PLANT FORMS

Crop is a plant grown for a definite purpose. Plants were probably the earliest form of life to appear on this earth. All crop plants constitute what is known as “higher plants.” Higher plants are plants which are able to exist on their own food from basic elements such as water and carbon (IV) oxide. This process of food manufacture in the higher plants is called “photosynthesis.”

This chapter will include a review of the structure of plants, their function, growth and reproduction. Higher plants are made up of two types: monocotyledons and dicotyledons.

Parts of a crop plant

The organs of plant can be divided into two parts. These are the vegetative organs and the reproductive organs. The vegetative parts of a plant are the root, stem, and leaves while the reproductive parts are the flowers. The leaves, stem and flowers are usually seen above the ground while the root is usually below ground. All the parts of a plant that are above the ground make up the shoot.

Root

A root is the part of a plant which grows down into the soil away from the light and towards water. It never bears leaves or buds. The roots are of different types, the tap and fibrous root systems.

Functions: The root has three main functions:

1. It absorbs water and salts from the soil.
2. It fixes the plant to the ground.
3. In some plants it acts as a storage organ, e.g. cassava.

Stem

A stem is the part of a plant which bears buds, leaves and flowers. Normally, but not always, the stem is above the ground.

Functions: The stem has four main functions.

1. It bears the leaves.
2. It holds the leaves so that they may obtain plenty of light.
3. It conducts water and salts to the leaves, and manufactured substances from the leaves to other parts of the plant.
4. In some plants, it stores food as in the case of yam tubers.

Leaves

The leaf is usually attached to the stem by a leaf stalk called the petiole. A typical leaf is a flat, green lamina or blade made from a soft tissue of thin-walled cells, supported by a stronger network of veins.

Functions: The important function of leaves is to make food, in the form of carbohydrates, by photosynthesis. The water necessary for this process is conveyed through the vessels which run in the vascular bundles branching from the stem, and through the petiole and midrib, dividing repeatedly to form a network of tiny veins throughout the lamina. The leaf needs a supply of carbon (IV) oxide from the air. This diffuses through the stomata in one or both of its surfaces. For respiration, all living cells need a supply of oxygen, which also enters through the stomata.

Flowers

This is the part of the shoot specially modified for reproduction and contains the ovary. The ovary will develop to form fruit after the process of pollination and fertilization, while the ovules which it contains will develop to form the seeds.

Fig. 3(a) Parts of a plant (Crotalaria)

The diagram labels include:

• Flower
• Fruit (pod)
• Leaf
• Internode
• Bud at node
• Node
• Stem
• Soil level
• Root

 Comparison between Monocots and Dicotyledons

Feature	Monocotyledons	Dicotyledons
Root	Always fibrous root system	Usually tap root system
Stem	The vascular bundles are scattered randomly through the stem.	The vascular bundles are arranged in a radial pattern like the spokes of a wheel.
Leaves	Long and narrow; leaf veins parallel	Short and broad; leaf veins are netted.
Flowers	Usually green, inconspicuous and attractive and unscented.	Usually colourful, sometimes scented.
Seeds	One seed leaf; do not naturally break into two halves.	Two seed leaves; break naturally into two halves—each is a cotyledon.
Pollination	Usually by wind	Usually by insects or other animals.
Major crops	Oil palm, ginger, rice, sugarcane, millet, maize, banana, grasses, guinea corn, coconut, sisal, pineapple.	Rubber, tobacco, yams, okro, citrus, peppers, cassava, cocoa, coffee, melon, pumpkin, groundnut, potato.

Monocotyledons

Plants are generally classified according to the structure of their seeds. In some plants, the seeds have only one seed leaf called a cotyledon. In other plants, the seeds have two cotyledons. Plants which have seeds with one seed leaf are called monocotyledons. The cotyledons are the stores of food for the germinating seedlings during the early stages of growth. Eventually, the cotyledons wither and finally drop off.

Fig. 4: Monocot plant showing roots, stems and leaves

Labels in the diagram:

• Staminate flowers
• Secondary axis
• Peduncle
• Pistillate flower
• Leaf blade
• Leaf sheath
• Node
• Culm
• Soil level
• Prop root
• Fibrous roots

Dicotyledons

Dicotyledons are characterized by the possession of two cotyledons in the embryo. The group includes hardwood and softwood trees, shrubs and many herbaceous plants.

Fig. 5(a): Parts of dicot plant

Labels in the diagram:

• Bud
• Flower
• Leaf
• Stem
• Lateral root
• Main root