Onion | Allium cepa

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Classification

  • Kingdom – Plantae
  • Division – Angiospermae
  • Class – Monocotyledonae
  • Order – Liliales
  • Family – Liliaceae
  • Genus – Allium
  • Scientific name – Allium cepa

Important points

  • Onion consists of both an aerial and an underground stem.
  • The underground stem is highly reduced into a condensed, disc-shaped structure called the bulb.
  • The scale leaves arising from the bulb are modified to store food and become fleshy.
  • The fleshy scale leaves are surrounded by thin, papery outer scales called the tunic, which prevent them from drying out. This type of bulb is called a tunicated bulb.
Onion bulb – Allium cepa

NCERT gives onion and garlic as examples of fleshy leaves that store food – a leaf modification. Onion has a tunicated bulb, lily has a scaly bulb (scales not covered by a tunic), and garlic has a compound bulb made of many small bulblets called cloves.

Morphology of the onion plant

  • Habit – a biennial herb.
  • Root – a cluster of adventitious, fibrous roots arises from the lower surface of the disc-shaped stem.
  • Stem – the true stem is the reduced disc inside the bulb. At flowering, it produces a long, leafless, hollow flowering stalk called the scape.
  • Leaves – two types: the inner fleshy scale leaves that store food, and the green, hollow, tubular aerial leaves that carry out photosynthesis. The leaves show parallel venation, as in most monocots.
  • Inflorescence – small flowers are clustered in a rounded umbel at the tip of the scape.
  • Chromosome number – 2n = 16.

Features of family Liliaceae

Onion belongs to the family Liliaceae (the lily family), one of the three families described in NCERT. Its main features are:

  • Vegetative characters – perennial herbs with underground bulbs, corms or rhizomes. The leaves are mostly basal, alternate, linear, exstipulate, with parallel venation.
  • Inflorescence – solitary or cymose, often umbellate clusters.
  • Flower – bisexual and actinomorphic.
  • Perianth – six tepals in two whorls (3+3), often united into a tube; valvate aestivation.
  • Androecium – six stamens in two whorls (3+3), epitepalous (attached to the tepals).
  • Gynoecium – tricarpellary, syncarpous, with a superior ovary, trilocular, with many ovules and axile placentation.
  • Fruit – capsule, rarely a berry.
  • Seed – endospermous.

Floral formula:
Br ⊕ ⚥ P(3+3) A3+3 G(3)

NCERT’s economic importance of Liliaceae: many plants are grown as ornamentals (tulip, Gloriosa), as medicine (Aloe), as vegetables (Asparagus), and as the source of colchicine (Colchicum autumnale), which is used to induce polyploidy.

Onion in the laboratory

  • Onion root tips are used to study the stages of mitosis, as the cells in the root tip divide actively.
  • Onion peel (the thin epidermis of the fleshy scale leaves) is used to observe plant cell structure and the process of plasmolysis.
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