Tipo de contenidos difundidos a través de herramientas Web 2

Sexual reproduction is the fusion of two different sex cells which usually come from two different parents.

The sex cells are known as gametes. In flowering plants, the male gamete is the pollen grain, while the female gamete is the ovule. The fusion of these haploid gametes (pollen grain and ovule) is known as

fertilisation.

It results in the formation of a single-celled zygote which has a diploid number (2n) of chromosome.

The zygote then divides and develops to produce the offspring whose somatic cells are all diploid.

In flowering plants, the flowers is the reproductive organ It bears the stamen that produce the pollen grains and the pistil which contain the ovule in the ovary

3.2.1 ReproductiVe System in Flowering Plants

The flower is the reproductive structure of flowering plants. It contains the male and the female sex organs which enable them to exhibit sexual reproduction. Fertilization inside the flower often leads to the production of seeds which are capable of germinating into new plants.

The Structure of a Flower

A flower is a cluster of modified leaves which is borne on a shortened stem, the flower

stalk

or

pedicel.

The flower is made up of four floral parts. These are (i) the calyx (ii) the corolla, (iii) the androecium (iv) the gynoecium. These floral parts are usually arranged in concentric rings, one above the other, on the

receptacle or thalamus.

The swollen tip of the pedicel is shown in fig. 9.5. and fig. 9.6.

0 The Calyx:

The calyx consists of sepals which are usually small and green. This protects the flower which is in the bud. The outer most whorl of a flower is made up of three to five sepals. They may be separated

(polysepalous)

or joined to form a cup

(gamosepalous).

There may be an

epicalyx

of bract below the calyx as in Hibiscus flower.

Figure 9.5 Longitudinal section of pride of Barbados

Ovary Petal

Sepal Rcecptacl

Episepa

Ovule

Nectar P dicle

Stigmas

Filamcn

Fused styles

Reproduction in Plants 171

Figure 9.6 Longitudinal section of Hibiscus flower

The Corolla:

The petals collectively known as the corolla form the second whorl or floral part inside the sepals. In many plants, they are the eye-catching part of the flower. Most flowers have four to ten petals which may be separated

(polypetalous)

or joined to form a tube

(gamopetalous).

Generally, petals are brightly coloured and scented

(pollinators).

When both the petals and sepals look alike, e.g.

lilies, they are collectively known as the

perianth.

0i0

Androecium:

The androecium are the male reproductive organs of a flower. The whorl inside the petals is a group of stamens collectively known as the

androecium.

Most

stamens

have:

a long slender stalk called the

filament

a swollen end called the

anther

Flowers may have three to numerous stamens which may be free or united. In some flowers, the filaments are united while the anthers are free. In the Hibiscus, the fused filaments from a

stamina! tube.

In the sunflower, the anthers are fused while the filaments are free. Sometimes, the filaments of stamens are attached to petals

(epipetalous).

Anther and Pollen Grains

The anther is a 2-4 lobed structure. Pollen grains are produced in these lobes within the region called

pollen sacs.

When the pollen grains mature, the anther lobes split to release them;

Pollen grains are fine yellowish particles. Each pollen grain is made of

i. Two coats - a tough protective outer coat called exine and a thin inner coat called intine of cellulose.

ii. A haploid

generative nucleus

which gives rise to two male gametes.

haploid

tube nucleus

When a pollen grain lands on the mature stigma of the same type of flower, it germinates to form a pollen tube.

(iv)

Gynoecium:

The gynoecium are the female reproductive organs of a flower. It is the innermost whorl of the floral parts of the flower. The gynoecium consists of

carpels.

It may consist of one, a few or many carpels. A separate carpel or a single structure of several fused carpels is called a

pistil.

A pistil with a single carpel is described as

monocarpous,

e.g. flamboyant, while one with two or more

carpels is known as

polycarpous,

e.g. Hibiscus. When the carpels are free from one another, the pistil is

(i)

said to be apocarpous, e.g. rose flower, but when they are fused together, it is described as syncarpous, e.g. lilies.

