Onion is a biennial or perennial herb. Root string linear, thick green cylindrical hollow leaves, with waxy surface; The leaf sheath is thick and scaly, and is dense around the shortened stem to form a bulb (commonly known as onion); Umbels, small white flowers; Capsule. The rhizome is wrapped with a thin layer of skin (white, yellow or red), and inside is a layer of meat, usually white or light yellow.
The radicle of onion will shrink soon after it is buried in the soil, so there is no main root. Its root is a stringed fibrous root, which is located at the base of the short stem plate. The root system is weak and has no root hairs. The root system is mainly densely distributed in the surface soil layer of 20cm, so the drought resistance is weak and the ability to absorb fertilizer and water is weak. The growth temperature of the root system is lower than that of the ground. When the ground temperature is 5 ℃, the root system starts to grow, the optimum temperature is 10-15 ℃, and the growth is slow at 24-25 ℃.
During the vegetative growth period, the stem of onion is shortened to form a flat conical stem plate, the lower part of the stem plate is a heel, the upper part of the stem plate is surrounded by a circular cylindrical leaf sheath and branch buds, and the lower part is covered with fibrous roots. The disc heel tissue of mature bulb is dried and hardened, which can prevent water from entering the bulb. 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 the conditions of low temperature and long sunshine, the growth cone starts to flower bud differentiation, and the flower bolt is tubular, hollow, the middle is expanded, there is wax powder, the top forms inflorescence, and can flower and bear fruit. Because of the degeneration in flower period, the aerial bulb was formed in the flower bud.
The leaves of onion are composed of leaf body and leaf sheath, which form pseudostems and bulbs. The leaf body is dark green, cylindrical, hollow, and has a concave groove in the abdomen (it is one of the morphological signs that distinguish onion from young seedlings). The tubular leaves of onion grow upright, with a small leaf area and a thick wax powder on the leaf surface, which is an ecological feature of drought resistance.
The scape is thick and strong, up to 1m high, hollow cylindrical, expanded below the middle, gradually narrowed upward, and the lower part is covered with leaf sheaths; Involucre 2-3-lobed; Umbel globose, with many dense flowers; The peduncle is about 2.5cm long. Pollen white; Perianth segments with green midvein, oblong ovate, 4-5mm long and 2mm wide; Filaments are equal in length, slightly longer than the perianth piece, connate at about 1 / 5 of the base, 1 / 2 of the lower part of the connate part is adnate to the perianth piece, the base of the inner whorl filaments is extremely enlarged, the enlarged part has 1 tooth on each side, and the outer whorl is conical; Ovary: subglobose, concave honeycombs with curtain at the base of ventral suture; The style is about 4mm long. The flowering and fruiting period is from May to July.
