perpetuation of life

EARTH AND LIFE SCIENCE | GRADE 11/12

Unit 17 Perpetuation of Life

All living organisms has a certain limit to their existence. Some live for many years, others live for few years, and some are even limited to a few days. Lifespan pertains to the length of time an organism lives. An example is an adult mayfly that has a lifespan of one day, a mouse that lives for 1-2 years, and a tortoise that lives for a very long period of time that could reach about 152 years. In this process, can you imagine what will happen to a species if there are no new generation of individuals that will replace them? What will happen to the plants if seeds fail to germinate and form new plant individuals? No more species live, and it gets extinct. Reproduction in living things is essential for the survival of all species. Reproduction is the process where new individuals from the same species are created. As an individual, living organism does not need reproduction to survive,

Copyright © 2018 Quipper Limited 3

2. Put four toothpicks on four sides of the onion bulb. 3. Place the onion bulb on the mouth of the jar with water. 4. Make sure that the bottom portion of the jar will be submerged in water. 5. Adjust the toothpick to make sure that the onion will not be totally soaked in

the water. 6. Observe the growth of the roots at the base of the onion for three days.

Guide Questions:

1. What is asexual propagation? 2. Did the onion root emerged after three days? How plants are capable to

asexually propagate? 3. Why do think onion was used for the activity? Does all plants are capable to

undergo asexual propagation? 4. What do you think is the importance of asexual reproduction in plants?

Learn about It

Asexual Reproduction in Plants Asexual reproduction occurs when new plants arise from only one parent. Asexual reproduction form genetically identical plants that are similar to the parent plant because no fertilization happen where mixing of male and female gametes or sex cells takes place. Commonly, these plants survive in stable environmental conditions as compared to plants produced from sexual reproduction because they carry genes identical to their parents. Types of Asexual Reproduction in Plants The types of asexual reproduction in plants are apomixis and vegetative propagation. The following discussion will focus on these particular types of asexual reproduction. Apomixis Apomixis is a type of asexual reproduction in plants that produce seeds without fertilization. Unfertilized flowers are able to produce offspring. Apomixis always yields offsprings that are genetically identical to their parents. Hence, this type of reproduction ensures that the parent plant is able to reproduce an offspring that is well suited to the environment where it is acclimated. One way to enable the


survival of apomictic offspring is through seed dispersal. When the seeds are dispersed in similar habitats as their parents, they have a good chance of survival. Examples of plants that undergo apomixis include orchids, some citrus species, and dandelions. In dandelions, bright yellow flowers attract more insects that aid in pollination; thus, triggering seed formation but not fertilization.

Fig. 1. Life cycle of dandelions showing its apomictic nature.

Vegetative Propagation Also known as vegetative reproduction, it is the most common type of asexual reproduction. It involves plant parts that produce buds which develop into new individuals. Vegetative propagation involves two processes: fragmentation and regeneration. Fragmentation is the separation of plant parts from the parent plant. Regeneration, on the other hand, is the development of the separated plant parts into new individuals.

Fig. 2. A fragment of a parent plant that is planted in soil which undergoes

regeneration and produces a new individual plant.


Vegetative propagation involves reproduction through vegetative plant parts and do not utilize any sex cells. Lower nonvascular plants like mosses and liverworts use spores as their vegetative structures while higher vascular plants can use their roots, leaves, or stems. This form of asexual reproduction can either occur by natural methods or artificial methods. Natural methods of vegetative reproduction can be through roots, stems, or leaves. Natural Vegetative Propagation Natural vegetative propagation refers to the development of a new plant from leaves, roots, or stems of a mature plant. The produced plants grow and develop without human intervention.

Fig. 3. Vegetative structures of wild strawberry, onion, and potatoes.

Roots One common vegetative propagation is the generation of new individuals from roots. Most roots that develop asexually form adventitious roots that can form plant structures other than the root, like stems or leaves. Through adventitious roots, new plants can arise from the stems, roots, or leaves of a parent plant. So when roots are planted in the soil, new plants are produced. Some common examples of plants that generate from roots include sweet potatoes and dahlia.


Fig. 4. Asexual reproduction in sweet potato through the

swollen adventitious roots. Stems Vegetative reproduction through the stems is considered as the most efficient type of propagation. In most perennial plants, buds grow in stems. The stem with buds serves as the vegetative structure for propagation. The types of vegetative structures include rhizomes, bulbs, corms, tubers, runners, and suckers.

● Rhizomes are underground stems that alternate with lateral and adventitious roots. They grow below the ground and are commonly mistaken for roots or bulbs. One example of plants with this structure is ginger.

