TOPIC 5: FLOWERING PLANTS: STRUCTURE
The flowering plants are the members of the plant kingdom with which we are most familiar. Nearly all our crops, hardwood trees, and garden plants are members of this group.
Flowering plants are also called angiosperms.
They have two major parts: one above the ground called the shoot and another below the ground called the root.
But not all parts below the ground are called roots. For instance, spear grass has an underground stem.
The root has a main root called the tap root and branching roots called lateral roots.
The shoot grows from the plumule of a seed while the root grows from the radicle of a seed.
Not all roots are found below the ground for example the prop roots of maize.
The shoot is made of a stem with leaves, buds and flowers.
The stem has leaves joining it by their bases at the nodes.
The interval from one node to the next is called the internode.
A shoot has a growing point called the terminal bud.
The angle made by the leaf to the stem is called the axil and any bud that arises out of it is called the axillary bud.
Lateral buds form lateral stems or branches.
A bud may grow into flowers; such a bud us called a floral bud.
Flowers growing on the same floral branch form an inflorescence.
LEAVES
A typical leaf like that of the hibiscus, consists of three main parts
1 The lamina or leaf blade,
2 The petiole or leaf stalk.
3 The leaf base.
In many plants (e.g. the rose) the leaf base bears a pair of outgrowths called stipules. These are not found in Crotalaria, and in the Acacia they are modified to form spines.
The edge of the lamina is called the margin. In the hibiscus it is toothed (or serrated), but in other plants (e.g. the orange) it is smooth (or entire). The petiole is continued up the centre of the lamina to form the mid-rib or main vein, which gives off branches on either side.
Typically, leaves are green, and they manufacture the plant's food from raw materials (carbon dioxide, salts and water). This function is chiefly carried out by the lamina, but in herbaceous plants like Crota-laria, it is also performed by all the green parts including the stem. The lamina of a typical leaf is broad and flat so that the exchange of gases between its cells and the air easily takes place.
The leaves described above are called foliage leaves. Some underground stems bear scale leaves, which are not green and do not carry out the function of typical leaves.
Functions of leaves
1. They carry out photosynthesis i.e. they manufacture food like starch from carbon dioxide from the atmosphere and water from the soil using sunlight energy trapped by chlorophyll; oxygen is given off as waste product.
2. They allow transpiration to take place; so that heat can be lost at the atmosphere and more water and salts can be pulled from the soil.
3. They carry out gaseous exchange. At night oxygen enters leaves and carbon dioxide leaves the leaves due to respiration taking place. During day, carbon dioxide enters the leaves and oxygen leaves due to photosynthesis taking place.
4. Some leaves store food for example cabbage, onions.
5. Some leaves carry out vegetative reproduction for example bryophyllum, cactus.
Venation
The arrangement of the veins is called the venation of the leaf, and there are two main kinds.
Reticulate venation: This is where the veins form a network for example in mango, hibiscus. This type of venation is characteristic of dicotyledonous plants.
Parallel venation: This is where the veins run along the leaf and do not meet for example in maize, grass. This type of venation is characteristic of monocotyledonous plants.
Plants like bananas have a main vein with branching veins perpendicular to it and parallel to each other.
Leaf shape
Types of leaves:
Simple leaves: These have one lamina or leaf blade for example mango, hibiscus.
Divided simple or simple palmate leaves: These have one lamina but it is deeply cut into tubes but is not completely divided for example pawpaw.
Compound leaves: This is where the lamina is completely divided into leaflets.
There are four types of compound leaves.
Compound pinnate leaves: This is where there are two leaflets opposite each other along a main stalk for example cassia, eucalyptus.
Compound bipinnate leaves: This is where each pinnate leaflet is divided into pinnate leaflets for example jacaranda, flambouyant, acacia.
Compound digitate (palmate) leaves: This is where the leaflets radiate from the end of the stalk like fingers of the palm for example cassava.
Compound trifoliate leaves: This is a leaf where there are three leaflets for example legumes like beans, peas.
Leaf margins:
The margins of leaves may be all round. Such leaves are called entire leaves for example mango, maize.
Some margins may be rough or saw-like. Such leaves are called serrated leaves for example hibiscus.
Arrangement of leaves on the stem:
Leaves are arranged on the stem in such a way that they all have access to sunlight.
Spiral arrangement: This is where there is one leaf at each node round the stem for example hibiscus.
