Modifications of Aerial Stems: 6 Types | Plants

The following points highlight the six main types of modification of aerial stems. The types are: 1. Stem Tendrils 2. Hooks 3. Stem Thorns 4. Phylloclade’s 5. Cladodes 6. Thalamus.

Type # 1. Stem Tendrils:

Tendrils are thread-like sensitive structures which can coil around a support and help the plant in climbing. They may be branched or un-branched. Branched stem tendrils may bear scale leaves in the region of forking (e.g., Grape Vine).

Stem tendrils are of four types:

(a) Axillary, e.g., Passiflora (Fig. 5.45. A),

(b) Extra- Axillary, e.g., Cucurbita, Lagenaria, Luffa (Fig. 5.45. B)

(c) Leaf Opposed, e.g., Grape Vine (Fig. 5.45. C).

In Grape Vine (Vitis vinifera) the main stem is sympodial due to scorpioid type of uniparous cymose branching. The terminal buds of the successive branches which are pushed to one side develop into tendrils,

(d) Floral Bud or Inflorescence Tendrils, e.g., Antigonon, Cardiospermum (Balloon Vine). In Antigonon the flowers occur in bunches in the axils of scale leaves on the floral shoot. The upper floral buds develop into tendrils instead of forming flowers (Fig. 5.46).

Type # 2. Hooks:

The pedicels or floral stalks of Artabotrys are modified into stiff curved thorns or hooks for helping in climbing. Hook is formed by peduncle in Uncaria gambier.

Type # 3. Stem Thorns:

They are stiff, sharp structures which have lost their growing point and become hard. Thorns not only reduce transpiration but also check browsing by animals. Axillary stem thorns occur in Citrus, Bougainvillea, Duranta, Pomegranate, Alhagi, etc. (Fig. 5.47).

In Bougainvillea the thorns are curved. They help the plant in climbing. In Duranta the stem thorns bear small foliage leaves. In Alhagi they possess flowers. Thorns are terminal and branched in Carissa.

Thorns, Spines, Prickles and Bristles:

They are sharp, pointed, and straight or curved hard structures which protect the plant from grazing animals and excessive transpiration. Thorns are modified stem structures (a vascular cylinder surrounded by a bark of thick- walled cells).

Spines are modified leaves or leaf parts (a vascular strand without a well developed bark). Prickles are superficial outgrowths of stem or leaves which do not possess a vascular cylinder. They can be easily pulled off. Bristles are stiff hair which becomes thickened due to deposition of silica or calcium carbonate.

Type # 4. Phylloclade’s:

They are flattened (e.g., Opuntia, Muhlenbeckia = Coccoloba) or cylindrical (e.g., Casuarina, Euphorbia royleana, E. tirucalli) green stems of unlimited growth which have taken over the function of photosyn­thesis. True leaves are caducous.

Formation of phylloclade’s helps the plant to grow in dry habitats because transpiration is little from the stem. In some cases phylloclade’s store water, mucilage and latex, e.g., Opuntia, Euphorbia royleana, E. tirucalli. In Opuntia the flattened, thick and fleshy stem is segmented or jointed (Fig. 5.48).

Each segment develops in the axil of a caducous leaf which falls down and leaves behind a scar. The segment may possess flowers and fruits. It bears many raised areas called areoles. An areole is actually a node. It has a leaf scar (scar left by a small fleshy but caducous leaf), one or two large spines and a number of stiff hair called bristles or glochidia.

The spines are modified leaves of suppressed axillary branches. They protect the plant against animals. In Euphorbia royleana, the phylloclade is a cylindrical, grooved, green stem. It bears drought deciduous leaves on spirally arranged ridges having stipular spines.

Type # 5. Cladodes (Cladophylls):

They are green stems of limited growth (usually one internode long) which have taken over the function of photosynthesis from the leaves. The true leaves are reduced to scales or spines. In Ruscus aculeatus (Fig. 5.49).

The cladodes are leaf like in appearance with spiny tip, ovate outline and roughly parallel veins. The cladodes are borne in the axils of scale leaves. Moreover, a floral bud with a basal scale leaf develops in the middle of a cladode showing that the area represents a node.

The stem nature of the cladode of Ruscus is clear from:

(a) Origin in the axil of scale leaf,

(b) Bearing a scale leaf in the middle,

(c) Forma­tion of a bud in the axil of the scale leaf borne in the middle,

(d) Development of flowers from the axillary bud produced in the middle of cladode. In Asparagus (Fig. 5.50)

The cladodes are slightly flattened, fleshy, and straight or curved pointed structures which develop in clusters in the axils of scale leaves. Each cluster represents a Suppressed cymosely divided branch with a few smaller scale leaves bearing cladodes in their axils. The main stem possesses leaf spines. Like phylloclade’s, cladodes are a modification to reduce transpiration.

Type # 6. Thalamus (Torus):

Flower is a specialised reproductive shoot which possesses a highly condensed axis called thalamus or torus. It forms a broadened tip of pedicel or floral stalk. Nodes and internodes are not distinct.

Thalamus bears four types of floral organs (sepals, petals, stamens and carpels), each from their own nodes. In certain cases the thalamus nodes become clear due to elongation of internodes between sepals and petals (anthophore, e.g., Silene), petals and stamens (androphore, e.g., Passiflora) or stamens and carpels (gynophore, e.g., Cleome).