Saturday, July 20, 2024
General Agriculture

Vegetative Structure of Seed Plants: The Stem

In this article, we shall be looking at the morphological and anatomical forms of seed plants, especially the flowering plants. Plant Morphology or Phytomorphology is the general term for the study of the physical form and external structure of plants (Raven et al, 2005), while Plant Anatomy or Phytotomy is the study of the internal structure of plants.

In this article, we shall be looking at a seed plants vegetative structure, the STEM. The vegetative or somatic structure of vascular plants includes two major organ systems namely:

The Shoot System composed of stems and leaves and The Root System.

1. Stem

A stem is one of two main structural axes of a vascular plant. The stem is normally divided into nodes and internodes, the nodes hold buds which grow into one or more leaves, inflorescences (flowers), cones or other stems etc.

The internodes act as spaces that distance one node from another. The term shoots is often confused with stems; shoots generally refer to new fresh plant growth and does include stems but also to other structures like leaves or flowers.

The other main structural axis of plants is the root. In most plants stems are located above the soil surface but some plants have underground stems.

Vegetative Structure of Seed Plants: The Stem
Fig 5.1 Stem showing internode and nodes plus leaf petiole and new stem rising from node.

According to Raven et al, 1981, stems have four main functions which are;

  • Support for and the elevation of leaves, flowers and fruits. The stems keep the leaves in the light and provide a place for the plant to keep its flowers and fruits.
  • Transport of fluids between the roots and the shoots in the xylem and phloem.
  • Storage of nutrients.
  • The production of new living tissue. The normal life span of plant cells is one to three years. Stems have cells called meristems that annually generate new living cells.
Vegetative Structure of Seed Plants: The Stem
Fig 5.2 Stem showing internode and nodes plus leaf petioles

Specialized Terms for Stems

Stems are often specialized for storage, asexual reproduction, protection or photosynthesis. Other specialized functions include:

Acaulescent – plants with very short stems that appear to have no stems. The leaves appear to rise out of the ground, e.g. some Viola.

Arborescent – tree like with woody stems normally with a single trunk.

Bud – an embryonic shoot with immature stem tip.

Bulb – a short vertical underground stem with fleshy storage leaves attached, e.g. Onion, Daffodil, Tulip. Bulbs often function in reproduction by splitting to form new bulbs or producing small new bulbs termed bulblets. Bulbs are a combination of stem and leaves so may better be considered as leaves because the leaves make up the greater part.

Caespitose – when stems grow in a tangled mass or clump or in low growing mats.

Cladophyll – a flattened stem that appears leaf like and is specialized for photosynthesis, e.g. Asparagus, Cactus pads.

Climbing – stems that cling or wrap around other plants or structures.

Corm – a short enlarged underground, storage stem, e.g. Taro, Crocus, Gladiolus.

Decumbent – stems that lie flat on the ground and turn upwards at the ends.

Fruticose – stems that grow shrub like with woody like habit.

Herbaceous – non woody, they die at the end of the growing season.

Pseudostem – A false stem made of the rolled bases of leaves, which may be 2 or 3 m tall as in Banana.

Rhizome – a horizontal underground stem that functions mainly in reproduction but also in storage, e.g. most Ferns, Iris.

Runner (plant part) – a type of stolon, horizontally growing on top of the ground and rooting at the nodes. E.g. Strawberry, Spider plant.

Scape – a stem that holds flowers that comes out of the ground and has no normal leaves. Hosta, Lily, Iris.

Stolons – a horizontal stem that produces rooted plantlets at its nodes and ends, forming near the surface of the ground.

Tree – a woody stem that is longer than 5 meters with a main trunk.

Thorns – a reduced stem with a sharp point and rounded shape. e.g. honey locust, Hawthorn.

Tuber – a swollen, underground storage stem adapted for storage and reproduction, e.g. Potato.

Woody – hard textured stems with secondary xylem.

Read Also : Gray Leaf Spot (Stemphylium spp) – Symptoms and Damage Control

Stem Structure

Stem usually consist of three tissues, dermal tissue, ground tissue and vascular tissue. The dermal tissue covers the outer surface of the stem and usually functions to waterproof, protect and control gas exchange.

The ground tissue usually consists mainly of parenchyma cells and fills in around the vascular tissue. It sometimes functions in photosynthesis. Vascular tissue provides long distance transport and structural support.

Most or all ground tissue may be lost in woody stems. The dermal tissue of aquatic plants stems may lack the waterproofing found in aerial stems. The arrangement of the vascular tissues varies widely among plant species.

Vegetative Structure of Seed Plants: The Stem
Fig 5.3 Photomicrograph of the cross-section of Flax stem, showing locations of underlying tissues. Ep = Epidermis; C = Cortex; BF = Bast fibres; P = Phloem; X = Xylem; Pi = Pith.

Dicot stems

Dicot stems with primary growth have pith in the center, with vascular bundles forming a distinct ring visible when the stem is viewed in cross section. The outside of the stem is covered with an epidermis, which is covered by a waterproof cuticle.

The epidermis also may contain stomata for gas exchange and hairs. A cortex of parenchyma cells lies between the epidermis and vascular bundles.

Woody dicots and many nonwoody dicots have secondary growth originating from their lateral or secondary meristems: the vascular cambium and the cork cambium or phellogen.

The vascular cambium forms between the xylem and phloem in the vascular bundles and connects to form a continuous cylinder. The vascular cambium cells divide to produce secondary xylem to the inside and secondary phloem to the outside.

