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Subject: Agricultural Science
Topic: Crop Propagation
Lesson Objectives: by the end of the lesson, the learners should be able to:
- Say the meaning of Crop Propagation;
- Mention and explain the types of crop Propagation with examples;
- State the advantages and disadvantages of the each type of crop Propagation.
Lesson Summary / Discussion
Meaning of Propagation
We increase the number of plants or crops by planting their seeds or
by using the cuttings or part of their stem, root or leaves. This method of increasing the number of crops or plants is called propagation
Methods of Crop Propagation
There are two methods of crop Propagation. They are:
- Seed Propagation
- Vegetative propagation
These are discussed below:
Crop Propagation by Seed (Sexual Propagation)
This is the use of seeds to increase the number of plants in a farm land. This process is also known as sexual reproduction, because maize seeds result from the fusion (union), joining together, of the male and female sex reproductive cells usually from two reproductive parent plants. The seeds develop, mature and dry up.
Activities: Teacher should take the learners on a visit to the school farm for observation of the crops planted there. You can ask them to write down the names of some other crops which are propagated by seeds. Ask them to take a close look at the crop plants. Most school farms have maize, tomatoes, leafy vegetables like tete, ewedu, okro and soko, while a few have fruit crops such as sweet orange, pawpaw and cashew. Before the maize on the school farm was planted, the seeds had been removed from the cobs, dried and stored. What did you observe about the height of the new maize plants? They are of different heights, some are very tall, most are of average height, while some others are short. Did you also observe that the new maize cobs (fruits) were at different heights on the maize plants. You will probably have noticed that most of the maize plants had one cob, while a few had two cobs.
Take a close look at the fruits produced by the tomato plants on your school farm. You would notice that the fruits come in various sizes large, medium and small. Many of the tomato plants come from seeds that were sown from one large tomato fruit.
Advantages of Seed Propagation
There are many advantages for using seeds to propagate a crop.
Seeds are easy to carry from place to place, they also store well after they have been dried to reduce their moisture content. They must however, not be too dry so that they are viable (i.e alive). Since there are farmers who
specialize in the growing of seeds for sale to other farmers i.e Seed Growers, improved crop seeds are readily available from the National
Seed Service (NSS), State Ministries of Agriculture and Private Seed Growers, Seeds are also easy to sow, more so as they can be drilled into the soil with seed drills or planters.
Disadvantages Seed Propagation
1. From the examples of the maize and tomato plants, it must be clearly stated that plants that are sown from seeds never grow up to look exactly
alike. They always show different characteristics of their parents or even grandparents. Remember that seeds are formed by the joining of a male sex cell from one parent with a female sex cell from another parent. So, if one of the original maize or tomato parent plants was tall and the other short, do not be surprised that some of the plants from the seeds planted are short or tall, while others are of medium size. This is to make it clear to you that even though you may select seeds from the tallest maize plants with the biggest maize cobs, you are not sure of the size of the new maize plants and cobs that they will produce. All the same, it is best to select seeds from the best looking plants for propagation since there is a better chance of increasing the numbers of the best looking plants that way.
2. By using seeds for propagation, the farmer reduces the quantity of seeds
that will be available for consumption.
3. Also, many seeds do not germinate easily due to dormancy.
Crop Propagation by Vegetative Organs (Asexual)
Asexual propagation involves reproduction from the vegetative parts of plants since these parts can grow again (regenerate). For instance, stem cuttings have the ability to form adventitious roots, while root cuttings can grow a new shoot system. Leaves can in fact grow new roots and shoots. A stem and a root or two stems can be grafted (joined) together to produce a new plant. This form of Vegetative propagation is called “grafting”. Avoid example can be seen in the grafted mango plant below:
Farmers, gardeners and horticulturists all make use of vegetative propagation to produce new plants from virtually any vegetative part.
Another good example of Vegetative propagation can be seen in the cassava plant which is propagated by part of its stem. When cassava is ready for harvest, its tubers which are below the ground, are dug out. The tuber may be used for making different food items such as fufu, gari or lafun. It can also be used for making starch or paste (gum).
The leaves on the cassava stems are removed and the stems are cut into
pieces of about 20 – 30cm and up to 1metre in length. The short stems
which are known as stem cuttings are stuck into the ground at the
beginning of a new planting season to propagate cassava. Each stem
cutting has at least four to six buds. The longer stem cuttings of up to
1metre in length give a bigger yield of cassava tubers. When we use part
of a plant other than its seed to get (produce) more types of the same
plant, this is called “Vegetative Propagation“. Can you now name three other crop plants that can be propagated in this way?
Advantages of Vegetative Propagation
1. Vegetative propagation has the advantage of producing new crops which look exactly like the original parent plant. If for instance, you propagate mango trees from a mango plant which is tall and has very many big succulent, sweet, non-fibrous fruits, all the new mango trees will also be tall and carry very many big, succulent, sweet, non-fibrous fruits.
2. Crops that are vegetatively propagated normally have uniform yields, similar crop quality and come into bearing (.e fruiting) early and at the same time.
3. In addition, you can get many new plants from one plant, and very
quickly too. Plants which look differently from all others, but which are special or are considered of valuable material can be easily and quickly multiplied by using this method.
The crops which are propagated vegetatively have some disadvantages
too. For instance, the new plants may not grow uniformly if some of
them are planted on poor soil. In the case of crops like cassava, some of
the cuttings could readily spread the much-leared (dreaded) disease of
cassava Mosaic virus if the parent plant was infected by the disease. This
shows that the disease that was existing in the parent stock i.e. cassava stem cuttings in this case, shows up (is manifested) in the new offspring plants. It is therefore very important that only matured, healthy, disease free and vigorous parent plants are used tor vegetative propagation.
2. Vegetative propagation is generally more expensive than the use of seeds, because the planting materials e.g. stem cuttings are usually bulky, they are not easy to transfer or carry from place to place.
3.They are not easily stored and they require a lot of space for their storage
Done studying? See previous lessons in Agricultural Science
Lesson Evaluation / Test
- What is propagation? Name two methods by which crops can be propagated.
- Give reasons reasons why it is desirable to propagate crops vegetatively.
- What are the disadvantages of propagating crops vegetatively?
- What characters do plant breeders select in an effort to improve the type of crops that they grown.
- How can you prevent the spread of diseases in vegetative propagation?
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