Structure of The Flowering Plant

Introduction

All flowering plants are called angiosperms.

All plants are:

  • All multicellular
  • All photosynthetic

Plants are divided into two groups:
1. Monocotyledonous
2. Dicotyledonous

  • A cotyledon is an embryonic seed leaf.

During early development of the plant, cotyledons are used as a food source or for photosynthesis (depending on the type of plant).

Monocotyledonous plants have one embryonic seed leaf.
Dicotyledonous plants have two embryonic seed leaves.

Tissue types in flowering plants:
1. Dermal: outer covering – functions in protection, gas exchange or absorption of water and minerals (depending on its location).
2. Ground: makes up the bulk of the plant – functions in photosynthesis, storage or support (depending on its location).
3. Vascular: composed of xylem and phloem tissue – functions in water and mineral transport (xylem) and food transport (phloem).
4. Meristematic: develops into each of the above three types of tissue.

  • Meristem: plant tissue composed of rapidly dividing unspecialised cells. These cells are dividing by mitosis.

Structure of the flowering plant

The diagram shows the structure of a typical flowering plant.

All plants are composed of a root system and shoot system.

Shoot system:

The shoot system consists of the following organs: stem, branches, petioles, buds, leaves, flowers, seeds and fruits.

Functions of the shoot system include:

  • Photosynthesis
  • Support
  • Transport
  • Sexual reproduction
  • Food storage
  • Gas exchange

Stem:

Stem functions mainly in support and transport – but can function in photosynthesis (if it is herbaceous). The stem also functions in growth.
The stem is divided into nodes and internodes.

  • Nodes are regions where branching occurs.
  • Internodes are where no branching occurs.
Nodes and internodes of a stem

Stems also have lenticels.

Lenticels:

  • Lenticels are small pores on a stem that function in gas exchange.

Internal structure of the stem:

The structure of the stem differs between monocots and dicots (see diagrams below – click into each diagram to see a larger view.

Leaf:

  • Thin organ with a large surface area.
  • Organ that makes food for the plant and functions in transpiration (loss of water from the plant).
  • Contains the green pigment, chlorophyll.
  • The outer dermal layer secretes a waxy cuticle to prevent excess water loss.
  • The leaf contains many internal air spaces that gives greater surface area for gas exchange.
  • The cells that make up the upper layer of ground tissue within the leaf contain many chloroplasts that carry out photosynthesis.
  • The lower epidermal layer has many pores, called stomata.
  • Stomatal opening and closing is controlled by guard cells.

External and internal structure of leaf:

Leaf venation:

The veins on leaves are of two types:

Parallel:

  • The veins run parallel, the entire length of the leaf; e.g. grass, tulip, daffodil.
  • Most monocot plants have parallel venation.

Net/Reticulate:

  • The veins spread out from a central vein called the midrib (see above).
  • Most dicot plants have net or reticulate venation.

Bud:

  • A bud is an undeveloped shoot

There are three types of bud:

  1. Axillary bud: present at the axil of leaf (between the stem and the petiole).
  2. Apical bud: present at the tip of the plant or the tip of a branch.
  3. Adventitious bud: present in an unusual area of the plant – such as the stem, branch, root or even leaf.

Flower arrangement between monocots and dicots

  • Monocots have flower parts arranged in multiples of three.
  • Dicots have flower parts arranged in multiples of four or five.

Root system:

A network of underground branches that have various functions:

  • Anchorage
  • Absorbing water and minerals
  • Transport of absorbed water and minerals to the shoot system
  • Storage of food
  • Support

Types of root system:

  1. Tap root system: one main root growing downwards with small secondary roots; e.g. carrot, dandelion.
  2. Fibrous root system: main roots of equal size; e.g. grass.
  3. Adventitious root system: roots that grow in unusual places such as the stem or branches; e.g. bayan tree, ivy.

Root structure:

There are four zones of a young root.

  1. Zone of protection: consists of a root cap, enabling the root to push its way through the ground.
  2. Meristematic zone: consists of meristematic tissue that divides rapidly by mitosis, creating new root tissue.
  3. Zone of elongation: area of the root affected by growth regulators, where the cells increase in size.
  4. Zone of differentiation: area of the root where unspecialised cells start to become specialised, becoming ground, dermal and vascular tissues.

The transverse structure of a root is shown below:

Vascular system

The vascular system in a plant is composed of two types of tissue: xylem and phloem.

Xylem:

  • A dead tissue – there is no cytoplasm and no nuclei in xylem tissue.
  • Contains lignin that gives the tissue high strength.
  • Transports water and dissolved minerals upwards.
  • Composed of two types of cell: tracheids (structure A in the diagram below) and vessels (structure B in the diagram below).

Phloem:

  • A living tissue.
  • Transports food (in the form of sucrose) upwards and downwards, depending on where food is needed.
  • Composed of companion cells and sieve tube cells.

Mandatory Activity: to prepare and examine microscopically a transverse section of a dicot stem at 100x and 400x.

Equipment/apparatus:

  • Geranium stem
  • Backed blade
  • Carrot
  • Clock glass
  • Paint brush
  • Water
  • Glass slide and coverslip
  • Light microscope

Method:

  • Cut a section of stem from an internode of a herbaceous stem such as a geranium.
  • Cut a triangular shaped slit along the length of a carrot and place the internode stem into the slit.
  • Wet a backed blade and carefully cut (cutting away from fingers) a few sections through the stem and carrot (avoid wedge-shaped sections).
  • Transfer the sections to a clock glass of water using a paint brush.
  • Place a section on a glass slide with a drop of water (to prevent the section drying out).
  • Carefully lower a glass coverslip from a 45º angle (this prevents the formation of air bubbles).
  • View the section firstly under low magnification and then under high magnification.
  • Make a sketch of each and label.

Results:

Conclusion:

  • Vascular bundles are arranged in a circle with the stem of dicot plants.

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