Alternate arrangement: This is where there is one leaf at each node but the leaves are on opposite sides of the stem for example bougainvillea.
Opposite arrangement: This is where there are two leaves at each node opposite each other for example Lantana.
Opposite
decussate arrangement: This is where there
are two leaves at each node opposite each other and each pair of leaves is at
right angles to the one below it for example guavas.
Whorled arrangement: This is where there are many leaves at each node for example umbrella tree.
Modifications of leaves:
Insectivorous leaves: These are leaves found on plants which grow in areas where there is no nitrogen in the soil; so these plants trap insects, produce an enzyme and digest the insect to get protein for example the pitcher plant.
Storage leaves:
These are fleshy leaves which store food for example cabbages, onions.
Succulent leaves:
These leaves preserve large quantities of water in them for example sisal, cactus. They are mainly found in deserts.
Internal structure of a leaf:
If a leaf's vertical section were taken, it would look like this:
The outer layer is called the epidermis. It has cells which have no spaces between them i.e. it has no intercellular spaces.
There are two epidermal layers: the upper epidermis and the lower epidermis.
The epidermis secretes a waxy layer called the cuticle.
The lower epidermis has small holes called stomata each bounded by two guard cells.
Epidermal cells do not have chloroplasts except the guard cells.
The inner layers of the leaf are called mesophyl.
The palisade mesophyl consists of palisade cells, which carry out photosynthesis because they have many chloroplasts.
Palisade cells have intercellular spaces between them.
The chloroplasts contain chlorophyll pigment.
The spongy mesophyl contains round spongy cells with very large intercellular spaces.
The spongy cells have less chloroplasts than the palisade cells.
FLOWERS
A flower is the reproductive part which produces seeds that are dispersed to grow into new plants.
A half-flower:
A flower has parts arranged in rings or whorls. The four main floral whorls are:
- The calyx which consists of the sepals;
- The corolla which consists of the petals;
- The androecium which consists of the stamens; and
- The gynoecium or pistil which consists of the carpels.
The Calyx:
This consists of sepals which protect the flower in the bud stage. The calyx form the outer most whorl. They are green in colour but in plants like Pride of Barbados they are the same colour as the petals. Such a calyx is said to be petaloid.
In a polysepalous calyx the sepals are not joined to each other for example in allamanda.
In a gamosepalous calyx the sepals are completely joined together for example in hibiscus.
Any floral whorl found outside the calyx is called the epicalyx for example in hibiscus.
The corolla:
This consists of the petals and is the most conspicuous part of the flower due to its bright colours. Some petals have scent for example in rose and coffee.
In a polypetalous corolla the petals are not joined together for example hibiscus.
In a gamopetalous corolla the petals are joined together to form the shape of a tube for example sweet potatoes.
In flowers like cana lily and monocotyledonous plants where the calyx and corolla are not differentiated, the two floral whorls are called perianth.
The androecium
This consists of the male organs called stamens. Each stamen consists of a filament at the end of which is an anther which produces pollen grains. Stamens grow from receptacle but in some flowers like hibiscus, they grow on the style. So they are epicarpelous. In cana lily they are attached to the petals, so they are epipetalous.
The gynoecium (pistil):
This consists of the carpels which are the female part of a flower.
A carpel consists of:
- A stigma which receives pollen grains;
- A style through which the pollen passes; and
- An ovary with ovules which contains the female egg nucleus.
In a monocarpous pistil, there is only one carpel for example in beans, peas, crotalaria, mango.
In an apocarpous pistil, there are more than one carpels which are entirely separate from each other for example in rose flower.
In a synocarpous pistil, there are more than one carpels which are all or at least their ovaries are joined together for example hibiscus, orange, tomato.
PLACENTATION:
This is the arrangement of ovules in the ovary. An ovule gets nutrients through a placenta.
There are four main types of placentation:
Marginal placentation:
This is where the ovary is from a single carpel (monocarpous) and the ovules are attached to its edges for example in beans, mango, crotalaria, cassia.
In monocarpous ovaries with one large seed for example drupes, there is basal placentation.
Axile placentation: This is where the ovaries of a synocarpous pistil have edges which are pushed in to form a central axis in the middle on which the ovules are born for example tomatoes, oranges, lemons.
Parietal placentation: This is where the ovaries of a synocarpous pistil are joined at the edges and the ovules are attached to the walls of the ovaries forming single chamber for example in the pawpaw.