Pistils have the following (il ovary (ii) style and (Hi) stigma. The ovary contains one or more ovoid structures called ovules, Each ovule houses a female gamete or egg cell.

After fertilization, the ovary develops into a _fruit while the ovules develop into seeds.

Table 9.1 - Summary of Parts of a Flower and their functions

Parts of a flower Function

(ii Pedicel Attaches the flower to the stem

(ii) Receptacle Carries and holds together the other parts of the flower (iii) Sepals (calyx) Enclose and protect the other floral parts when the

flower is in the bud stage. If brightly coloured, they also attract insects, if green, they make plant food (photosynthesis).

(iv) Petals (corolla) attract insects which pollinate flower (v) Filament holds or carries the anther.

(vi) Anther contains the pollen grains

(vii) Pollen grains produce the male gametes that fertilise the ovules (viii) Stigma receives pollen grains at pollination

(ix) Style connects the stigma to the ovary and it is the passage for the pollen tube to reach the ovules

(x) Ovary contains the ovules, develops into fruit (xi) Ovules produce the female games, develop into seeds

Types of Ovary

The position of an ovary in the receptacle dictates how it is classified. An ovary can be described as superior, inferior and half-inferior (fig. 9.7).

Supreme ovary: An ovary is described as superior when it is placed above the other floral parts, namely the calyx, corolla and stamens on the receptacle e.g. Hibiscus. The flower having this type of superior ovary is described as hypogynous flower.

(ii) Half inferior ovary: An ovary is described as half inferior when the ovary lies inside a cup-shaped receptacle and other floral parts appear to be attached slightly above it or almost at the same level. e.g.

rose flower. The flower having this type of half inferior ovary is described as perigynous flowers.

(iii) Inferior ovary: An ovary is described as inferior when it is placed below the other floral parts, namely the calyx, corolla and stamens on the receptacle. That is, other floral parts are:

Half interior ovary

Figure 9.7 Types of ovary

stamen

vary petal sepal receptacle Superior ovary

petal stamen

ovary sepal receptacle (cup-shaped)

stamen petal sepal ovary receptacle

Inferior ovary

Reproduction in Plants 173

above it on the receptacle, e.g. cana lily, sunflower. The flower having this type of inferior ovary is described as epigynous flower.

Terms Used in Describing Flowers

Inflorescence: Inflorescence is a group of flowers which attached themselves to a common stalk or axis, e.g. Pride of Barbados.

(ii) Solitary flower: A solitary flower is one that is attached singly either to the leaf axis or to the tip of a branch e.g. Hibiscus and pawpaw flower.

(iii) Perfect flower: A perfect flower is one that has both carpels and stamens in it, e.g. pride of Barbados.

(iv) Imperfect flower: An imperfect flower is one in which either stamens or carpels are naturally missing, e.g. maize flowers.

(v) Complete flower: A complete flower is the type that has naturally all the four floral parts namely, calyx, corolla, androecium and gynoecium, e.g. Hibiscus, pride of Barbados.

(vi) Incomplete flower: An incomplete flower is the type which lacks one or more of the floral parts, e.g.

maize and pawpaw flowers.

(vii) Regular flower: A flower is regular if it has all members of a whorl on it, i.e. petals identical in shape and in size and are evenly arranged on the receptacle. Such a flower can be cut vertically into two similar halves through any one of several vertical places (radial symmetry) and is described as actinomorphic flower, e.g. Hibiscus.

(viii) Irregular flower: An irregular flower is one in which the members of a whorl, e.g. petals are hot similar either because some parts are fused, some are smaller than others or because one or more parts are missing. The flower can be cut vertically into two similar halves through only one plane (bilateral symmetry) and is described as zygomorphic flower e.g. pride of Barbados and Delonix.

(ix) Axillary flower: Axillary flowers are the those which are borne in the axils of leaves.

(x) Terminal flowers: Terminal flowers are borne at the end of stems or branches.

(xi) Bisexual flower: A bisexual flower also called a hermaphrodite has both the carpels (female) and tamens (male) on it, e.g. pride of Barbados, Hibiscus.

