
The rachilla, sterile lemmas and the rudimentary glumes all support the floret. The floret includes the lemma, palea, and the flower.
The larger protective glume covering the floret is called the lemma and the smaller one is referred to as the palea.
Both the lemma and palea have ridges referred to as nerves. The lemma has five while the palea has three. The middle nerve of the lemma can be either smooth or hairy.
The lemma has a constricted structure at its end called the keel. In some varieties, the keel is elongated into a thin extension, the awn.
The floret contains a flower. The flower consists of a pistil (female organ) and six stamens (male organs).
The stamens have two-celled anthers borne on slender filaments.
The pistil contains one ovule and bears a double-plumed stigma on a short style.
At the flower’s base near the palea are two transparent structures known as lodicules. The lodicules thrust the lemma and palea apart at flowering to enable the elongating stamens to emerge out of the open floret. The lemma and palea close after the anthers have shed their pollen.
The rice grain is the ripened ovary, with the lemma, palea, rachilla, sterile lemmas and the awn firmly attached to it.
The rice hull includes the lemma and palea and their associated structures – the sterile lemmas, rachilla, and awn.
The dehulled rice grain is called caryopsis, commonly referred to as brown rice because of three brownish pericarp layers that envelope it. Next to the pericarp layers are the two tegmen layers and the aleurone layers.
The remaining part of the grain consists of the endosperm and the embryo. The endosperm provides nourishment to the germinating embryo. The embryo lies on the belly side of the grain and is enclosed by the lemma. It is the embryonic organ of the seed.
The embryo contains the plumule (embryonic leaves) and the radicle (embryonic primary root).