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The stem is geotropically negative. It is a sturdy plant that tends to grow out of the ground. The stems of many plants are not as sturdy as they may be. They can either be tangled around a post or let to trail.
The embryo’s plumbule extends outward to produce the stem, which is an ascending portion of the plant. In addition to flowers, it produces leaves and twigs.
Stems are divided into various sections by nodes. The branches and leaves of a node are the result of the node’s growth. The Internode is the space between the two nodes.
Plants are classed according to their size, sturdiness, and lifespan. It is possible that herbs are tiny plants with a flexible stem. There are many types of shrub, which are medium-sized plants with woody stems that branch from the base, creating a bushy look.
In the developing apex, there are many little leaves in the process of growing. In addition, the axils and laterals of leaves can generate lateral or axillary buds. A branch or flower can grow out of these buds.
A tree’s solid, long trunk and numerous branches make it a tree. In botany, annuals are those plants whose entire life cycle is finished within one growing season. Rice, groundnuts, and other agricultural products are examples of what we refer to as food crops. All of the stages of a Biennial’s development can be completed in just two seasons! (cabbage or radish).
Perennials are plants that live for several years without losing their flowers or fruits, therefore the name. The stems, in addition to bearing leaves, branches, and flowers, also serve as a means of vegetative propagation, perennation, maintenance, and the storage of food reserves.
Working of Stem
As they receive light and continue to exchange gases, leaves are carried on stems in a way that allows them to perform their essential role.
Pollination of other plants is facilitated by fruit and flowers produced by the stem’s flora.
Flowers, fruits, and leaves are all held in place by this structure.
To get water and minerals from the roots to the fruits, the stem is the passageway.
Supplementary roles
Photosynthesis
As climbers
Perennation
Propagation by the use of plants.
Storage of food
There are three types of stems: underground, aerial, and sub-aerial.
1. Stems in the sub-aerial
Occasionally, lower stem buds can be seen sprouting from the base of the plant and forming new branches. According to their origin, nature and mechanism of reproduction, these are classified.
(i) Sucker
It is similar to the stolon, has a lateral root except but that it grows in an upward direction and gives rise to a new plant, such as Mentha,
(ii) Runner
Prostrate growth can occur at any height above the soil’s surface. Scale leaves grow on nodes. Long internodes and a spreading stem characterise this plant. The adventitious stems sprout from the lower side of nodes. The axils at the base of the aerial stem are what give way to the runner. Runners are formed by slender, horizontal branches. Branches form independent plants when the older components of a plant die off. Doob grass
(iii) Offset
Offset is typically shorter and thicker in comparison. Pistia and water hyacinth are two examples of aquatic plants that contain this compound. This plant begins by growing as a bunch of leaves near the water or the ground, and then produces adventitious roots that grow in the water or the ground from all other nodes, such as Pistia.
(iv) Stolon
From the lower half of the main axis, a thin lateral branch grows. This lateral branch climbs higher and finally arches outwards. Terminal buds then fall to the ground, causing new shoots to grow out of the ground. Soil stabilisation is facilitated by its roots, which are also present. E.g – Jasmine.
2. Underground stem
The stems of some plants remain in the ground for storage and perennation purposes. Every year, they produce aerial shoots. The presence of scale leaves and buds at nodes allows you to tell them apart from roots. This stem can also be used to propagate plants.
This is a list of underground stalks that have undergone modifications:
(i) Corm
A subterranean stem, known as a corm, is thick and short, and it holds a lot of stored food. Scales, which are thin, sheathing foliage bases composed of dead leaves, cover the plant’s vertical growth. At the nodes of the corm, there are buds. Adventitious roots are produced by releasing these buds. Food storage; vegetative multiplication; and perennation are all functions of the corm. The bottom of the corm can be either flattened or somewhat rounded depending on where it is placed in relation to the stem. Colocasia.
(ii) Bulb
An underground stem with scale-like leaves known as a bulb. When the stem is severely decreased, it takes on a disc-like form. Conical or convex, the discoid stem’s internodes are deeply squeezed. Scales of flesh cover these nodes. Many leaves surround the terminal bud on the top disc. The axillary buds are located between the leaf axis. The adventitious root grows on the disc’s bottom surface. Onions, are a good example.
(iii) Tuber
The growth of the underground stems is caused by the build-up of food at the root. A potato is a good illustration of this. Potato eyes are nodes that contain one to three buds. New plants grow from these buds developing in the axils of the small scale-like leaves of the tuber.
(iv) Rhizome
Nodes or internodes are distinguishable on the rhizome, which is a thicker, prostrate underground stem. At the nodes, it may develop scaly or axillary leaves. Terminal buds and adventitious roots may be present, as well as unbranched or branching rhizomes. Ginger is a great example of this.
3. Aerial stems
Aerial stalks that have been changed can serve a variety of roles. These stems come in a wide range of varieties.
