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The Plant

They not only help to provide the air we breath, but clean it as well!

Plants have been divided into two Categories based on how they reproduce. These groups are called - Gymnosperms and Angiosperms.

Gymnosperm, or Naked Seed: The Category of seed-bearing plants without flowers. There are several types, but the most common are the coniferous plants. The seeds are not enclosed in a fruit, but are borne upon seed scales arranged in Cones. The seeds develop within the Cones and shortly after the seeds mature, the protective scales of the Cone open up and the seeds are released. Distributed world-wide with the majority in Temperate and Sub-Arctic regions, this Category are the most ancient of our seed bearing plants.

Angiosperm, or Enclosed Seed: These are the flowering plants. Appearing after the Gymnosperms, it is believed that the Magnolia was the first of this Category that now includes countless shrubs, herbaceous plants, grasses, broadleaf trees and specialized plants such as succulents, and aquatics. With well over a quarter of a million known species, Angiosperms have colonized and adapted to every region in the world. They have become so successful they are now the most dominant form of plant life and form a major food source. Following fertilization of the Ovule, the seeds develop within the protection of the Ovary. The Ovary enlarges into a fruit as the fertilized seeds grow. Perhaps the secret to their success is that Angiosperms have two sub-classes that have formed as a result of environmental adaptations:

Dicotyledons; have floral organs in multiples of four or five. The leaves have a netlike vein pattern, and the vascular tissue inside the stem is arranged in a ring. This group of plants consists of trees, shrubs and most herbaceous plants. Seedlings have two seed leaves or Cotyledons, for nourishment until its true leaves develop. The stem and root expand with secondary growth.
Monocotyledons; with the floral organs generally occurring in multiples of three. The veins in the leaf are parallel, the vascular system inside the stem is scattered, and as such the plants produce no secondary growth. Consist of grasses, bulbs and palms. Germinated seedlings have one Cotyledon.

Vascular Plants (Tracheophytes), are made up of Rigid Walled Cells. These Cells exist in many shapes, variations and sizes, and when organized into functional units form Tissue. Each type has a particular function to perform.

At the center of a Cell is the Vacuole, filled with Cell Sap - containing nutrients, metabolites, and waste products. Surrounding the Vacuole is the living 'soup' of the Cell, Cytoplasm, within which are varying Organelle, proteins, carbohydrates and salts, including; Mitochondria - converts energy into forms that are usable by the cell, Nucleus - contains the DNA responsible for providing the Cell with its unique characteristics and Chloroplast - (see below). A semi-permeable membrane allows water and salts in, whilst keeping the Sap and Cytoplasm within.

Eukaryotes Cell

New Cells and Tissue are formed at growing points called Meristems. This tissue of actively dividing cells is located at the tips of the stem or roots, where they are responsible for the primary growth of plants, or in the Cambium layer, (see below) where they are responsible for secondary plant growth or 'thickening'.

The Vascular System:
At the center of the Dicotyledonous stem is the Pith, which is surrounded by Cambium Tissue. It is in this layer that new Cells are formed. During the growing season the Cells in the Cambium Layer divide actively, producing new Cells that differentiate into Xylem, toward the inner side of the Cambium, and Phloem toward the outer side. Arranged in columns called Vascular Bundles, this is the Conducting Tissue that is responsible for transporting water and nutrients to all the Cells of the plant. The Vascular Tissue is surrounded by the Cortex or Ground Tissue, which is all protected by a layer called the Epidermis. Growth of trees & shrubs depend on successive layers of Woody Tissue to the stem of the original young seedling due to the increase in Xylem Cells. A new Cambium Layer, called the Cork Cambium develops outside the Phloem and produces successive layers of Cork Cells that protect the stem.

The Tissues of a Monocotyledonous plant stem are similar, the big difference is the arrangement of the Cortex and Vascular System. The Cortex and Pith is replaced by Parenchyma Cells. The Parenchyma has air spaces within it's Cells allowing for an exchange of gases to take place. Surrounded by a layer of Sclerenchyma Cells, the stem is encased within the Epidermis. As the Sclerenchyma Cells die off on reaching maturity, the thick, heavily lignified wall that encased them remains to strengthen and support the stem. There is a layer of tough Sclerenchyma around the Vascular Bundles that are scattered throughout the Parenchyma. The Vascular Bundles do not contain Cambium Tissue resulting in no secondary thickening or Wood to form.

