Lesson Objectives:

God willing, at the end of this lesson you will have learnt how to:

• understand the importance of learning about vegetative organs and their functions.

• identify the different parts of a plant.

• identify the functions of the various parts of the plant.

LESSON SIGNIFICANCE

Why learn about the different parts of a plant and their functions?

How could that help you in farming or even in life in general?

Plants are very important to life on earth providing many essential ecological services such as purifying water, maintaining ideal oxygen and carbon dioxide levels in the atmosphere as well as regulating climate and soil development, among other things. Plants provide foods, timber and a whole host of other products useful to man.

Understanding the different parts of plants and their functions and how plants have adapted to their environment is important to understanding how to better cultivate those plants that are useful. An understanding of roots, he different types, how they grow and their functions e.g. have allowed farmers to make the steps necessary to prepare the soil to enhance root growth. An understanding of flowers and their functions have allowed farmers to identify ways of promoting flowering and hence improving fruit production or delaying flowering and enhancing leaf production.

Understanding plants parts, their structure and their functions has also helped in understanding principles that can be further applied in other fields of study such as biology, engineering (producing stronger materials), architecture (producing more durable buildings), etc.

All in all, understanding the parts of plants and their functions is very important not only to crop productions and soil management but also to other fields.

Plant Vegetative ORgans

Together, the organs of a plant allow it to carry out the seven processes of life.

• Movement - all living things move, even plants

• Respiration - getting energy from food

• Sensitivity - detecting changes in the surroundings

• Growth - all living things grow

• Reproduction - making more living things of the same type

• Excretion - getting rid of waste

• Nutrition - taking in and using food

The phrase MRS GREN is one way to remember them:

Vascular plants have two distinct organ systems:

• a shoot system: generally grows above ground, where it absorbs the light needed for photosynthesis.

• a root system: supports the plants and absorbs water and minerals, is usually underground.

The Shoot System

The shoot system consists of two portions:

• the vegetative (non-reproductive) parts of the plant, such as the leaves and the stems, and

• the reproductive parts of the plant, which include flowers and fruits.

• Stem

Stems are usually above ground, although the stems of some plants, such as the potato, also grow underground.

Stems may be herbaceous (soft) or woody in nature. They may be classified as:

Shoot: A young stem (1 year old or less) with leaves.

Twig: A young stem (1 year old or less) that is in the dormant winter stage (has no leaves).

Branch: A stem that is more than 1 year old, typically with lateral (side) stems radiating from it.

Trunk: A woody plant's main stem.

Their main function is to provide support to the plant, holding leaves, flowers and buds; in some cases, stems also store food for the plant.

A stem may be unbranched, like that of a palm tree, or it may be highly branched, like that of a magnolia tree.

The stem of the plant connects the roots to the leaves, helping to transport absorbed water and minerals to different parts of the plant. It also helps to transport the products of photosynthesis, namely sugars, from the leaves to the rest of the plant.

There are two important tissues responsible for this:

Xylem tissue transports water and nutrients from the roots to different parts of the plant. Phloem tissue, which transports organic compounds from the site of photosynthesis to other parts of the plant.

Unlike xylem conducting cells, phloem conducting cells are alive at maturity.

The xylem and phloem always lie adjacent to each other. In stems, the xylem and the phloem form a structure called a vascular bundle;

Growth in plants occurs as the stems (and roots) lengthen. This is referred to as primary growth resulting from the mitosis of special cells called apical meristems.

Secondary growth is characterized by an increase in thickness or girth of the plant, and is caused by cell division in the lateral meristem. This type of growth is more common in woody plants and some dicotyledons, but rarely occurs in monocotyledons.

Stems can continue to grow throughout a plant’s life: a phenomenon called indeterminate growth.

The thickening of the stem that occurs in secondary growth is due to the formation of secondary phloem and secondary xylem by the vascular cambium, plus the action of cork cambium, which forms the tough outermost layer of the stem. The cells of the secondary xylem contain lignin, which provides hardiness and strength.

The bark protects the plant against physical damage and helps reduce water loss. In some plants, the stem has many openings, known as lenticels, which allow the interior cells to exchange gases with the outside atmosphere

Stem Modifications

Some plant species have modified stems that are especially suited to a particular habitat and environment (Figure

30.2.10

30.2.10). A rhizome is a modified stem that grows horizontally underground and has nodes and internodes. Vertical shoots may arise from the buds on the rhizome of some plants, such as ginger and ferns. Corms are similar to rhizomes, except they are more rounded and fleshy (such as in gladiolus). Corms contain stored food that enables some plants to survive the winter. Stolons are stems that run almost parallel to the ground, or just below the surface, and can give rise to new plants at the nodes. Runners are a type of stolon that runs above the ground and produces new clone plants at nodes at varying intervals: strawberries are an example. Tubers are modified stems that may store starch, as seen in the potato (Solanum sp.). Tubers arise as swollen ends of stolons, and contain many adventitious or unusual buds (familiar to us as the “eyes” on potatoes). A bulb, which functions as an underground storage unit, is a modification of a stem that has the appearance of enlarged fleshy leaves emerging from the stem or surrounding the base of the stem, as seen in the iris.