● Tubers are swollen, underground stem which contains axillary buds on the scale leaves or ‘scars’ (‘eyes’) present on the stem. An example of a tuber is the potato.

● Bulbs are modified stems which have one or more axillary buds which form the shoots of the new plant. Onion is an example of a bulb.

● Corms are swollen, underground stems that are vertically oriented. They consist of a swollen base surrounded by scale leaves. One or more buds may grow into a new plant. One example of plants that propagate through corms is Colocasia.

● Runners or stolons arise from the body of a plant. They grow above ground and occasionally give rise to roots. The runners decay once the new plant arises. An example of a plant exhibiting this structure is strawberry.

● Suckers are lateral branches coming from the underground part of the stem. They grow upwards, giving rise to a new plant. Common examples of plants with this structure include bamboo, pineapple, and chrysanthemum.


rhizome tuber bulb

corm runner sucker

Fig. 5. Different types of modified stem for vegetative propagation.

Leaves Leaves can also reproduce new individuals via vegetative propagation. An example of plant that exhibits this type of propagation include the mother of thousands plant (Bryophyllum daigremontianum). In this plant, the plantlets arise from the leaf margins. These plantlets may compete with the root system of the parent plant when they are able to establish themselves in the soil.


Fig. 6. Vegetative propagation through the leaves that create leaf buds of new

individual plant. Artificial Vegetative Propagation Artificial vegetative propagation is an asexual reproduction requires human intervention. In agriculture, artificial methods of vegetative propagation are often done. The crops are reproduced by vegetative means for product quality or ease in propagation. They include potatoes, which are propagated from tubers, and sugarcane, which are propagated by stem cuttings. Artificial methods include cutting, grafting, and layering. Cutting is the most common method of artificial propagation. Types of cutting include stem cutting, root cutting, and leaf cutting.

● Stem cutting uses stems that are cut into smaller pieces of about three inches to two feet long. Each cutting must have at least two nodes and internodes. The cut stems are placed in moist soil or partially immersed in water. They give rise to adventitious roots, and they grow into new plants. Some examples of plants propagated by stem cuttings include roses, bougainvillea, and potatoes.

● Root cutting involves roots from dormant plants which are cut into pieces of about one to three inches long. The cut roots are placed horizontally in moist soil. They give rise to roots and shoots, forming new plants. Some examples of plants that are propagated through root cutting include tamarind and lemon.

● Leaf cutting involves cutting succulent leaves which are collected with their petioles. The leaves are placed in a jar filled with water for one week for adventitious roots to grow at the base of the petioles. They are then planted in moist soil, exposing the leaf blades. Eventually, new plants grow from the leaves. Some examples of plants propagated using leaf cutting include begonia and peperomia.


stem cutting in bougainvillea

root cutting leaf cuttings in begonia

Fig. 7. Examples of cuttings. Layering is a type of propagation where the roots are developed in the stem while the stem is still intact to the plant. The branch of a plant is bent towards the ground and then covered by soil while the tip of the branch is still exposed. New roots arise from the part of the branch which is covered with the soil. This portion with the roots is separated from the parent plant and grown as a new plant. Examples of plants propagated by layering include climbing roses, honeysuckle, and lilac.

Fig. 8. Layering method for vegetative propagation.

Grafting is a type of propagation that involves joining tissues of plants and allowing them to grow together. The upper part of the joint plant is called the scion while the lower part is called the stock. Examples of plants propagated by grafting include watermelon, eggplant, and tomato.


Fig. 9. Grafting of stem for vegetative propagation.

Key Points

● Asexual reproduction occurs when new plants arise from only one parent. The new plant is genetically identical to the parent plant due to the absence of fertilization.


● The types of asexual reproduction in plants are apomixis and vegetative propagation.

● Apomixis is a type of asexual reproduction in plants that produce seeds without fertilization. Unfertilized flowers are able to produce offspring.

● Vegetative propagation is the most common type of asexual reproduction. It involves plant parts that produce buds which develop into new individuals. Vegetative propagation involves two processes: fragmentation and regeneration.

● Vegetative propagation can either occur by natural methods or artificial methods. Natural methods of vegetative reproduction can be through roots, stems, or leaves.

