Onions are biennial or perennial herbs. Root string-like, thick green cylindrical hollow leaves with waxy surface; leaf sheaths are thick and scaly, densely packed around the shortened stems, forming bulbs (commonly known as onions); umbels, small white flowers; capsules. The rhizome is covered with a thin layer of skin (white, yellow or red), and inside is a layer of flesh, usually white or light yellow.
The radicle of onion will shrink soon after it enters the soil, so there is no tap root. Its root is a string-like fibrous root, which grows at the base of the shortened stem disc. The root system is weak and has no root hairs. The root system is mainly densely distributed in the topsoil layer of 20cm. Therefore, drought tolerance is weak, and the ability to absorb fertilizer and water is weak. The root growth temperature is lower than that on the ground. When the ground temperature is 5 degrees Celsius, the root system begins to grow. It is optimal at 10-15 degrees Celsius, and grows slowly at 24-25 degrees Celsius.
During the period of vegetative growth, the stem of onion shrinks to form a flat conical stem disc. The lower part of the stem disc is the disc heel. The upper part of the stem disc is surrounded by circular cylindrical leaf sheaths and branch buds, and fibrous roots grow underneath. The disc heel tissue of mature bulbs shrinks and hardens, which prevents water from entering the bulbs. Therefore, the disc heel can control the premature growth of the root or the premature germination of the bulb. During the reproductive growth period, the plant is subjected to low temperature and long-day conditions, and the growth cone begins to differentiate the flower buds, and the flower spurs are formed. Wax powder, forming inflorescences at the top, can bloom and bear fruit. Due to the degradation of the flowering period, the top bulb onion forms aerial bulbs in the buds.
The leaf of an onion is composed of a leaf body and a leaf sheath. The leaf sheath forms a pseudostem and bulb. The leaf body is dark green, cylindrical, hollow, and has a groove on the abdomen (it is one of the morphological signs that distinguish it from green onions in the seedling stage) . The tubular leaves of onions grow upright, with a small leaf area, and thick wax powder on the surface of the leaves, which is an ecological feature of drought resistance.
Scape is stout, up to 1m in height, hollow cylindrical shape, swelled below the middle, tapered upward, and sheathed in the lower part; involucre 2 to 3-lobed; umbel-shaped inflorescence with many dense flowers; small pedicel long About 2.5cm. The pollen is white; the perianth has a green midrib, oblong-ovate, 4～5mm long and about 2mm wide; the filaments are the same length, slightly longer than the perianth, connate at about 1/5 of the base, 1/half of the lower part of the connate 2 Adjacent to the tepal, the base of the inner ring filament is extremely enlarged, the enlarged part has 1 tooth on each side, and the outer ring is tapered; ovary: nearly globular, with a concave honey hole on the base of the abdominal suture; style length is about 4mm. Flower and fruit period from May to July.