As the stem increases in diameter due to production of secondary xylem and secondary phloem, the cortex and epidermis are eventually destroyed. Before the cortex is destroyed, a cork cambium develops there.

The cork cambium divides to produce waterproof cork cells externally and sometimes phelloderm cells internally. Those three tissues form the periderm, which replaces the epidermis in function. Areas of loosely-packed cells in the periderm that function in gas exchange are called lenticels.

Secondary xylem is commercially important as wood. The seasonal variation in growth from the vascular cambium is what creates yearly tree rings in temperate climates. Tree rings are the basis of dendrochronology, which dates wooden objects and associated artifacts.

Dendroclimatology is the use of tree rings as a record of past climates. The aerial stem of an adult tree is called a trunk. The dead, usually darker inner wood of a large diameter trunk is termed the heartwood. The outer, living wood is termed the sapwood.

Vegetative Structure of Seed Plants: The Stem
Fig 5.4 The trunk of this redwood tree is its stem.

Vascular bundles are present throughout the monocot stem, although concentrated towards the outside. This differs from the dicot stem that has a ring of vascular bundles and often none in the center.

The shoot apex in monocot stems is more elongated. Leaf sheathes grow up around it, protecting it. This is true to some extent of almost all monocots. Monocots rarely produce secondary growth and are therefore seldom woody. However, many monocot stems increase in diameter via anamolous secondary growth.

Gymnosperm stems

All gymnosperms are woody plants. Their stems are similar in structure to woody dicots except that most gymnosperms produce only tracheids in their xylem, not the vessels found in dicots.

Gymnosperm wood also often contains resin ducts. Woody dicots are called hardwoods, e.g. Oak, Iroko Mahogany, Teak and Walnut. In contrast, softwoods are gymnosperms, such as pine, spruce and fir.

Economic Importance of Stems

Vegetative Structure of Seed Plants: The Stem
Fig 5.5 White and green asparagus – crispy stems are the edible parts of this vegetable.

There are thousands of species whose stems have economic uses. Stems provide a few major staple crops such as potato and taro. Sugarcane stems are a major source of sugar. Maple sugar is obtained from trunks of maple trees.

Vegetables from stems are Asparagus, Bamboo shoots, cactus pads or Nopalitos, kohlrabi, and water chestnut. The spice, cinnamon is bark from a tree trunk. Cellulose from tree trunks is a food additive in bread, grated Parmesan cheese, and other processed foods. Gum arabic is an important food additive obtained from the trunks of Acaciasenegaltrees. Chicle, the main ingredient in chewing gum, is obtained from trunks of the chicle tree.

Medicines obtained from stems include quinine from the bark of Cinchona trees, Camphor distilled from wood of a tree in the same genus that provides cinnamon, and the muscle relaxant curare from the bark of tropical vines.

Wood is a used in thousands of ways, e.g. buildings, furniture, boats, airplanes, wagons, car parts, musical instruments, sports equipment, railroad ties, utility poles, fence posts, pilings, toothpicks, matches, plywood, coffins, shingles, barrel staves, toys, tool handles, picture frames, veneer, charcoal and firewood.

Wood pulp is widely used to make paper, cardboard, cellulose sponges, cellophane and some important plastics and textiles, such as cellulose acetate and rayon. Bamboo stems also have hundreds of uses, including paper, buildings, furniture, boats, musical instruments, fishing poles, water pipes, plant stakes, and scaffolding. Trunks of palm trees and tree ferns are often used for building. Reed stems are also important building materials in some areas.

Tannins used for tanning leather are obtained from the wood of certain trees, such as quebracho. Cork is obtained from the bark of the cork oak. Rubber is obtained from the trunks of Heveabrasiliensis.

Rattan, used for furniture and baskets, is made from the stems of tropical vining palms. Bast fibers for textiles and rope are obtained from stems include flax, hemp, jute and ramie. The earliest paper was obtained from the stems of papyrus by the ancient Egyptians.

Amber is fossilized sap from tree trunks; it is used for jewelry and may contain ancient animals. Resins from conifer wood are used to produce turpentine and rosin. Tree bark is often used as a mulch and in growing media for container plants (Raven et al, 1981).

In conclusion, stems are structural axes of a vascular plant with specialized functions, and of highly economic importance. A stem is a structural axes of a vascular plant. A stem is normally divided into nodes and internodes Stems have four major functions: support, transportation, storage and production of living tissues. Stems have specialized functions.

Stems consist of three tissues: dermal, ground and vascular tissues. Stems normally undergo secondary growth. Stems are of highly economic importance.

Read Also : Ways To Generate Money From Bulky Wastes


Benadine Nonye is an agricultural consultant and a writer with several years of professional experience in the agriculture industry. - National Diploma in Agricultural Technology - Bachelor's Degree in Agricultural Science - Master's Degree in Science Education - PhD Student in Agricultural Economics and Environmental Policy... Visit My Websites On: 1. - Your Comprehensive Practical Agricultural Knowledge and Farmer’s Guide Website! 2. - For Effective Environmental Management through Proper Waste Management and Recycling Practices! Join Me On: Twitter: @benadinenonye - Instagram: benadinenonye - LinkedIn: benadinenonye - YouTube: Agric4Profits TV and WealthInWastes TV - Pinterest: BenadineNonye4u - Facebook: BenadineNonye

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