For many of you, this will be your first exposure to rice as a plant. This chapter provides a brief introduction to the physical parts of the rice plant - what botanists call the morphology It is intended to be neither highly technical nor comprehensive. A basic understanding of the physical characteristics of the rice plant will facilitate your understanding of the rice growth cycle and should prove useful in many aspects of cultivation.
I. General Information
Rice (oryza sativa) belongs to the family of cereal grasses, along with wheat, corn, millet, oats, barley, rye, and numerous others. The grass family provides the world with over 60% of its caloric intake and over 75% of the protein for developing nations.
The rice plant is an annual grass (it normally grows for only a year and then dies) with round, hollow, jointed culms (stems), flat leaves, and a terminal panicle (flower cluster). It is the only cultivated cereal plant adapted to growing in both flooded and non-flooded soils. Grown under a wide range of climatic and geographical conditions on all five continents, it serves as the staple food throughout much of the world.
The parts of the rice plant may be divided as follows:
- stem and leaves
- reproductive organs
As the underground portion of the plant, the roots serve as support, draw food and water from the soil, and store food. They are fibrous and consist of rootless and root hairs. The embryonic roots, or those which grow out of the seed when it germinates, nave few branches. They live for only a short time after germination. Secondary adventitious roots (i.e. roots appearing in an irregular pattern) emerge from the underground nodes of the young culm and replace the embryonic roots. Although a few adventitious roots grow straight down to a depth of over 15", most spread out under flooded conditions into the shallow oxidized soil layer near the surface to form a broad, dense network.
III. Stem and Leaves
The role of the stem (or culm) is to support the leaves and reproductive structures, and to transfer essential nutrients between the roots, the leaves, and the reproductive structures. The stem is made up of a series of nodes and internodes in alternating order. The node (corresponding to the "joint" between two sections of the stem) bears a leaf and a bud which, if it is on the lowermost node, may grow into a tiller, or shoot. The mature internode is hollow and finely grooved. Its outer surface is hairless. It varies in length, generally increasing from the lower to the upper internedes. The lower internodes at the base of the stem are short and thickened into a solid section. The internodes have the capacity to elongate in deep water in order to keep a portion of the plant above water to carry on photosynthesis.
The leaves function as the principal organs of photosynthesis and respiration (i.e. they contain chlorophyll-containing cells which convert sunlight to chemical energy and synthesize organic "fuel" compounds from inorganic compounds). The leaves are borne at an angle on the stem in two ranks - one at each node. The blade, or extended part of the leaf, is attached to the node by the leaf sheath. The sheath envelops the internode toward, and in some cases even beyond, the next node. On either side of the base of the blade are pairs of small, earlike appendages known as auricles. Just above the auricles is a tissue-like, triangular structure called the ligule. Rice plants have both auricles and ligules and a ligule at every internode; this characteristic is often helpful in differentiating between rice and grassy weeds, which can have auricles or a ligule but not both.
The uppermost leaf below the panicle, the flag leaf, provides the most important source of photosynthetic energy during reproduction.
Fig. 1: Stem and Leaves of Rice
III. Reproductive Organs
The panicle, or flower cluster, contains the reproductive organs of the rice plant. Borne atop the uppermost node on the stem, the panicle is divided into primary, secondary, and sometimes tertiary branches bearing the spikelets. The branches may be arranged singly or in pairs. The panicle stands erect at blooming, but it usually drops as the spikelets fill, mature, and develop into grains. Varieties differ greatly in the length, shape, and angle of the primary branches, as well as in the weight of the overall panicle
Fig. 2: The article (partly shown)
Each individual spikelet contains a set of floral parts flanked by the lemma and palea. The flower consists of six stamens and a pistil. The stamens (which contain pollen, or "sperm") are composed of two-celled anthers borne on slender filaments. The pistil consists of the ovary (containing the ovule, or "egg"), the style, and the stigma. During reproduction, the stigma catches pollen from the stamens and conducts it down to the ovary, where it comes into contact with the ovule and fertilization occurs.
Fig. 3: The Spikelet
The grain is the seed of the rice plant, a fertilized and ripened ovule containing a live embryo capable of germinating to produce a new plant. It is composed of the ripened ovary, the lemma and palea, the rachilla, the sterile lemmas, and the awn (not always present). The lemma and palea and their associated structures constitute the hull or husk. The embryo lies at the ventral side of the spikelet next to the lemma and contains the embryonic root. The rest of the grain consists largely of endosperm (the edible portion), containing starch, proteins, sugar, fats, crude fiber, and inorganic matter.
Fig. 4: The Grain