aerial modifications of stems are tendrils and thorns (Figure

30.2.11

30.2.11). Tendrils are slender, twining strands that enable a plant (like a vine or pumpkin) to seek support by climbing on other surfaces. Thorns are modified branches appearing as sharp outgrowths that protect the plant; common examples include roses, Osage orange and devil’s walking stick.

Root Systems

The roots of seed plants have three major functions: anchoring the plant to the soil, absorbing water and minerals and transporting them upwards, and storing the products of photosynthesis. Some roots are modified to absorb moisture and exchange gases.

Primary and secondary growth occurs as the roots lengthen and widen. Some plants, especially those that are woody, also increase in thickness during their life span.

Along with having different systems, there are different types of roots:

• primary root originated from the root of the seedling,

• secondary (lateral) roots originate from the primary roots, and

• adventitious roots originate on stems (sometimes also on leaves), the example are prop roots of screw pine (Pandanus).

In roots, the xylem and phloem are termed the vascular stele or vascular cylinder.

Root systems are mainly of two types (Figure

30.3.1

30.3.1). Dicots have a tap root system, while monocots have a fibrous root system. A tap root system has a main root that grows down vertically, and from which many smaller lateral roots arise.

A tap root system penetrates deep into the soil. In contrast, a fibrous root system is located closer to the soil surface, and forms a dense network of roots that also helps prevent soil erosion.

Some plants have a combination of tap roots and fibrous roots. Plants that grow in dry areas often have deep root systems, whereas plants growing in areas with abundant water are likely to have shallower root systems.

Root Structure

Figure 16.1.2.1 The root

The Root Tip

The root tip consists of a

• root cap — a sheath of cells that

  • protect the meristem from abrasion and damage as the root tip grows through the soil;

  • secrete the growth hormone auxin;

  • detect water and nutrients in the soil;

  • detect gravity and respond with gravitropism.

• meristem - a region of rapid mitosis, which produces the new cells for root growth.

Because of the frequency of mitosis in the meristem, root tips are often used to demonstrate mitosis in the laboratory The inset is a photo (courtesy of Carolina Biological Supply Co.) of anaphase in the meristem of an onion root tip.

The Region of Elongation

Here the cells produced by mitosis undergo a period of elongation in the direction of the axis of the root.

The Region of Differentiation

Here develop the differentiated tissues of the root.

• Epidermis - A single layer of flattened cells at the surface. When first formed, epidermal cells have extensions — the root hairs — which greatly increase the surface area available for the uptake of nutrients from the soil. The photo below shows the root hairs in the region of differentiation of a germinating radish seed.

Figure 16.1.2.2 Radish root

• Cortex - A band of parenchyma cells that develops beneath the epidermis. It stores food. Its inner surface is bounded by a single layer of cells, the

• Endodermis

• Stele

  • Pericycle - the outer boundary of the stele. Secondary roots branch from it.

  • Xylem - arranged in bundles in a spokelike fashion

  • Phloem - alternates with xylem

• Cambium - In older parts of the root, another meristem forms between the xylem and phloem. Mitosis in the cambium produces new "secondary xylem" to the inside and secondary phloem to the outside.

Water Uptake

Water enters the root through the epidermis. Once within the epidermis, water passes through the cortex, mainly traveling between the cells. However, in order to enter the stele, it must pass through the cytoplasm of the cells of the endodermis.

Once within the stele, water is free again to move between cells as well as through them. In young roots, water enters directly into the xylem. In older roots, it may have to pass first through a band of phloem and cambium. It does so by traveling through horizontally-elongated cells, the xylem rays.

Mineral Uptake

One might have expected that minerals would enter the root dissolved in water. But, in fact, minerals enter separately:

• Even when no water is being absorbed, minerals enter freely - mostly through the root hairs.

• Minerals can enter against their concentration gradient; that is, by active transport. For example, plants can take up K⁺ from the soil against a ten-thousand-fold concentration gradient; e.g., from as little as 10 µM in the soil to 100 mM in the cell.

• Anything that interferes with the metabolism of root hairs interferes with mineral absorption.

• The root hairs are also the point of entry of mycorrhizal fungi. These transport minerals - especially phosphorus - to the root hair in exchange for carbohydrates from the plant.

• In legumes, the root hairs are the point of entry of rhizobia that will establish the mutualistic partnership enabling the plant to convert atmospheric nitrogen into protein.

