Relevance of Biology to Agriculture – SS1 Biology Lesson Note (Second Term)

Plant classification is the grouping of plants into categories because plants exist in various forms and types. Plants can be classified by botanical grouping, agricultural use, life cycle and size. The lesson also covers how farming practices such as bush burning, overgrazing, tillage, deforestation, fertiliser use and pesticide use affect the ecological system.

Behavioural objectives

By the end of the lesson, students should be able to:

• state the four bases on which plants are classified;
• describe the botanical classification of plants into thallophytes, bryophytes and tracheophytes;
• classify plants agriculturally and give examples of each class;
• classify plants by life cycle and by size, with examples;
• explain the effects of bush burning, overgrazing, tillage, deforestation, fertiliser application and pesticide application on the ecological system.

Why are plants classified?

Plants exist in various forms and types, hence the need for classification. Plants can be classified on the following bases:

1. Botanical classification
2. Agricultural classification
3. Classification based on life cycle
4. Classification based on size

What is botanical classification of plants?

Botanical classification subdivides the plant kingdom into divisions, classes, orders, families, genera and species. This is based on the structures, functions and evolutionary trends of the plants. Plants are then generally classified into three broad groups:

1. Thallophytes (A)
2. Bryophytes (B)
3. Tracheophytes (C)

Tracheophytes are further grouped into pteridophytes (D) and spermatophytes (E). The spermatophytes can also be grouped into gymnosperms (F) and angiosperms (G). The angiosperms are subdivided into dicot (H) and monocot (I).

A and B are non-vascular plants, and C refers to vascular plants. D refers to non-flowering plants; E refers to seed plants, while G refers to flowering plants proper.

What is agricultural classification of plants?

Agricultural classification of plants is based on:

1. The product obtained from the plants
2. The parts of the plant that are useful
3. The economic importance of the plants

Plants are therefore classified agriculturally into the following:

• Crops and weeds: plants that are needed on the farm are called crops, while other unwanted plants are called weeds.
• Food crops and cash crops: crops grown mainly for human consumption are called food crops, e.g. maize and yam. Those grown mainly to earn money are cash crops, e.g. cocoa and coffee.
• Root crops: plants which store mainly starch in edible underground stems or roots, e.g. yam and cassava.
• Cereal crops: monocotyledonous plants of the grass family, whose grains are eaten, e.g. maize, millet, guinea corn, rice and wheat. They are rich in carbohydrates.
• Fruit crops: plants whose fruits are eaten. They are rich in vitamins and minerals, and fruits are also rich in sugar. Examples are oranges, mangoes, avocado pears and cashews.
• Vegetable crops: herbaceous plants whose vegetable (leafy) parts are eaten. They include spinach, lettuce, carrots, cabbage, okra, tomato, onion and pepper. They are also rich in vitamins and minerals.
• Legumes: plants of the beans family, such as cowpeas and groundnuts, whose seeds are eaten. They are rich in proteins.
• Spices: plants whose parts are used for seasoning food, such as pepper, curry, thyme and ginger.
• Latex plants: plants that are grown for their useful latex (a milky fluid), e.g. rubber plants, used for making natural rubber in tropical countries.
• Fibre plants: plants which produce fibre for textile and bag production, e.g. cotton and hemp.
• Beverage and drug plants: plants whose parts are taken as stimulants or drugs, e.g. tea, coffee, cocoa, kola nut and the quinine tree (for medicine).
• Oil plants: plants that produce oil of economic value, e.g. oil palm, shea nut, groundnut, coconut, castor oil plant and melon.

How are plants classified by life cycle and size?

Classification based on life cycle

Based on life cycle (period of existence), plants can be classified as:

1. Annuals: plants which complete their life cycle within one growing season or a year, e.g. maize, yam, melon, cowpea and tomato.
2. Biennials: plants which complete their life cycle within two years, e.g. banana, plantain and pineapple.
3. Perennials: plants which persist for more than two years, producing their yields every season, e.g. orange, mango, oil palm and cocoa.