Web Links

To learn more about asexual reproduction, you can check the following web links:

● Want to know some interesting facts about asexual reproduction in plants? Visit this site: Encyclopedia.com. 2015. ‘Ásexual reproduction.’ https://www.encyclopedia.com/plants-and-animals

● Craving for more? Here are more information on asexual reproduction in plants. Visit this site: 7 Active Studio. 2017. ‘Asexual Reproduction in Plants.’ Video. https://youtu.be/7zzp37y5DFg

● Want to watch an actual video of plant grafting? Visit this site: NativNurseries. 2014. ‘’Grafting trees.’ Video. https://youtu.be/Ryf0248Od60

Check Your Understanding

A. Using the table below, compare the given topics below.


Lesson 17.2: Sexual Reproduction in Plants

Objectives In this lesson, you should be able to:

● understand the general mechanisms of sexual reproduction in plants; and

● identify the different forms of sexual reproduction in plants.

Sexual reproduction is a form reproductive strategy where new individual is formed following the union of two gametes or sex cells. This is very normal to animals like humans and also exhibited in plants. How does sexual reproduction happen in plants?

Warm-Up Flower Dissection All flowers are important in order to allow sexual reproduction in plant. Flower structure varies from one species to another. It contains parts that are essential for pollination to properly occur. In this activity, you will be introduced to sexual reproduction in plants. Materials:

● gumamela flower ● magnifying glass ● razor

Procedure:

1. Locate the sepals of the flower. Count the number and describe its features. 2. Identify the petals. Count the number and describe its features. 3. Find the stamen which is the male part of the flower. Record the number and

describe its features.


4. The last remaining part is the female parts which is the pistil. Record the number and describe its features.

5. Complete the table by putting the number of each individual part present, its illustration, and description of its function.

Flower part Number Illustration Function

Sepal

Petal

Stamen

Pistil

Guide Questions:

1. What do you think is the advantage of having a colorful flower? 2. Why do you think sepals and petals referred to as “accessory parts” (of the

plant)? 3. Why is it important to elevate the anthers? 4. Why do you think flowers contain higher number of stamen than pistils?


Learn about It

Angiosperms or flowering plants contain seeds enclosed in an ovary that is in the form of a fruit. On the other hand, gymnosperms have no flowers or fruits, instead, it has its seeds unenclosed or “naked” on the surface of scales that form the cone. Other things that differentiate angiosperms and gymnosperms are the flowers, fruits, and endosperm in the seeds. There are two types of plants that propagate through sexual reproduction. Sexual Reproduction in Angiosperm Parts of a Flower Flowers are the reproductive organs in angiosperms. They consist of the androecium (male structure) and gynoecium (female structure). Androecium is the male structure of a flowering plant that consists of whorls of stamen. The stamen is comprised of the filament and the anther. The filament is a long, slender stalk that holds the anther. The anther produces the pollen grains (male reproductive cells). Gynoecium is the female reproductive structure of a flowering plant. It may consist of a single carpel (monocarpellary) or may have several carpels (multicarpellary). The carpel is made up of the stigma, the style, and the ovary. The style is a slender stalk that supports the stigma. The stigma is the sticky part that receives the pollen. The ovary is the basal sac that contains the ovules (female reproductive cells). Pollination in Angiosperm Pollination unites the male and female reproductive cells or gametes. It takes place once the pollen grains reached the female stigma. There are three types of pollination. They are autogamy, geitonogamy, and xenogamy.


Fig. 10. Sexual reproductive structures of a flower.

Autogamy is a form of pollination where pollen are transferred to the stigma of the same flower. Geitonogamy is a form of of pollination where pollen are transferred to the stigma of another flower but of the same plant. Xenogamy, also called cross-pollination, is a form of pollination where pollen are transferred to the stigma of a different plant. These forms of pollination are illustrated in the figure below.


Fig. 10. Forms of pollination.

Pollination is made possible through different agents in pollination. Abiotic agents are nonliving things that aid in the transfer of pollen grains from the anther to the stigma. Examples of abiotic agents include wind and water. Biotic agents are living things that aid in pollination. Examples of biotic agents include animals and humans. After pollination, if the received pollen is of the right type, the gynoecium recognizes and accepts the pollen to promote post-pollination events that lead to fertilization.

Fig. 11. A bee getting nectar from a flower where pollen grains gets attach to the

bee’s body.


Fertilization in Angiosperm Fertilization takes place when the sperm (germinated pollen) unites with the egg (ovule) forming a fertilized egg called a zygote. The process of fertilization involves the following steps:

1. The pollen grain attaches to the stigma. 2. Each pollen grain becomes a part of the pollen tube, which grows down the

neck of the style and reaches the ovary. 3. Sperm cells are discharged into the embryo sac, fertilizing the egg cell.