• Xylem Tissue:
Conducts soil solution up the plant from the roots.
Builds up in woody perennials to form the wood
• Phloem Tissue:
Conducts manufactured food from the leaves.
Transports this food throughout the plant.

Stem Cross-Section:

Fig. 1
Monocotyledon Stem

Fig. 2
Dicotoyledon Juvenile Stem

Fig. 3
Dicotoyledon Mature Stem

Monocot Stem
Young Dicot Stem Mature Dicot Stem

The Dermal System:

Dermal Tissue forms the Epidermis or skin of the plant. The Lower Epidermis of the Leaf contains Stomata. These are openings through which water is lost and the gases Carbon Dioxide and Oxygen are exchanged with the atmosphere; given the term Transpiration. These openings are surrounded by specialized Cells called Guard Cells. The Guard Cells expand in extreme weather conditions or contract in desirable conditions. This will alter the size of the openings and thus regulate Transpiration. The Epidermis is covered with a waxy coating called the Cuticle, a waterproof layer reducing water loss from the plant surface through Evaporation.

Leaf Cross-section
A large Palisade Parenchyma layer within the body of the leaf contains columns of Cells containing Chloroplasts (see below). Beneath the Palisade Layer is the Spongy Parenchyma layer - these are cells of various shapes forming a three-dimensional mesh. The spongy layer encase the Leaf Veins housing the vascular bundles.

The body of many plants are organized into three main organs: Root, Stem and Leaf.

Plants contain Cells called Chloroplasts. The Chloroplast Cells contain the green coloured Chlorophyll, (which is why most plants appear green). Apart for being responsible for providing the usual colour of plants, Chloroplast generates the most important chemical reaction on the planet, Photosynthesis. This is the process by which plants manufacture their own food; Chloroplast cells absorb energy from sunlight. The Suns Energy combines with water and Carbon Dioxide from the air. The energy is then converted into chemical energy, or Carbohydrates. It is Carbohydrate that not only sustains the plant but most other life as well. The by-product of this process is Oxygen.

The Leaf:
The main area for
Transpiration, (water-loss).
Respiration (energy)
Gas exchange.

The Stem:
The main function of which is
Supporting the leaves and flowers.
Transporting soil solution from the roots to the leaves
Transporting manufactured sugars (Carbohydrates) throughout the plant.
Food storage.

The Root:
The main function of which is
Uptake of water and nutrients (Soil-Solution).
Transporting Soil Solution to the stem.
Food storage.

Water Movement

Water and Carbohydrate Movement in a Plant

At the tips of young roots are very fine hairs. These root hairs absorb Soil Solution from between soil particles as the result of Osmosis. The Soil Solution passes into the Xylem Tissue in the center of the root and continues through the root, into the stem and into the leaves. Water moves out of the plant as a vapour via the Stomata. The Soil Solution combines with the Carbohydrates manufactured by Photosynthesis and moves into the Phloem Tissue. These sugars and mineral salts are the food the plant needs for healthy and sustained growth, and are transported from the leaves throughout the plant.

The Flower.
There is another plant organ on angiosperms, and one which has a great significance for the survival of all life on this planet ...Flowers. Their function is to produce fertile seed, which they do by attracting insects to pollinate it. Flowers are modified shoots, consisting of up to four types of 'leaves' arranged in layers.
These are:

Sepals - protect the bud before it blooms, are outermost.

Petals - the next layer, attracts pollinators by using colour and scent.

Stamens -  the third layer and the flowers' male reproductive organs, produce the Pollen. Near their base are the Nectary Glands that produce Nectar to feed pollinating insects.

Pistil or Carpel - the innermost and female part, composed of Stigma, Style, Ovary and Ovule. There may be one Carpel or many Carpels (Compound Flowers), depending on the species.

