Onion is a perennial herb of Liliaceae and Allium.
Onion is a biennial or perennial herb. Root string, dark green cylindrical hollow leaves with waxy surface; The leaf sheath is thick and scaly, and it is dense around the shortened stem to form a bulb (commonly known as onion); An umbrella with small white flowers; The root of the capsule is covered with a thin layer of skin (white, yellow or red) and the inside is a layer of meat, usually white or light yellow.
The radicle of onion will shrink soon after being buried, so there is no main root. Its roots are chordal fibrous roots, which are planted at the base of shortened stem disk. The root system is weak and hairless. The root system is mainly distributed in the top soil of 20cm, so it has weak drought tolerance and weak ability to absorb fertilizer and water. The growth temperature of root system is lower than that of aboveground part. When the ground temperature is 5℃, the roots begin to grow, and the optimum temperature is 10 ~ 15℃, and the growth is slow at 24 ~ 25℃.
During the vegetative growth period, the stem of onion becomes shorter, forming an oblate conical stem disk. The lower part of the stem disk is the heel of the stem disk, and the upper part of the stem disk is surrounded by cylindrical leaf sheaths and branches and buds, and fibrous roots grow below. The heel tissue of mature bulb shrinks and hardens, which can prevent water from entering the bulb. Therefore, the heel can control the premature growth of roots or the premature germination of bulbs. In the reproductive growth period, plants are affected by low temperature and long sunshine, and the growth cone begins to differentiate into flower buds, which are tubular and hollow, with wax powder and inflorescences at the top, and can blossom and bear fruit. Due to the degradation of flowering period, onion bulbs form aerial bulbs in buds.
The leaves of onion are composed of leaf body and leaf sheath, forming pseudostems and bulbs. The leaves are dark green, cylindrical and hollow, and there is a concave groove in the abdomen (this is one of the morphological signs that distinguish them from green onions at seedling stage). The tubular leaves of onion grow vertically, with small leaf area and thick wax powder on the surface, which is an ecological feature of drought resistance.
Scape is stout, as high as 1m, hollow and cylindrical, expanding below the middle, tapering upward, and the lower part is covered by leaf sheath; Involucre 2 ~ 3-lobed; Umbellies are spherical with many dense flowers; Pedicel is about 2.5 cm long. Pollen white; Tepals with green midvein, oblong and oval, 4 ~ 5 mm long and 2 mm wide; Filaments are the same length, slightly longer than perianth segments, and combine at the base of 1/5. 1/2 is attached to the lower part of the joint, the base of the inner filament is extremely expanded, and there are 1 teeth on both sides of the expanded part, and the outer wheel is conical; Ovary: nearly spherical, with curtain at the base of abdominal seam; The style is about 4mm long. The flowering and fruiting period is May-July.
Onion is a very common and cheap home-cooked dish. Its meat is tender, its juice is spicy and light, its quality is good, and it is suitable for raw food. The edible part of onion is the underground hypertrophy bulb (onion head). High nutritional value. Onion is rich in nutrition, which contains not only potassium, vitamin C, folic acid, zinc, selenium, fiber and other nutrients, but also two special nutrients-quercetin and prostaglandin A. These two special nutrients make onion have many irreplaceable health care effects.