The Stem : Structure of Stem, Functions of Stem and Shoot Modifications

In most plants, stems are present above the soil surface but did you know that some plant have underground stems too? Surprised? Well, the ginger, the potato, the onion, the garlic that you have so often in your meals are actually modified underground stems. 

You must have seen cacti. Did you know that the leaf-like green and flat part of a cactus is actually a stem? By now you must be wondering how stems are of so many different kinds?

Come let us help you understand more about the stem.

Table of Contents:

• Structure of stem
• Functions of stem (shoot system)
• Shoot modifications
• Practice Problems
• FAQs

Structure of Stem

Stem refers to the ascending part of the plant axis that bears branches, leaves, flowers, and fruits. Together they form the shoot system. The shoot develops from the Plumule of the seed. 

Young stems are green and slender while older ones are thick, woody, and dark brown. Stem comprises nodes, points from which new leaves and branches arise, internodes, regions between two nodes, buds, which are underdeveloped young shoots with a condensed axis and young leaves that are closely placed and fold over the apex. These buds can be present terminally at the apex of the stem or at the axil of leaves. Buds can grow into branches, new shoots or flowers.

Functions of Stem (shoot system)

It supports the leaves and branches and holds them to expose them to maximum light. The stem and its branches have flowers and fruits. It helps in the conduction of water, minerals, and products of photosynthesis. Stems can be modified to store food. Example: Potato. Few plants can propagate vegetatively using stems. Examples: Rose, Hibiscus. In plants like Opuntia, the stem is modified to perform photosynthesis. Stems of climbers such as cucurbits, Passiflora, etc are modified to provide support while climbing. Thorns of lemon, Bougainvillaea, etc are modified stems which provide protection.

Learn more about parts of plant.

Shoot Modifications

Shoots modifications can be of three types:

A. Underground
B. Sub-aerial
C. Aerial

Underground Modifications

These help to store reserve food, propagate vegetatively and also help in perennation.

Rhizome

Rhizome is a stout, fleshy and horizontal underground stem which stores food. Nodes possess axillary buds which are covered by scaly leaves. Internodes are prominent. Adventitious roots develop at the nodes. Examples: Banana, ginger, turmeric, etc.

Tuber

Tubers are swollen ends of special underground branches of stem. These are stouter than rhizomes, with slender internodes and no adventitious roots. They bear axillary buds covered with scaly leaves in depressions known as eyes. Examples: Potato, Dahlia.

Corm

Corm is a short, stout, and usually unbranched stem that grows vertically. It bears many buds in axils of scale leaves. Buds grow into aerial shoots and new corms are formed when food gets stored at the base. Examples: Amorphophallus (zaminkand), Colocasia (arbi).

Bulb

Bulb is a reduced disc-like stem with adventitious roots present at its base. The stem is covered with overlapping leaves or leaf bases known as scales which enclose an apical bud. The scales may be fleshy and tunicated or scaly.Examples: Onion, garlic, tulip.

Sub-aerial modifications

Sub aerial stem modifications are seen in herbaceous plants with thin and weak stems. These mainly help in vegetative propagation.

Runner

Runners arise from the basal internodes of the aerial branches known as crown and run horizontally over the ground. A runner has long and thin internodes and branches creep over the surface of soil. Adventitious roots develop at the nodes and aerial branches arise from the axils of scale leaves present at the nodes. On getting separated from the parent plant, the daughter plants propagate in a similar manner. Example: Strawberry, grass.

Stolon

Stolon is a weak lateral branch that arises from the base of the main stem. After they grow aerially for sometime they bend downwards to touch the ground, where their terminal bud gives rise to a new shoot and adventitious roots. Stolons bear nodes and internodes. They are similar to runners except for the fact that they are produced just beneath the surface of the ground. Example: Mint.

Offset

Offsets are short, more or less thickened, horizontal branches with short internodes. These arise from the axil of the lower leaves of the main shoot. At the nodes, the offset produces a tuft of leaves, arranged in a rosette fashion, above and a cluster of roots below. Upon detaching from the parent plant, each branch can grow into a new plant. Examples: Pistia, Eichhornia.

