Question

Explain different aerial modifications of stem?

Answer 1

1. Stem tendrils:

Tendrils are long, thin, win and spirally coiled climbing organs of the plants.They coil around the nearby support and help the plant to climb.They may be branched or unbranched. Stem tendrils are of the following four types-

1. Axillary , e.g., Passiflora.
2. Extra-axillary; e.g., Luffa (vern. Ghiatori), – Cucurbita (vem. Kaddoo), Lagenaria (vern. Lauki).
3. Apical bud tendrils, e.g., Grape Vine (litis vinifera). The main stem of the weak-stemmed plant stems shows sympodial branching of scorpoid type. The apical buds of successive branches are pushed to one side and modified into tendrils.
4. Floral bud or inflorescence tendrils, e.g., Antigonon In Antigonon, the flowers occur in bunch which the upper floral buds are modified to tendrils.

2. Stem thorns :

The axillary buds of some plants become arrested and get modified into stiff, sharp and pointed structures, called thorns. The are deep seated -structures having vascular comiections with stem. Besides reducing transpiration, they protect the plants from browsing animals. Examples – Citrus, Duranta, Bougainvillea, Pomegranate.

3. Phylloclades:

In some xerophytes, the leaves are highly reduced or modified to spines. The function of leaves (mainly photosynthesis) is taken over by the stems which get modified into flattened (e.g., Opuntia, Muehlenbeckia = cocoloba) or cylindrical (e.g., Casuarina. Euphorbia royleana, E. tirucalli, etc.) structures, called phylloclades. They are green in colour due to presence of photosynthetic pigments, show, unlimited growth and coated by thick cuticle in or wax surface to reduce transpiration.
Some phylloclades store water mucilage and latex. In Opuntia, the stem is modified into jointed, branched, flat, flesh and succulent leaf like structure.

• The leaves are small, fleshy and caducous.
• The leaves of axillary branches are modified into spines to check the loss of water.
• Each segment of phylloclade arises in the axil of a caducous leaf.
• The later falls down leaving behind a scar.
• The segment bears several nodes or areoles. The flowers are borne on the nodes.

In Euphorbia tirucalli. the phylloclade is green, cylindrical, shining and succulent. It is much branched. The leaves are delicate and formed during rainy season but shed in dry season, i.e., they are drought deciduous. The plants possess a milky latex which helps them in retaining moisture and plugging the area of injury.

4. Cladodes:

These are the modifications of stem and branches of limited growth. Each cladode is green, flat or cylindrical, leaf-like structure which per-forms photosynthesis. The cladode of Asparagus consists of single intemode. They arise in clusters in the axils of scale leaves. The true leaves are modified to spines. In Riiscus aculeatus, the leaf – like cladodes are borne in the axils of scale leaves. They are the cladodes of two intemodes. The nodal portion between the two internodes is sometimes provided with a floral bud arising in the axil of bract in the middle of cladode.