Stem – Morphology, Types, and Modifications
Stem – Morphology, Types, and Modifications
CHARACTERISTICS OF STEM
- The stem grows positive phototropic, negative hydrotropic and negative geotropic.
- The chlorophyll is found in the young stem, but it is absent in woody stem.
- The stem haves nodes and the parts found between them are known as internodes.
- Various types of plant organs such as leaves, branches, buds, flowers etc. originate in exogenous form from the meristematic tissue of internodes.
- An angle formed between stem and leaf is called leaf axil. In this axis, the axillary bud is found and the apical bud is found on the tip of branches.
SHAPE OF STEM
There are different types of stem found in different plants, like:
- Cylindrical – in almost all plants
- Ribbed – In Casuarina
- Flat – In Hawthorn (Opuntia)
- Jointed – in sugarcane, bamboo
- Triangular – In Cyprus
- Quadrangular – In Tulsi (Ocimum or Basil)
stem – Morphology, Types, and Modifications
FORMS OF STEM
There are two types of the stem depending on the form of the stem –
This type of stem is strong and hard, which stands straight on the ground. It is of two types –
This type of stem is green, soft, and woodless. In the annual plant, wheat; in the biennial plant, beet; and perennial plant, canna lily have these types of the stem.
Such stems are long, strong, and woody. It is of two types. –
In these plants, many woody branches have emerged simultaneously in place of the main stem. Example Leptadenia, Capparis, Calotropis, Rose etc.
The stem is thick and woody in the trees. Only one main stem is found in them and branches emerge from this main stem. They are of four types:
This type of stem contains branches. It is thickest at the base and gradually tapers towards the apex, due to which tree appears cone-shaped. Example Pinus palustris and Ashok (Polyathia longifera).
This type of stem is without branches and the leaves on top of the stem are crown-shaped. Example Date (Phoenix dactylifera) and coconut (Cocos nucifera) etc.
This type of stem contains branches which spread out in different directions. The axillary buds are more active than the apical bud. Examples Banyan (Ficus bengalensis), Neem (Azadirachta indica) and Peeple (Ficus religiosa).
This type of stem contains distinct nodes and internodes. The nodes are swollen which gives the stem a jointed appearance. Bamboo (Bambusa arundinacea).
The weak stems cannot stand straight on the ground, so they fall down or climb on other objects for support. Weak stems are long and thin and are of two types:
They grow up crawling on the ground. The adventitious roots, leaves, and branches emerge from their nodes and separate itself from the original plant to form a new offspring. Examples Doob grass (Cynodon dactylon), Oxalis, Centella, Colocasia. Creepers are four types :
These stems craw on the ground and grow horizontally. They do not form roots at the nodes. These are of three types:
In this type of stem, after growing horizontally the tip of the stem rises upwards without any support. Example Portulaca grandiflora, Lindenbergia.
The stems of these plants grow on the ground only. Their tips do not rise upwards. Example convolvulus pluricaulis
This type of stem climbs upwards with the help of some support. These are the following types:
c.1. Tendril Climber
Spiral, soft, and fast-moving structure emerge from the nodes of the stems. These are called Tendril Climber. They help the plant to climb up objects for support. They may be modified leaf or the part of the leaf. Like Stipule in Smilex, Leaf apex in Glauarosa (Gloriosa superba), Leaf petiole in Clematis, Leaf lamina (Leaf blade) in Pea, complete foliage (leaf) in Lathyrus aphaca.
c.2. Stem Climber
In this type of plant, many branches of the stem interlock each other and climb up. These are two types:
c.3. Root Climber
In plants like betel vine(Piper betel), adventitious roots are modified to form the threadlike structure that helps them to climb.
c.4. Hook and thorn climber
Prickle in Hawthorn (Opuntia humifusa), Spine in Roses, Thorn in Berry (Ziziphus mauritiana), Hook in Bignonia helps the plant to climb up. Thorn, prickle, and spine usually have straight edges while the hook is further twisted.
Adhesive discs are found on the stem of the plant. These discs help to climb up on the support. Example Ficus repens.
MODIFICATIONS OF STEMS
We can find some modifications in the stem which help them to perform specific tasks.These include aerial, semi-aerial and underground modifications.
Aerial Modifications of the Stem
In some plants, aerial stems are converted to perform specific functions. These are the following types.
1. Stem Tendril
Spiral fibers are found in the place of axillary bud or apical bud which helps in climbing up. Example grapes (Grapevine, Vitis vinifera)
2. Stem thorn
This is the Modifications of the axillary bud. It is thick and spiny. Example Lime (Citrus) Bougainvillaea. Stem thorn work as protection from animals.
Stems become fleshy or green and form a flat leaf-like structure. It does photosynthesis. Example Hawthorne (Opuntia). Interns and nodes are present on the stem.
A phylloclade stem with single internode is called cladode. That is flattened, green and photosynthetic. Example Asparagus and Ruscus aculeatus.
The vegetative or flower buds collects food and gets swollen. These help in vegetative propagation. Examples include Agave and Yam.
Semi-aerial Modifications of the Stem
In these types of stem, the branches grow parallel, below or above the surface of the ground. It is of four types:
Stems increase on crawling on the ground. On the stems, the nodes are found to have adventitious roots and leaves. New plants are produced by the separation of the internodes. Example dub grass.
Lateral branches emerge from the base of the main stem, they initially grow upward and then arch down to develop new plants. Example Strawberry
The underground branches of this type of stem arise from the axillary buds of the underground part of the aerial stem. Example mint and chrysanthemum
It is found in aquatic plants. The stem grows crawling in water which is similar to the runners found on land. Their branches are small, thick and have one internode. Examples Water hyacinth and Pistia.
Underground Modifications of Stem
Underground part of the stem is converted for food collection, physiological functions, reproduction etc. Due to the presence of nodes, internodes, buds etc. they are called stem. They are of the following types:
It is thick, fleshly and irregular underground stem. The stem grows horizontally below the soil surface and produces aerial leaves above the ground. Examples Zingiber officinale and turmeric (Curcuma longa) etc.
The stem tubers are swollen ends of specialized underground branches that contain nodes and internodes. Example: potato (Solanum tuberosum).
It is fleshy, condensed vertically growing root that is usually unbranched. Elephant foot yam (Amorphophallus paeoniifolius), taro (Colocasia esculenta), saffron (Crocus sativus) etc.
Some plants have the very small disc-like stem which contains fleshly scaly leaves. These leaves are known as the bulb. They are modified underground leaves which are fleshy, swollen and spherical due to the storage of food. Example: Onion (Allium cepa), Garlic (Allium sativum) etc.
FUNCTIONS OF STEMS
- The stem bears branches, leaves, flowers, etc.
- The stem conducts water and mineral nutrients from roots to other parts of the plants.
- It helps in food storage, reproduction, protection, and support.
- The plant growth regulators (plant hormones) are also synthesized by the stem.
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