- REASONS FOR THE STUDY OF PLANTS
Plants are the most important organisms that inhabit the earth. They are important because they have the ability to convert inorganic Carbon and inorganic Nitrogen into organic Carbon and Nitrogen, i.e. Proteins, Sugars, Fats and Oils. No other group of organisms have this ability. Animals are not able to use inorganic carbon and nitrogen in this way and therefore have to take in ready- made, sugars and proteins which they can only get from plants or other animals, who have in turn, received theirs from plants.
This type of relationship of an animal eating a smaller animal which in turn eats grass is called a food chain. The following is an example of a food chain:
The Impala eats veld grasses; The Lion then eats the Impala;
The veld grass is termed a producer; the Impala is called the primary consumer and the Lion the secondary consumer.
Apart from plants being the primary source of feed for most animals, they also have many other benefits which are outlined as follows:
Fuels : 90% f the fuels used by man come from plants and are divided into:
- Fossil fuels; and
- Non – fossil fuels.
Fossil fuels are those such as Coal, Oil and Natural Gas which were, in fact, plants millions of years ago,
Non – fossil fuels are those such as Wood and Alcohol (derived from sugar cane). These fuels are the result of plants that are very much alive today.
Fibers: Many of our fibers used in rope, sacking, and clothes are derived from plants such as Flax, Jute, Sisal, Cotton, Hemp, and Coconut.
Chemicals: Many drugs, oils and pesticides are obtained from plants. Examples of crops grown for oils are: Groundnuts, Cottonseed, Sunflowers and Soybeans.
Luxuries: These include such delights as Tea, Coffee and Tobacco.
Timber: This is a very important commodity obtained from plants: namely trees:
It is used in the construction and furniture industry and for the production of paper.
It should by now be apparent that plants are very important to man and the animal kingdom. The efficient production of plant material is one that is reliant on the farmer. An educated farmer should have a good knowledge of plants, their structure and how they work and how they are classified. To possess a good knowledge of plants, they must be studied. The study of plants is called Botany.
- COMPARISON OF THE STRUCTURE OF PLANTS AND ANIMALS
Before one can study the structural difference between plants and animal’s one must first examine the difference between a plant cell and an animal cell.
Table 1: A comparison between an animal cell and a plant cell.
|An Animal Cell||A Plant Cell|
|They do not posses a cell wall||Plant cells do posses a cell wall|
|Animal cells rarely possess a vacuole||Plant cells often possess a vacuole|
|Animal cells never contain chloroplasts. Chloroplasts are small structures that are contained in some plant cells which are responsible for making food.||Plant cells often but not always contain Chloroplasts.|
However, apart from the difference in the cells, plants and animals have other structural differences.
Table 2: A structural difference between animals and plants.
|Plants synthesise their own food stuffs by a process called photosynthesis. This is why most plants are green.||Animals are unable to synthesise their own food stuffs and have to take in ready-made food.|
|Plants are to some extent indefinite in shape. For example, if one examines two individual trees of the same species, although the basic structures are the same, e.g. flower and leaves, the two individuals are not the same shape.||Animals, on the other hand, are very definite in shape. For example, if one examines two individual cows their basic shape is the same. They have a head, four legs and a tail.|
|Plants are not mobile, i.e. they are unable to move from place to place. Most plants are not capable of voluntary movement.||Most animals are mobile, and even those that are not, are still capable of some degree of voluntary movement.|
Botanical terminology is very vast so it is necessary to introduce a few essential basic terms as you need to become familiar with them:
- Botany – This is the study of plants
- Autotrophic – This term refers to the mode of nutrition of an organism; it means that the organism’s source of carbon is inorganic.
- Taxonomy – This is the classification of organisms into similar categories.
- Organism – A living thing.
- Physiology -This is the study of plant functions and processes.
- Anatomy – This is the structure of the body of the plant.
- Biology – The study of living things.
A wide range of botanical vocabulary will be encountered in the course; however, their meanings will be explained when they appear.
- PLANT TAXONOMY
This refers to the classification of plants. All living things are classified according to similarities in structure and form. There are three kingdoms that all living things are grouped into.
- Animal kingdom – to be discussed in Zoology.
- Plant kingdom
- The kingdom of the Protists.
The basic differences between plants and animals have already been discussed. The third kingdom of protists is a kingdom that contains many organisms that both botanists and zoologists lay claim to. Classical biologists will recognise only the two kingdoms of the Plant and Animal. The third kingdom contains single celled organisms and primitive multi-cellular organisms showing little or no cellular specialisation. (Cellular specialisation occurs where certain cells become specialised to perform special functions.)
The third kingdom includes algae, protozoa, bacteria, fungi and viruses. These will be discussed later.
The first serious attempt at classification was made by a Greek, (Theophrattus) at around 350 B.C. However it was not until 1737 that a Swede, Linnaeus, published his book “Systema Naturae”, in which he proposed a system of classification which still forms the basis of modern classification.
