Question

Onions are modified ?
A.Stem
B.Leaves
C.Root
D.All of the above

Answer 1

Leaves develop from the meristematic tissue called leaf primordium, which is a localised exogenous extension near the apical meristem. Leaf is a thin outgrowth of the stem. Based on posture, two kinds of leaves can be recognized, namely dorsi-ventral or bifacial leaves, and isobilateral or equifacial leaves.

Complete answer:
A typical dicot leaf is a dorsiventral leaf with reticulate venation. It has three main parts, namely leaf base, petiole, and leaf lamina. Basal portion of the leaf that attaches to the stem is called leaf base. It protects the small buds in the axil. Petiole is the leaf stalk. It connects the lamina to the stem, exposes the lamina to sun light and transports water and nutrients. Lamina or leaf blade is the thin, green, broad pard part of the leaf. It is involved in photosynthesis, transpiration and gas exchange.
There are a lot of modifications of leaves. Leaves are modified for climbing, protection and nutrition. These modifications include tendrils, spines, hooks, scale leaves, pitchers, phyllodes etc.

Leaf tendrils: These are long, spirally coiled and spring like leaves. They help weak-stemmed plants to climb on a support. Examples are Pisum, Gloriosa, Lathyrus. In Pisum, leaf tendrils are modified terminal leaflets, in Gloriosa they are modified leaf tips and in Lathyrus, they are entire leaves.

Leaf spines: They are sharply pointed structures characteristic of xerophytic plants, such as cacti, Asparagus etc. Leaf spines are used for defence, protection, and control of transpiration. In Opuntia, ordinary leaves are modified into scale leaves, and axillary buds are modified into spines.

Leaf scales: They are thin, stalkless and membranous. Examples are Onion and Garlic. In Onion, scale leaves protect axillary buds. They are fleshy due to the storage of water and food.

Pitcher: It is a flask-shaped modification of the leaf lamina for trapping insects. It is found in some insectivorous plants, such as Nepenthes and Sarracenia. In Nepenthes, leaf lamina is modified into a pitcher. The mouth of the pitcher is covered by a lid, called operculum. It is a modified part of the leaf tip. The basal portion of the petiole is leaf-like for photosynthesis. Its upper portion is coiled like a tendril to hold the pitcher in vertical position. The whole pitcher is beautifully coloured to attract insects.

Phyllode: It is a flat, leaf-like modification of the petiole. It is a xerophytic adaptation to reduce transpiration. In this case, normal leaflets fall at an early stage. Then petiole develops to a phyllode. Phyllodes are common among Parkinsonia, Acacia etc.

Modified stems are seen in Potato, Ginger, Turmeric, Zaminkand, Colocasia. In them, the stem is modified to store food in them.
Modified roots are seen in Asparagus, Turnip, carrot, Sweet potato etc.

Hence the correct answer is OPTION(B)

Note: Leaf venation or pattern of arrangement of veins in the lamina is two types, namely reticulate venation and parallel venation. In reticulate venation, veins and veinlets form an interconnecting network. In parallel venation, veins are parallel to one another. Parallel venation is horizontal in the case of Plantain and longitudinal in Bamboo and Lilies. Reticulate venation is characteristic of dicot leaves and parallel venation is characteristic of monocot leaves.