Free-central placentation: This is where the ovaries of a synocarpous pistil have a large chamber and the ovules are borne on a knob which projects from the base of the ovary inwards for example in jack fruits, cucumber.
N.B: In central placentation, the projecting knob touches the roof of the ovary for example in pepper.
Variation in the shape of receptacles:
There are there shapes of receptacles:
In a hypogynous flower the receptacle is conical with all the four floral whorls arranged in order on it so that the ovary is on top. Such an ovary is said to be superior for example in cassia, beans, hibiscus.
In a perigynous flower the receptacle is cup-shaped with the ovary at the centre and the other floral whorls are attached at the edge of the receptacles for example the rose flower. The ovary in this case is also superior.
In an epigynous flower the ovary is completely sunk into the receptacle and the other floral whorls are on top of the receptacle. The ovary in this case is said to be inferior for example in apples, guavas.
Some terms used in describing flowers:
A complete flower is a flower which has all four floral whorls present for example hibiscus. An incomplete flower is a flower which has one or more of the floral whorls missing for example pumpkins, pawpaw.
The non-essential organs of a flower are the calyx and the corolla.
The essential organs of a flower are the reproductive organs i.e. the androecium and the gynoecium.
An irregular flower is a flower which has only one line of symetry for example dutchman's pipe, bean, crotalaria.
A regular flower is a flower which has more than one line of symetry for example sweet potatoes, hibiscus.
A hermaphrodite flower is a flower which possesses both stamens and carpels for example hibiscus, canalily.
A unisexual flower is a flower which possesses either stamens alone or carpels alone for example pawpaw, pumpkin.
A staminate flower is a flower which possesses stamens alone for example pumpkins, maize, pawpaw.
A pistillate flower is a flower which possesses carpels alone for example pumpkin, maize, pawpaw.
A monoecius plant is a plant which possesses both staminate and pistillate flowers for example maize, pumpkin.
A dioecius plant is a plant where the staminate flower and pistillate flowers are born on separate plants for example pawpaw.
Floral diagram:
This is a diagrammatic representation of flower parts using concentric circles for each floral whorl. A floral diagram gives more information about a flower than would a diagram of a half flower.
Below is a floral diagram of a hibiscus flower:
The gynoecium is shown having five carpels joined together (synocarpous).
The androecium is shown having numerous stamens joined to the pistil (epicarpalous).
The corolla is shown having five separate petals (polypetalous).
The calyx is shown having five joined sepals (gamosepalous).
The floral formula:
This is the use of symbols to represent flower parts and their arrangement. It is quicker and gives more information about a flower.
POLLINATION
This is the transfer of pollen grains from the anthers to the stigma. It is carried out by pollinating agents like wind, insects, birds and bats. There are two types of pollination:
Self pollination: This is the transfer of pollen grains from the anthers to the stigma of the same flower or another flower on the same plant.
Advantages of self pollination:
¨ Good characteristics of the plant are maintained.
¨ There are high chances of it taking place.
Disadvantages of self pollination:
¨ No new characteristics are got.
¨ Poor characteristics are maintained.
Characteristics of self-pollinated flowers/plants:
- They have hermaphrodite flowers.
- The stamens and carpels ripen at the same time.
- The stigma and the anthers are close to each other.
- The anthers or male flowers are higher than the stigma or female flowers.
- The anthers and stigma are enclosed.
Cross-pollination: This is the transfer of pollen grains from the anthers of a flower to the stigma of another flower on a different plant but plants of the same species.
Advantages of cross pollination:
New characteristics are acquired.
Poor characteristics can be eliminated.
Hybridization can be carried out.
Disadvantages of cross-pollination:
Poor characteristics can be acquired.
Good characteristics of a plant can be lost.
It requires agents because there are fewer chances of it taking place.
Characteristics of cross-pollinated plant:
The anthers ripen earlier than the stigma (protandry).
The stigma ripens earlier than the anthers (protogyny).
They are dioecius i.e. male and female flowers are on separate plants.
The stigma is higher than the anthers.
They have self-sterility or incompatibility. This is where pollen grain of the same plant fails to germinate on the stigma because stigma inhibits it.
They have two types of flowers (heterostyl) for example primrose with pin-eyed flowers and thrum-eyed flowers.
Characteristics of insect-pollinated flowers:
- They have large and brightly coloured petals.