Botanists divide the plant kingdom into five divisions. Each division is divided into Classes; each class is divided into Orders and each order is divided into Families. The Families are divided into Genera, and these are divided into Species. When plants are given a botanical name, they are given two names; the first name is the genus to which it belongs, and the second is the species name. This system of nomenclature is called the Binomial System. Plants are put into the above groups on the basis of numerous naturally similar characteristics. This system of choosing natural similarities is called Natural
A species can be defined as a group of individual plants or animals in which the sum total of their characters constantly resemble each other to a greater degree, than members of other groups. These form a true interbreeding assemblage and will not under natural conditions produce viable offspring with members of another group.
Two of the Divisions used by classical botanists are, in fact, placed in the kingdom protista by most modern biologists.
THEY ARE THE DIVISIONS OF:
- Myxomycetes – the slime fungi
- Thallophyta – fungi and algae, e.g. mushroom and seaweeds.
THE REMAINING THREE DIVISIONS CAN SAFELY BE REGARDED AS PLANTS AND THESE ARE:
- Bryophyta – These are very simple plants that show very little Cellular specialisation. They do not possess stems roots and leaves; however, they do possess similar analogous structures. Their reproductive systems are primitive and flowers are not produced. This division includes liverworts and mosses.
- Pteridaphyta – These organisms do exhibit stems, leaves and root structures, their reproductive system is not very advanced however and flowers and seeds are not produced.
- Spermatophyta – These organisms have well developed root, stem and leaf
structures and their reproductive system is considerably more advanced as they produce seeds. This includes all agricultural crops.
THE DIVISION IS DIVIDED UP INTO TWO SUB-DIVISION:
- Angiosperms – these are the plants that produce flowers that contain an ovule (this is the structure that contains a female egg cell (gamete), enclosed in an ovary. (In plants this is the structure that contains one or more ovules) which forms part of the flower (or inflorescence).
- Gymnosperms – these are those plants that do not produce flowers and their ovules are not found protected in an ovary but lie unprotected on the bracts (shelf like structures) of cones. The gymnosperms are, in fact, known as the cone bearing plants. They are important economically because they constitute a major source of timber.
This group of plants is divided into two classes
a) Monocotyledons, and b) Dicotyledons.
The former includes all the grasses e.g. wheat, maize, sorghum etc., and the latter includes most trees and crops such as tobacco and potatoes. The differences between these two classes are outlined in the Table below:
|Their leaves are parallel veined and the edges are usually smooth surfaces.||Their leaves are not veined in structure and there are a variety of leaf shapes.|
|The embryos of these seeds have only one Cotyledon (seed leaf)||Dicots’ seeds have two cotyledons (seed leafs)|
|Vascular tissue (this is the tissue that is responsible for the movement of fluid in the||Vascular tissue is arranged as a ring of open bundles in the stem.|
|plant) Is in the centre of the stem.|
|Floral parts (these are the non-reproductive components of a flower, i.e. Petals, Sepals.) are singular||Floral parts are arranged in fours or fives or multiples thereof.|
Figure 1: A comparison between Monocotyledons and Dicotyledons.
Most agricultural crops belong to the sub-division Angiospermae An example of the complete classification of one of them is – the Potato:
|Variety||Up To Date|
Its botanical name is; Solanum tuberosum Var. “Up To Date”
It has been noticed for centuries that certain organisms appear, to have become adapted to their environment. For example, some plants have adapted to living in salt water and some to living at high altitudes. Some plants can live in very cold regions, others in very hot regions. The question one asks is ‘why was this’?
In answer to that, there were a few people, like Lamarck, who believed that the environment modified those organisms living in it by causing them to adapt to it and passing on such adaptations to subsequent generations.
For example, he believed that originally, the giraffe was a short-necked creature and the environment changed such that the only food available was tree leaves. The giraffes then had to stretch their necks in order to reach food. The necks became longer and as a result the offspring were born with long necks also. Lamarck lived in the late 18th Century – early 19th Century. Nowadays his theory is obviously absurd.
However, in 1859, a gentleman naturalist named Charles Darwin proposed a more realistic theory. Darwin had voyaged around the world and noticed how different organisms were very similar and yet were best suited to different environments. After a great deal of thought and careful consideration, Darwin proposed his theory of “Evolution by Natural Selection”. He proposed that every now and again an individual would be born which was slightly different from the parents in some significant characteristics. Normally, this would be a deformity but occasionally the “deformity” would give the individual a better chance of survival in the environment. As that individual was born with this difference, a substantial number of its progeny would be born with the defect as well and they would eventually become the norm because all the original organisms would not have been able to compete. That is, the best suited individuals would have been selected naturally by their environment. Taken to its logical conclusion, the theory means that all life could have evolved from one cell by a process of natural selection. This theory of evolution has been accepted by modern biologists. The theory of evolution has a mass of archeological and fossil evidence of long-extinct creatures to support it.
Plant breeder’s today attempt to breed “super plants” by imposing unnatural selection. Evolution is happening in agriculture every day. One example is insecticide or herbicide resistance. This is where one individual suddenly, by chance, is born or hatched possessing resistance and survives the spray; so, too, do its entire offspring. Prolonged use of chemicals will cause resistant organisms to be selected and the chemical becomes useless.