Classification based on size

Plants fall into three categories based on size:

1. Herbs are small plants with fleshy stems, e.g. spinach and waterleaf.
2. Shrubs are medium-sized plants with woody stem branches very close to the ground (soil), e.g. hibiscus.
3. Trees are big plants with woody trunks, which branch at the top, e.g. iroko, mahogany, cashew and coffee.

How do agricultural activities affect the ecological system?

The following farming practices carried out by farmers have some consequences on the ecological system.

1. Bush burning

Bush burning involves the setting of fire in the bush to clear out the vegetation. Effects of bush burning include:

• Destruction of the organic matter in the soil
• The atmosphere is polluted with smoke
• Many of the micro-organisms are killed
• It exposes the soil to erosion and leaching
• It reduces the water-holding capacity of the soil
• It leads to the extinction of some animals
• The ash produced gives the soil a slightly alkaline nature

2. Overgrazing

Overgrazing is a situation where more animals than a particular pasture can support are put there to graze. It is a way of exceeding the carrying capacity of the soil. Effects of overgrazing include:

• It removes the vegetative cover of the soil
• It exposes the soil to erosion
• It destroys the soil structure
• More faeces are dropped on the soil, which could improve the fertility of the soil
• Weeds can be eradicated from such lands
• It leads to compactness of the soil, resulting from the continuous trampling of animals
• It causes poor growth and poor regenerative capacity of vegetation

3. Tillage

Tillage is the working, digging or breaking up of the soil in preparation for the planting of crops. Effects of tillage include:

• It encourages leaching
• It helps to loosen the soil
• It enhances proper aeration of the soil
• It exposes the soil organisms and may kill some
• It changes the structure and texture of the soil
• It leads to changes in the ecology of the land
• Intensive tillage can lead to loss of soil fertility
• It exposes the soil to erosion

4. Deforestation

Deforestation is the continuous removal of forest stands (trees), either by bush burning or by indiscriminate felling, without replacing them. Effects of deforestation include:

• It reduces water percolation, due to the absence of humus and dead leaves on the soil
• It reduces the amount of rainfall in the area
• It hinders the activities of micro-organisms in the soil
• It results in loss of nutrients through leaching and erosion
• It reduces the wildlife population in the area concerned
• It reduces the humus content of the soil

5. Fertiliser application

This involves the application of certain chemicals or substances to the soil to improve its fertility. Effects of fertiliser application include:

• It brings about the loss of organic matter or humus
• It deteriorates the structure of the soil
• It increases the porosity of the soil
• It supplements the nutrient content of the soil
• Excessive application can cause soil acidity
• The productive capacity of the soil is enhanced
• It stimulates vegetative growth, hence it reduces soil erosion

6. Application of pesticides and herbicides

Pesticides are chemical substances used to destroy or kill pests. Herbicides are chemical substances, in the form of solutions or gases, capable of destroying weeds. Effects of pesticide application include:

• It causes pollution of the environment
• It affects or destroys other useful plants and animals
• It reduces the population of the target insects or plants
• Pesticides may leave undesirable residues in the environment
• When such chemicals are washed into rivers or lakes, they can cause the death of aquatic animals

Evaluation

1. State the four bases on which plants can be classified.
2. Name the three broad groups of plants in botanical classification.
3. What are cereal crops? Give two examples.
4. Distinguish between annuals and perennials, giving one example of each.
5. State three effects of bush burning on the ecological system.

Assignment

1. Which of the following is not an example of a classification of plants?
   (a) Herbs and shrubs (b) Annuals and perennials (c) Gramineae and Euphorbiaceae (d) Monocot and dicot
2. Plants can be classified based on all these except
   (a) botanical (b) size (c) agricultural use (d) planting season
3. Fruit crops are rich in
   (a) vitamins and minerals (b) vitamins and protein (c) minerals and carbohydrate (d) protein and carbohydrate
4. Spices include
   (a) pepper and ginger (b) palm oil and ginger (c) lettuce and carrot (d) yam and maize
5. Which of these does not harm the ecological system?
   (a) fertiliser application (b) crop rotation (c) tillage (d) bush burning
6. State three effects each of the following farm practices on the ecosystem:
   • Bush clearing
   • Shifting cultivation
   • Monocropping
7. What are the classes of plants based on uses?