Plants absorb their nutrients in inorganic form

For examples:

• nitrogen enters as nitrate (NO₃⁻) or ammonium ions (NH₄⁺)

• phosphorus as PO₄³⁻

• potassium as K⁺

• calcium as Ca²⁺

When you hear of the virtues of organic fertilizers, remember that such materials meet no nutritional need of the plant until their constituents have been degraded to inorganic forms. Organic matter does play an important role in making good soil texture, but only to the extent that it can yield inorganic ions can it meet the nutritional needs of the plant.

Once within the epidermis, inorganic ions pass inward from cell to cell, probably through plasmodesmata. The final step from the cytoplasm of the pericycle cells to the xylem is probably accomplished once again by active transport.

Contributors and Attributions

• John W. Kimball. This content is distributed under a Creative Commons Attribution 3.0 Unported (CC BY 3.0) license and made possible by funding from The Saylor Foundation.

ROOT MODIFICATIONS

Roots employ many different modifications which help to protect them, interact with their environment and store nutrients.

Roots of parasitic plants are modified into haustoria which sink themselves into the vascular tissue of a host plant and live off of the host plant’s water and nutrients.

Root nodules present on the roots of nitrogen-fixing plants, they contain bacteria and proteins that work together to change atmospheric nitrogen into ammonia:

2N2 + 6H2O → 4NH3 + 3O2

Mycorrhiza is a root modification started when a fungus penetrates the roots and makes them more efficient in mineral and water absorption in exchange for organic compounds. Endophytic fungi can also inhabit other plant organs and tissues.

Roots of mangroves (plants growing in ocean coastal swamps) are frequently modified into supportive aerial roots (“legs”). Since these swamp plants need oxygen to allow cell respiration in underground parts, there are pneumatophores, specialized roots which grow upward and passively catch the air via multiple pores.

Plants which grow on sand have another problem: their substrate constantly disappears. To avoid this, plants developed contractile roots which may shorten and pull plant body deeper into the sand.

Some orchid roots are green and photosynthetic!

Activity

1. __________, oxygen, and nutrients that are readily-available underground are absorbed by the roots.

2. The leaves of trees release __________ to the environment during the food-making process.

3. Photosynthesis is the process of converting sunlight, carbon dioxide, and water into usable __________.

4. The __________ of a plant attracts various insects using its scent and colorful display.

5. A __________ is the seed-bearing structure in flowering plants.

6. The __________ transports absorbed water and minerals to different parts of the plant.

7. The __________ of a plant are the sites where sugar is produced.

8. A __________ is a vegetable with a nutritious, juicy, sweet root that is often orange in color.

9. __________ contribute to anchoring the plant securely into the soil.

10. The __________ released by animals and humans are used by plants for food production.

Quiz

Complete the following quiz in your portfolio.

Discussion

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Post under the appropriate topic in the Forums what you learnt, what you would like to learn and what you do not understand.

References

• Grade 9 SCCP Curriculum Guide

• Biology. Libre Texts Website. <https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(OpenStax)/6%3A_Plant_Structure_and_Function/30%3A_Plant_Form_and_Physiology/30.1%3A_The_Plant_Body>

• Anatomy and Physiology of Plants. Libre TExts Website. <https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_Biology_(Kimball)/16%3A_The_Anatomy_and_Physiology_of_Plants/16.01%3A_Plant_Anatomy/16.1.02%3A_Roots>

General Biology. LibreTexts Website. <https://bio.libretexts.org/Bookshelves/Introductory_and_General_Biology/Book%3A_General_Biology_(OpenStax)/6%3A_Plant_Structure_and_Function/30%3A_Plant_Form_and_Physiology/30.3%3A_Roots>

• BBC. What are teh Organs of a Plant. BiteSize Biology website. <https://www.bbc.co.uk/bitesize/topics/znyycdm/articles/zjchsrd>

• OSU Extension Service. Vegetative Plant Parts. Organ State University. <https://extension.oregonstate.edu/gardening/techniques/vegetative-plant-parts>

• Image of Plant Organs <https://biology-forums.com/gallery/18099_27_04_12_6_27_57.jpeg>

• Image of Cross Section of Vascular Bundle <https://bio.libretexts.org/@api/deki/files/1731/Figure_30_01_02f.jpg?revision=1>

• Image of Vascular Bundles of Monocots and Dicots <https://bio.libretexts.org/@api/deki/files/1737/Figure_30_02_06.jpg?revision=1>

• The Root. Introduction to Botany. LibreTexts. <https://bio.libretexts.org/Bookshelves/Botany/Book%3A_Introduction_to_Botany_(Shipunov)/05%3A_Tissues_and_Organs_-_How_the_Plant_is_Built/5.05%3A_The_Root>