(xii) Unisexual flower: A unisexual flower is the type that has either stamensor carpels at its sexual parts.

A flower that has only carpels is a female flower and such flower is described as pistillate, e.g. maize, pawpaw. On the other hand, any flower that has only stamens is a male flower hence it is described as staminate, e.g. pawpaw, maize.

(xiii) Monoecious flower: When male and female flowers are found on the same plants, the plant is said to be monoecious e.g. oil palm, maize.

(xiv) Dioecious flower: When male and female flowers are found on different plants, the plant is said to be

The various kinds of placentation (fig. 9.8).

Ovule

Placenta Placenta

free central dioecious plants, the plant is said to be dioecious, e.g. pawpaw.

(xv) Essential parts of a flower: The stamens and carpels are regarded as the essential parts of the flower because they produce the gametes required for fertilisation to take place resulting in the formation of seeds or fruits.

(xvi) Non-essential parts of a flower: The petals and the sepals are regarded as the non-essential parts of a flower because they are not required for gamete production in flowers.

Placentation in Flowering Plants

Placentation is defined as the arrangement of the ovules within the ovary. There are various ways in which ovules arc attached to the ridges of the ovary. These ovules arc attached to the ovary by fleshy structures called placentae (singular: placenta) through short stalks called funicles.

Kinds of Placentation

Marginal

Placenta

Placenta

Figure 9.8 Kinds of Placentation

Axile

Parietal

L

^.- --- Ovule Basal

1. Marginal placentation: In marginal placentation, the ovules are attached to the placenta along one margin of the ovary. Examples are beans, cowpea, pride of Barbados, flamboyant, cassia, crotalaria and Delonix.

2. Parietal placentation: In this arrangement, the ovules are attached to the sides of a syncarpous ovary havng a single chamber, e.g. pawpaw.

3. Free-central placentation: In this type of arrangement, the ovules are borne on a knob which projects from the base of the ovary, e.g. cana lily.

4. Axile placentation: In an axile placentation, the carpels of a syncarpous ovary meet in the centre to form the placenta to which the ovules are attached e.g. tomato.

STOMA OPEN STOMA CLOSED

0 ⁰⁰

O

guard cell thick wall

0 ₀

poky/o⁰

H 04iirsr:

chloroplasts

all

nuclei epidermal cells

stomatal pore (open)

stomata' pore (closed)

H 0

Respiration in Plants 159

while flaccidity of the guard cells causes the closing of the guard cells. Gaseous exchange, especially through the stomata, is made possible when the stomata open. see fig. 8.6.

A. Stoma: how the guard cells open and close a stoma

Figure 8.4 Gaseous exchange structures awl mechanisms in plants

3.4.1 Factors or Conditions Affecting Respiration

1. Oxygen: Presence of oxygen is essential for respiration (Kreb cycle only). The rate of respiration under varying concentration of oxygen varies. If the concentration of oxygen falls below 5%, the rate of respiration rapidly drops. Under this condition more carbondioxide is evolved than oxygen absorbed.

With gradual increase in oxygen concentration, there is a corresponding steady increase in the rate of respiration.

2. Temperature: The minimum rate of respiration is reached at 0oC or even at 10oC. With the rise in temperature, the rate of respiration increases. Beyond 400/u or 45% which is the maximum, protoplasm is injured, particularly affecting enzyme activity, and thus the rate of respiration decreases. The optimum temperature however lies between 30oC and 35oC.

3. Light: The effect of light is only indirect; in bright sunlight, the respiratory activity is greater than in subdued light. This may be due to the fact that in bright light stomata remain wide open and oxygen is easily and quickly absorbed.

4. Supply of Water: Protoplasm saturated with water respires more vigorously than that in a desiccated condition, as in dry seed. Thus with the supply of water the rate of respiration increases.

5. Vitality of Cells: Respiration in young active cells is more rapid than in old cellsAkgetative buds, floral

In document Uso de las tecnologías de información y comunicación de las organizaciones ecuatorianas (página 50-57)