(i) Cladode
Cladode refers to a phylloclade having only one or two interodes, as in asparagus. ‘Cladodes’ are green, needle-like structures found in Asparagus. Clusters of these can be found in the axils or scalys of the body. In the nodes of the main stem, you will see spines on the leaves. Scale leaves can be found just above the spine. Every branch of the main stem has scale leaves. Scale leaves have axillary clusters of cladodes.
(ii) Stem thorn
Hard, sharp and straight characterises the stem thorn. Using it to defend the body or as a means of ascent is possible.E.g Duranta
(iii) Stem tendril
This leafless spirally coiled branch aids climbers in gaining access to higher places. Passiflora.
(iv) Bulbia
A flower bud or a vegetative bud fusion is what is known as a bulbil. Food storage is to blame for the inflated appearance. It could be compared to a vegetative propagation organ like Dioscorea, for example.
(v) Phylloclade
Cylindrical, flattened or green stem with the function and form of a leaf known as Pylloclade. It includes chlorophyll, which is essential for photosynthesis to take place. Nodes and internodes can be organised in a variety of ways, from short to lengthy intervals. When the leaves of a xerophytic plant grow slowly or prematurely, or if the leaves are transformed into spines, phylloclades can be observed among the leaves. This includes Opuntia.
Anatomy for Stem
For Stem, a thin section of transverse section displays its internal structure when seen under the microscope:
The interior structure of a Dicot Stem
1) Cortex
It is located below the epidermis, which is the first layer of skin. It is possible to divide it into three sections.
(i) Hypodermis
Collenchymatous cells are found in a 4 to 5 cell thick layer. Chloroplasts, which are living cells, are found in these cells.
(ii) Endodermis
In the cortex, the innermost layer is the endodermis. Compact barrel-shaped cells line a single row without intercellular gaps. The endodermis is also known as a starch-sheath because it contains starch grains. Endodermal cells show Casparian strips.
(iii) General cortex
It is located below the hypodermis and is known as the general cortex. There are only a few thin-walled, parenchymatous cells with intercellular gaps that make up the structure. Chloroplasts are found in some cells. Chlorenchyma is the name given to the cells that make up chlorenchyma.
2. Epidermis
It is the stem’s outermost single-celled layer. The outer layer is made up of cells that have been cutinized. Multicellular hairs and stomata can be found in its structure. It is naturally protective.
3. Vascular bundles
Vascular bundles are also referred to as capillaries.
They are many and arranged in a ring-like structure, with the pericycle covering the entire thing. An open, conjoined and collateral vascular bundle is found. There are three vascular bundles in the human body: Xylem, Cambium, and Phloem.
4. Pericycle
It is located below the endodermis. It is made up of semilunar patches of sclerenchyma. Unlike other cell walls, the sclerenchyma cell walls are stiff and inflexible. Pericycle assist the plant’s mechanical support while also protecting its vascular bundles from damage.
(i) Phloem
Composed mostly of companion cells, sieve tubes, and phloem parenchyma, is the Phloem which is positioned below the pericycle. Protein, carbohydrates, and lipids are stored in the phloem tissues.
(ii) Cambium
There are two types of cells in the cambium: xylem and phloem. Only one layer of Meristematic cells can be found in the cambial cells.
(iii) Xylem
In the vascular bundles, the xylem is the deepest layer and is found toward the centre. Tracheids (wood-fibre) and parenchyma (wood-parenchyma) make up the majority of the structure. Vessels closer to the centre are known as protoxylems, and vessels further away as metaxylems.
5. The medulla or pith
The stem’s middle section is the medulla.With apparent intercellular gaps, it is made up of parenchymatous cells. Storage and transverse conduction of food items are carried out by this organ.
The Internal Structure of the Monocot Stem
1. Ground tissue
Parenchyma cells, which extend from the hypodermis to the centre, make up the structure. Unlike the cortex, pericycle, or endodermis, it does not have definite tissues. Intercellular gaps are seen in the ground tissue cells. Food reserves and vascular bundles are spread throughout the ground tissue.
2. Epidermis
Cells arranged in a square shape make up the outermost layer of the stem. The stomata impede the flow. Cuticle covers the entire surface of the skin, devoid of any hairs from the epidermis.
3. Vascular bundles
Vascular bundles are also referred to as capillaries.
A large number of vascular clusters have remained in the ground tissue. They are positioned closer to the periphery. The peripheral vascular packages are smaller in size than the central vascular packages. Each vascular bundle is surrounded by a bundle-sheath, which is a thick-walled layer of sclerenchyma cellular cells . It is responsible for protecting and supporting the vascular bundles. Vascular bundles are categorized as closed, conjoint, collateral, or endarch. Each vascular bundle is made up of either xylem or phloem fibres.
4. Hypodermis
It is found beneath the skin’s surface. Sclerenchymatous cells form two or more layers in this structure. It performs mechanically by supporting and strengthening the stem.
Conclusion
The study of the anatomy of different parts of flowering plants like the root, flower, stem, seed, leaf inflorescence is crucial to understanding the functioning of a plant. This article talks about it to boost your NEET UG biology preparations.