Sucker

The sucker originates as a lateral branch from the underground axillary bud of an aerial shoot. It grows underground obliquely for some distance before emerging upwards. Suckers bear axillary buds at the nodes, which can give rise to new plants. Examples: Chrysanthemum.

Aerial modifications

In some plants, the stem above the ground undergoes modifications to perform specialised functions such as providing support, protection and performing photosynthesis.

Tendrils

These are green thread-like and spirally coiled structures which are sensitive to touch. These help the weak stemmed plants to climb up against a solid support. Tendrils can be modifications of axillary buds or floral buds. Examples: Gourds (cucumber, pumpkins, watermelon) and grapevines

Thorns

They are hard, pointed structures that develop on the axils of leaves. Thorns are modified axillary buds. They provide protection against grazing animals. Examples: Citrus, Bougainvillaea.

Phylloclade 

Phylloclades are flattened or cylindrical green stems of unlimited growth. These are commonly found in xerophytic plants in which the leaves are modified to spines. Phylloclade can perform photosynthesis and can also store water and mucilage. Examples: Thick, fleshy, and succulent in Opuntia, cylindrical in Euphorbia, Casuarina

Cladodes

Cladodes are flattened green stems that are one or two internodes long. It develops from the axil of a scaly leaf. It performs photosynthesis as the true leaves are reduced to spines or scales. Example - Ruscus.

Practice problems of the Stem

Question1. Which of the following are examples of runners?

A. Grass,and banyan
B. Mint and sugarcane
C. Banana, pineapple and Chrysanthemum
D. Grass and strawberry

Answer: Grass and strawberry have sub-aerial stem modifications called runners. Runners arise from the basal internodes of the aerial branches and run horizontally over the ground. A runner has long and thin internodes and adventitious roots develop at the nodes. Aerial branches arise from the axils of scale leaves present at the nodes. On getting separated from the parent plant, the daughter plants propagate in a similar manner.

Thus, the correct option is d.

Question2. Potato is stem because it.?

A. possesses axillary buds (eyes) 
B. contains reserve food
C. does not bear roots 
D. lacks chlorophyll 

Answer: Potatoes have modified underground stems for food storage. 

These tubers have pits called eyes which bear axillary buds which can grow into a new plant. 

Presence of buds, nodes and internodes are characteristic features of this modified stem. 

Thus, the correct option is a.

Question3. Ginger multiplies vegetatively by which modified edible structure? 

. Tuber
B. Rhizome
C.  Bud
D. Stem

Answer: Rhizomes are modified thick and fleshy horizontal underground stems which have a large amount of stored food, as seen in ginger. They have distinct nodes and internodes and have thin brown scaly leaves at the nodes to protect the axillary buds. The buds help in vegetative propagation of ginger. The rhizome of ginger is edible. Thus the correct option is b.

Question4. In some plants, a thin lateral branch arises from the central axis and after it grows aerially, it arches down to touch the ground. In which of the following plants can we find such modified stems?

A. Pistia and Eichhornia
B. Mint and jasmine
C. Banana, pineapple and Chrysanthemum
D. Opuntia and Euphorbia

Answer: Mint and jasmine show sub-aerial stem modifications which are called stolons, where the lateral branch develops from the base of the stem. In stolon, the lateral branch is elongated, grows like an arch for some distance, and when it touches the ground the terminal bud gives rise to shoots and adventitious roots.

Thus, the correct option is b.

FAQs of the Stem

Question1. Are both potato and sweet potato roots of a plant? 

Answer: The edible part of the potato is a modified underground stem (Tuber) and that of sweet potato is a modified adventitious root.

Question2. How is a thorn different from a spine?

Answer: Thorns are sharp and pointed modified stems whereas spines are leaf modifications that are sharp and pointed and help in reducing surface area of the leaf to reduce water loss.

Question3. What is the edible part of an onion?

Answer:  Onion is a modified stem known as a bulb in which the stem appears as a reduced disc at the base and is covered by overlapping fleshy scale leaves which are edible.

Question4. How does the stem of the cactus help in water conservation?

Answer: Cacti have modified stems known as phylloclades which are flattened green structures that can grow up to several internodes. These stems are modified to perform photosynthesis and store water. This allows the leaves to be reduced to spines to reduce water loss by transpiration.