Modified aerial stem with branching of unlimited growth as the photosynthetic structure is

A)  Cylindrical in Opuntia and Euphorbia
B)  Fleshy and cylindrical in Euphorbia
C)  Flattened in Opuntia and Euphorbia
D)  Cylindrical in Opuntia

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Hint: The stem is the part of the plant which is found above the ground. The younger stems are green in colour and the bark of trees is brown in colour. It forms the basis of the shoot system and bears fruits, leaves, and flowers.

Complete Step-by-step answer: Stems of several plants are modified to perform various functions like protection, storage, support, photosynthesis, perennation, and propagation. Modifications assist in better adaptation and survival. In certain plants, stems are modified, which can be subaerial, aerial, or underground modifications. They are modified to carry out additional functions, which are not normally linked with the stem. In certain plants, the aerial part of the stem and buds get modified to perform different functions like support, photosynthesis, climbing, and vegetative propagation.
Tendrils:These are coiled and delicate parts, which are used for climbing. They evolve from axillary buds. Examples: watermelons, grapevines, cucumber, and pumpkin.

Thorns:These are developed from axillary buds. They are pointed, woody, and hard, spiny structures, providing protection from grazing animals. Examples: Lemon, and bougainvillea.

Bulbils:These are modified axillary bud. It stores food and becomes fleshy. It separates from the mother plant and evolves into a new plant. Example: Dioscorea

Cladode and Phylloclade:These are modified stems, which are green and carry out photosynthesis. They become fleshy. Leaves are decreased to spines. These are commonly found in xerophytic plants to decrease water loss. Examples: Euphorbia, Opuntia, Asparagus.

Therefore the correct answer is Option B.

Note: Stem supports elevation of leaves, flowers, and fruits. It stores nutrients and also assists in transportation. Stems have meristems that assist in the production of new living tissue. The primary function of the stem is to provide support to other parts of the plant and conduction of water, food, and minerals.