Chapter 5- Morphology of flowering plants- Characteristics, Functions, Types of Stem (Notes)
Posted on : 29-09-2018 Posted by : Admin

Modifications of stems


Shoot and the Shoot system

The system of the plant which grows aerially is called as shoot system. The axis of the shoot system is called stem. Stem bears branches, leaves, flowers and fruits. The branches develop on the stem either horizontally or obliquely.  Shoot system develops from the plumule of the embryo. The stem bears nodes and internodes. The regions of the stem where leaves are borne are called nodes while internodes are the portions between two nodes.

The buds on the stem may be terminal or axillary. Stem is generally green when young and later on becomes woody and dark brown. The main function of the stem is spreading out branches bearing leaves, flowers and fruits. It conducts water, minerals and the products of photosynthesis. Some stems perform the function of storage of food, support, protection and vegetative propagation.


Characteristics of Stem

  • Stem is the vertically growing axis of the shoot system. It is positively phototropic and negatively geotropic in growth.
  • The vertical growth of the stem is maintained by the apical bud.
  • Stem is divisible into nodes and internodes.
  • Leaves are developed on the stem at nodes. The upper angle between the leaf and the stem is called axil. The axillary buds developed in the axils produce branches.
  • Young stems are green and woody stems are brown in color.
  • The stems have multicellular hairs.
  • Branches have exogenous origin.
  • Stem-like axis formed by the aggregated concentric leaf bases is called pseudostem. For example, Banana.

Modifications of stems


Types of Stems

Mainly there are two types of stems namely, hard stems and weak stems. Let us discuss each of them.

Hard stems

  • They are also known as erect stems.
  • These stems are capable of growing erect without any help.
  • Some of these stems have branches and some they may not have branches.
  • Majority of the plants have hard and wordy stems.

Based on the strength of the stem hard stems can be of three types namely Herbs, Shrubs and trees.

Herbs- The stem of hers is green, delicate and short. Most often these plants are very small and may not even rise above the surface of the soil. Sometime they may be subterranean giving rise to some leaves at soil level. The flowering shoots bearing inflorescence spring up during the reproductive season. Such a plant is called acaulescent as it is superficially stemless.

Example: Wheat, Buttercup, Onion

Based on the duration of life herbs may be

Ephemerals: These are the plants which live only for few weeks. In other words, the growing season for these plants is very short. These kinds of plants are found in desert or very cold regions.

Example: Astragalus, Artemesia

Annuals: They complete their life cycle in one year and after which they flower and die. Annuals can be represented by seasonal flowers, crops, vegetable and fruits.

Example: Rice, Wheat, Brinjal

Bi-annuals or biennials: They complete their life cycle in two years. It is interesting to know that some of the annuals of India (cabbage, Radish etc.) behave as biennials in cold western countries due to the temperature which does not permit adequate growth in just one year.

Example: Radish, Turnip

Perennials: These plants grow for more than two years. In these plants, the aerial shoot dies every year but the underground parts remain alive.

Example: Musa, Ficus

Shrubs- Shrubs are larger than herbs. They are woody, branched and longer than herbs. They give out branches and rise above the level of soil to give a bushy appearance. Through the branches are woody, they are not as strong as trees. Generally shrubs are flowering plants.

Example: China-rose, Nerium

Trees- These are the biggest plants. The stem of trees is hard and woody. Almost all trees are perennial. Trees can be again classified into following types,

Based on the types of branches trees may be

Caudex: These are unbranched columnar stems with crown of leaves at the apex.

Example: Cocos, Borassus, Phoenix, Cycas

Ex-current: In this type the main stem grows indefinitely and branches arise acropetally. And finally this plant appears conical in shape.

Example: Polyalthia, Casuarina, Pinus

Deliquescent: These are short and strong branches arising on a short trunk giving umbrella or dome like appearance.    

Example: Ficus, Mangifera

Culms: These are unbranched erect long thin stems with articulated nodes also called Culms.

Example: Sorghum, Zea, Bambusa

Weak stems

  • These stems cannot stand erect without any help.
  • They grow either on the ground or creep on supports like walls, fences or ropes.
  • Usually these stems contain no timber in them and that is why they are so weak and feeble.
  • The number of plants with weak stems is comparably less.

Weak stems can be classified as Prostrate plants and climbers

Prostrate plants- They are also known as trailors. They trail along the surface and are rooted at one place. Prostrate plants are of three types. Let us study about them,

Procumbent plants: In these plants, Stem and branches lie flat on the ground throughout their length.

Example: Tribulus, Evolvulus, Basella, Alternanthera

Decumbent plants: In these plants, Basal part of the stem is prostrate while upper part becomes erect.

Example: Tridax, portulaca

Diffuse plants: Here, the plant is much branched and these branches grow profusely in all directions,

Example: Boerhavia

Climbers- These are weak stemmed plants that climb over the support. Depending upon the organ used for climbing they may be grouped as twiners, tendril climbers and stragglers.

Twiners: Stems of these plants are delicate and coil around the support, they are

Dextrose – The stems of these plants coil round the support in clock wise direction

Example: Dioscorea

Sinistrose – The stems of these plants coil round the support  in anti-clock wise direction

Example: Dolichos, Clitoria, Abrus

Tendril climbers: These plants climb up with the help of tendrils. The organs modified as tendrils and the corresponding examples are given below,

Organ Example
Roots Pothos, Piper betel, Vanilla
Terminal buds Vitis, Cissus
Axillary buds Passiflora
Peduncle Antipogon & Cardiospermum
Woody perennial twiners Lianes – Bominnis, Tinospora, Gnetum
Adhesive discs Ampelopsis
Woody twiners Calamus
Entire leaf except stipules Lathyrus
Leaflets Pisum
Petioles Clematis, Nepenthes
Stipules Smilax
Leaf tip Gloriosa

**Roots developing from the radicle are called as normal roots and the roots developing from any other part of the plant except radicle are called adventitious roots.

Stragglers: These plants are woody climbers. They are also called ramblers or scramblers. They climb up with the help of hooks and thorns.

Hooks are curved and woody structures. Terminal bud modified into hooks in Artabotrus. Axillary buds modified into hooks in Hugonia. Terminal leaflets modified into hooks in Bignonia.

Thorns are woody structures. Axillary bud modified into a thorn in Bougainvillea and Canthium.


Functions of Stems

Generally any normal shoot will show the following functions:

  1. Stem provides structural support to the plant
  2. Exposure: Stem helps in exposing the leaves and flowers to sunlight for efficient photosynthesis.
  3. Conduction: Stem is the two-way channel for the transport of water, minerals and food materials. The water and the minerals which are absorbed by the roots are conducted up to the leaves with the help of shoots. Conduction is physiological function.
  4. The stems also conduct the synthesized food material from the leaves to other storage organs of the plant.
  5. Stem bears the entire weight of the plant
  6. Stem bears the entire weight of the plant
  7. Stem bears the entire weight of the plant

Modifications of stems

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