When pollen comes into contact with the stigma, it travels down to the ovules and Fertilization occurs. The ovules develop to produce seed and the ovary swells to form the 'Fruit' as we know it.

There are many types of flowers produced, (see below). The diagram I have used here is a cross-section of a Single, Simple Flower - consisting of a single row of petals arranged in a whorl. Many of our cultivated plants' flowers are Double, meaning more than one row of petals.
Simple Flower

The sepals are collectively termed the Calyx, and is normally green. However, they may be colourfull such as the sepals found on Orchids, Fuchsia and Delphiniums for example. The petals are collectively termed the Corolla. When the calyx and corolla are very similar in colour and form and not easily distinguished, such as with Magnolias, Tulips Lilies and Clematis, they are given the collective term Tepals. 

The arrangement of flowers on a stem is called Inflorescence, of which there are two main classes; Terminal - where the inflorescence sits on top of the shoot and Axillary, in which the inflorescence is found in the axil of a leaf. There are many flower types.

Types of Flower

Single - an inflorescence composed of a single flower.
Raceme - an un-branched shoot with lateral flowers supported by stalks called pedicels, the flowers mature from the bottom upwards.
Spike - a raceme with un-stalked flowers, also maturing from the bottom upwards.
Panicle - branched clusters of flowers in which the branches are racemes.
Corymb - a flat-topped raceme; the lower branches of the raceme are long and the upper branches are short so that the overall shape is flat.
Umbel - a flat-topped inflorescence in which the pedicels all originate from a single point.
Composite – an inflorescence composed of many florets, (a small or reduced flower).

We have already discussed the plant categories, Gymnosperms and Angiosperms. The plants within these two categories have been given a Latin Botanical name and, based mainly on their floral structure, put into groups. These groups are:

• Family – made up of similar Genera. There are hundreds of plant families. As far as food production is concerned, the two most significant plant families are:
Rosaceae - Rose Family. The plants within this group have similar flower shapes based on five petals with many stamens, most of which produce succulent fruits. This huge family include many of our commonly grown flowering trees and shrubs such as; roses, cotoneaster, apple, plum, geum, kerria, potentilla, crataegus, blackberry, raspberry, spiraea and sorbus, to name just a few.
Poaceae or Gramineae - Grass Family. The distinctive characteristic of the plants within this group include feathery flower heads and the plants go on to produce grain. Apart from the grasses we grow in our gardens, this vast family includes the cereals, such as wheat, barley and corn for example.
A few other important families, including plant examples of each one:

Asteraceae - Daisies, Aster, Chrysanthemums and Achillea.
Brassicaceae – Brassicas, Wallflowers, Honesty, Stock and Radish.
Solanaceae – Potato, Tomato, Petunia, Peppers, Aubergine, Nightshades and Tobacco.
Apiaceae – Cow Parsley, Carrot, Celery, Fennel and Hemlock.
Ranunculaceae - Buttercups, Aquilegia, Nigella, Clematis and Delphinium.
Leguminosae – Clover, Vetch, Laburnum, Wisteria, Gorse, Broom and Lupin.
Lamiaceae - Nettle, Mint, Sage, Thyme, Basil, Coleus and Ivy.
Liliaceae – Tulip, Kniphofia, Onions, Leek Fritillaria, Bluebells, Daffodil, Lilly and Hosta.
Oleaceae – Olive, Fraxinus, Kerria, Forsythia, Jasmine, Privet and Lilac.

• Genus - made up of differing plant species, that whilst related, do not normally interbreed.

• Species (spp.) - individual plants of the same species that are able to breed amongst themselves and produce fertile seeds. The resulting offspring will closely resemble their parents, but very subtle, slow changes and adaptations may occur - or Evolve. The species name often describes a characteristic of the plant such as country of origin, colour, growth habit or person that discovered it, eg. japonica (Japan), alba (white), repens (creeping), forrestii (George Forrest).

• Cultivars (Cv.) - plants produced for the garden or table from a wild or natural plant species by hybridization and selection.

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