Fig. 12. Elongation of the pollen tube during pre-fertilization process in angiosperm. A double fertilization happens when the pollen grain enters the ovary and releases two sperm cells. One sperm cell unites with the egg cell forming a diploid cell or zygote. The other sperm cell bonds with two polar nuclei forming a triploid endosperm nucleus. This means that the fertilized endosperm has three set of genetic materials from the parent plant. The process of double fertilization involves the following steps:

1. The pollen grains stick to the stigma containing two cells, generative and tube cell.

2. The pollen tube cell grows along the style while the generative cell moves inside the pollen tube. It then divides into two sperm cells.

3. The pollen tube then penetrates the micropyle or the opening of the ovule.


4. Once inside the ovule or the embryo sac, one of the sperm fertilizes the egg, forming a diploid zygote. The other sperm then fertilizes the polar nuclei, and the triploid endosperm is formed. This endosperm will become the food source of the growing embryo.

Fig. 13. Double fertilization in angiosperms.

Post-fertilization in Angiosperm During post-fertilization, the zygote develops into an embryo while the endosperm nucleus develops into the endosperm. After fertilization, the ovule, which contains the embryo and the endosperm, matures into a seed while the ovary forms the pericarp of the fruit. Fruits are considered as the mature ovary or ovaries of one or multiple flowers. The pericarp is the fleshy part of the fruit that is often edible. These are tissues derived from the walls of the ovary. The pericarp has the endocarp, the inner layer; mesocarp, the middle layer, and the exocarp, which serves as the peeling of the fruits.

Fig. 14. Basic part of an angiosperm fruit.


The seed is the beginning of the next generation. It grows into a seedling, which then grows into a mature plant. The mature plant then produces flowers that contain the reproductive cells.

Fig. 15. Life cycle of a flowering plant.

Sexual Reproduction in Gymnosperm Gymnosperms have enclosed or “naked” seeds on the surface of their leaves or scales. Unlike angiosperms, gymnosperms do not have flowers and fruits. Their ovules, which become seeds, are on the surface of a scale or modified leaf. Examples of gymnosperms include cycads, conifers, gnetophytes, and ginkgos.

Fig. 16. The types of gymnosperms: gnetophyte (upper left); gingko (upper right);

cycads (lower left); and conifer (lower right).


Parts of a Cone Gymnosperms have reproductive parts called cones. They produce two kinds of cones: the male and the female cones. The male cones produce the pollen while the female cones contain at least one ovule.

Fig. 17 The male (left) and female (right) cones of conifers.

The male cone are usually smaller and more soft than its female counterpart. It clump in groups at the tip of the twigs of gymnosperms. Commonly, male cone has scales where the two pollen sacs are located. During spring or early summer, the pollen sacs open and release the pollen grains. Each pollen grain has two air bladders that aid in its dispersal. On the other hand, the female cone is usually attached to the branch of the stem of the gymnosperm. The scales of female cone grow in a helical fashion. The scales overlap to each other like fish scales.


Pollination in Gymnosperms In gymnosperms, the main pollinating agent is wind. The wind carries the pollen from the male cones to the female cones. A sticky substance secreted by the ovule collects the pollen. After pollination, the ovule closes and seals in the pollen. The sperm cell fertilizes the egg cell, and they form the zygote.

Fig. 19 Dispersal of pollen grain from male cone of gymnosperm for pollination.

Fertilization in Gymnosperms Most of the time, the female cones grow at the end of the branches for easy access by the pollen grain. When pollen grains are released through the action of the wind, some get stocked between the scales in the female cones where two ovules are located. When the pollen reaches an ovule through the elongation of the pollen tube, the egg becomes fertilized. The result is an embryo that starts to grow, protected by sporophytic tissue. A developing gymnosperm seed takes over a year to mature. Post-fertilization in Gymnosperms For seed dispersal, the scales on the cones of some species of gymnosperm escape from the cone when fully developed. In some instance, cones fall to the ground when it matures and then release the seeds it contain. Cone seeds have wings


attached to the seeds that helps in the proper dispersal of the seeds. When seeds land, it germinate if the conditions are favorable. Just like the angiosperm, the first structure to burst out of the seed will develop into a root to anchor itself on the ground and start getting nutrients. The stem will grow longer and upwards, lifting the rest of the seed off the ground.

Fig. 20. Fertilization process in male and female cone gymnosperm.


Fig. 21. Life cycle of a gymnosperm.

Key Points

● Angiosperms or the flowering plants have seeds enclosed within the fruit. On the other hand, gymnosperms have no flowers or fruits, instead, it has cones.

● Flowers are the reproductive organs in angiosperms. They consist of the androecium (male structure) and gynoecium (female structure).

● Fertilization may be a double fertilization. This happens when the pollen grain enters the ovary and releases two sperm cells. One sperm cell unites


perpetuation of life

Documents