The stem is the plant axis that bears shoots, leaves, buds, and the basal end-roots. It conducts water, minerals, and food to other parts of the plant body through various systems. In some plants, the stem is modified to perform other functions such as storage of food, support, vegetative propagation, and protection. These are simply known as modified stems and can be categorized into various classes. We will learn about each class with modified stem examples in this topic.
Based on the type of modifications, stem modifications can be classified into:
Plants that produce underground modifications of stem utilize it for perennation and storage of food. These plants produce aerial shoots annually. They look like roots but can be distinguished with the following characteristics. The salient features are:
Nodes and internodes are observed.
There is the presence of scale leaves, buds, and adventitious roots at the nodes.
The internal structure resembles an aerial stem and not of a root.
Below are some types of underground modifications of stems:
Rhizome: It's a non-green, fleshy underground stem with nodes and internodes. The nodes have dry scale leaves with axillary buds. Terminal buds can also be seen. The adventitious roots arise from the lower sides. The rhizome stem that grows obliquely is called a root-stock rhizome and when they grow horizontally they are known as straggling rhizomes. Examples of rootstock rhizome: Banana, Alocasia
Examples of straggling rhizome: Ginger, turmeric
Bulb Modification: This type of modified stem looks like a highly condensed discoid stem. The upper surface has a terminal bud and there are many fleshy scaled leaves present. At the base of the bulb, there are many adventitious roots present. These bulbs can be scaly or tunicated. A tunicated bulb will be covered by a sheath of dry membranous scale leaves which is called a tunic. In the case of scaly bulbs, there will be no tunic present. Examples of tunicate bulbs: Onions and garlic.
Examples of scaly bulbs: Lily.
Corm: This is a condensed rhizome that grows in a vertical direction. It has a flattening base and is more or less spherical in shape. It has distinct circular nodes and internodes. The nodes have scale leaves and axillary buds and the adventitious roots are seen at the base or all over the body area. Examples: Colocasia, crocus
Tuber: It is a swollen tip of an underground lateral stem and is covered by a corry. The skin has several depressions which are known as eyes. Each eye is a node and bears one or more buds subtended by a leaf scar. A big scar at one end also known as the heel end of a potato marks its attachment to a stolon. Adventitious roots are generally absent in tubers. Example: Potatoes
These types of modified stems are partially aerial and partially underground. Their general characteristics are:
They have short aerial branches.
Adventitious roots develop to nodes.
The node of the stem can grow into a fully developed plant.
Their sub aerial stem modifications are meant for vegetative propagation
The plants are known as creepers.
There are Four Types of Sub-Aerial Stems
Runner: Runners are a type of creeping stem with long internodes. They run horizontally on the soil surface and bear scale leaves, adventitious roots, and scale leaves. This type of steam rises from an axillary bud. A mother plant often produces several runners in all directions. Then they break off and grow into new plants. Examples: Wood sorrel, lawn grass, etc.
Sucker: Sucker stems rise from the basal underground part of the main stem. They grow horizontally for a distance below the soil and then grow upwards obliquely. The stem develops a leafy shoot and adventitious roots before separating from the mother plant. Examples: Chrysanthemum, mentha (Pudina)
Stolon: It's a weak lateral stem that rises from the main stem base. After growing aerially for a while it bends downwards to touch the ground. Here, its terminal bud produces a new shoot and adventitious roots. Example: Jasmine, Colocasia.
Offset: This is a short runner with one lone internode. It develops from a leaf axil and then grows as a short horizontal branch and produces a rosette of leaves above and adventitious roots below. This type of stem modification is generally observed in aquatic plants. Example: Pistia, eichhornia, etc.
Type III: Aerial Stem Modifications
Aerial stem modifications have special functions such as protecting the plant, food storage, vegetative propagation, climbing, etc.
There are 6 types of Aerial Stem Modifications:
Stem Tendrils: The stem of the plant or the branches are modified to green thread-like leafless structures known as tendrils and are used for climbing. These tendrils can be both branched and unbranched and a scale leaf can be seen at the point of branching.
Tendrils can be subdivided into 4 types:
Axillary tendrils: Example: Passiflora
Extra axillary tendrils: Example: Cucurbita
Apical bud tendrils: Also known as leaf-opposed tendrils and a common example is a grapevine
Floral bud tendrils: Also known as inflorescence tendrils. Example: Antigonon.
Thorns: These are pointed, hard, or woody structures and sometimes bear leaves and flowers. They can also be branched. In plants like Duranta and Citrus, thorns are modified axillary buds. Thorns are used for defense or climbing. They also regulate respiration.
Phylloclade: These are fleshy in nature and are flattened or cylindrical branches. The leaves transform to spine or scales and they also regulate transpiration. Phylloclades take part in photosynthesis and store water. Example: Coccoloba, euphorbia.
Cladophylls: Also known as cladodes, these flattened or cylindrical branches help in photosynthesis. In Ruscus, there are two long internodes observed whereas in asparagus (another cladode) one long internode is observed.
Bulbil: These are modified floral or vegetative buds with stored food for the plant body. They are meant for vegetative propagation. Bulbils detach to become new plants. Example: Dioscorea, where bulbils are condensed axillary buds, in lily, the floral buds transform to bulbils.
Thalamus: Thalamus is a condensed stem axis and bears floral organs such as the corolla, calyx, the androecium, and the gynoecium.
1. Why are stems modified?
Stems are modified to take part in various functions in the plant body. For example, aerial stem modification thorns regulate transpiration and keep the loss of water from the plant body in check. Their primary role is defending the plant from outside harm.
Another example can be the runner (subaerial stem modification) which plays a part in vegetative propagation.
Underground modified stems such as beetroots and potatoes act as food storage units for the plant body.
2. What are pseudobulbs?
In several aerial orchids one internode of the stem becomes tuberous and fleshy. This newly formed structure stores a good amount of moisture which enables the plant to survive in adverse unfavorable conditions such as drought. These structures are called pseudobulbs.
3. What are brachyblasts?
Generally all foliage leaves are borne by the nodes of an elongated stem or branch. However, in some plants, the leaves as concentrated on secondary stems that are located at axillary positions. These secondary stems are short and have close internodes so the leaves appear in clusters. They are called as brachyblasts or spurs or dwarf shoots. They are commonly seen in gymnosperms.