- They produce nectar.
- They have a sticky stigma.
- They produce relatively less quantities of heavy pollen grains.
- The pollen grains have spikes which make them easily attached to the bodies of insects.
- They have scent for example rose and coffee.
- The stigmas are higher than the anthers.
- The essential organs are enclosed in the flower.
- Their nectar is positioned strategically and there are pollen guides.
- An example of an insect-pollinated flower is the sweet pea (crotalaria).
The insect that pollinates the above flower must be heavy, like a bee in order to be able to depress the keel so that the anthers and the stigma burst out of it.
Characteristics of wind-pollinated flowers:
- The Perianth is dull coloured.
- They have long stigmas on feathery styles.
- They produce large quantities of light pollen grains.
- The anthers are higher than the stigma.
- The essential organs are not enclosed but are hanging out.
- The pollen grains are winged.
- They do not produce nectar.
- They do not produce scent.
An example of a wind pollinated flower is maize and panicum which is hermaphrodite.
Fertilisation in flowers:
When a pollen grain lands on the right stigma, the following events happens:
- The pollen grain absorbs water and nutrients from the stigma.
- It germinates to form a pollen tube.
- The pollen grain penetrates through the style.
- The penetration is led by a pollen tube nucleus.
- A generative nucleus in the pollen tube divides to form two male nuclei.
- The pollen tube reaches the ovary and heads towards the ovule.
- The pollen tube bursts open on reaching the ovule.
- The pollen tube nucleus disappears.
- The two male nuclei enter the ovule through the micropyle.
In the ovule is an embryo sac which contains the egg nucleus or ovum, two synergid cell which attracts the pollen tube, (two) polar nuclei and three antipodal cells.
The ovule is surrounded by layers called integuments and is attached to the ovary wall by the placenta which supplies it with nutrients.
On entering the embryo sac, the first male nucleus fuses with the egg nucleus or ovum to form a zygote.
The second male nucleus fuses with the polar nuclei to form a secondary nucleus or endosperm nucleus. These two fertilization results in what is termed as double fertilization.
Reproduction in flowering plants begins with pollination, the transfer of pollen from anther to stigma on the same flower or to the stigma of another flower on the same plant (self-pollination), or from anther on one plant to the stigma of another plant (cross-pollination). Once the pollen grain lodges on the stigma, a pollen tube grows from the pollen grain to an ovule.
Two sperm nuclei then pass through the pollen tube. One of them unites with the egg nucleus and produces a zygote. The other sperm nucleus unites with two polar nuclei to produce an endosperm nucleus. The fertilized ovule develops into a seed.
The results of fertilisation:
- When the first male nucleus fuses with the egg nucleus or ovum, a zygote is formed.
- They zygote forms the embryo and cotyledon of the seed.
- When the second male nucleus fuses with the polar nuclei, a secondary nucleus or endosperm nucleus is formed.
- The secondary nucleus or endosperm nucleus becomes the endosperm of the seed.
- The antiodal cells and the synergid cells disappear or degenerate.
- The integuments form the testa of the seed.
- The placenta later leaves a scar on the seed called the hilum.
- The ovule becomes the seed.
- The ovary becomes the fruit.
- The stigma, style, stamens and the petals whither off.
- The sepals may persist on the fruit.
FRUITS
A fruit is a structure which developed from an ovary whose ovules have been fertilized.
Functions of a fruit:
¨ It encloses and protects the developing seeds.
¨ It assists in seed dispersal.
¨ It supplies seeds with nutrients through the placenta.
Characteristics of fruits:
¨ They have a pericarp - this is a cover which develops from the ovary wall.
¨ They have a seed or seeds.
¨ They have a scar left by the style.
¨ They have a stalk or a scar left by the flower stalk.
Differences between fruits and seeds:
¨ A fruit contains seeds but a seed does not contain other seeds.
¨ A fruit has a stalk or a scar left by the stalk whereas a seed has only one scar called a hilum left by the placenta.
A seed has a micropyle whereas a fruit does not have it.
Types of fruits:
There are three major types of fruits:
- Simple fruits: These are formed from an apocarpous or synocarpous pistil for example beans, oranges.
- Aggregate fruits: These are formed from an apocarpous pistil for example rose.
- Multiple fruits: These are not formed from one flower but from a whole inflorescence of flowers for example pineapples, strawberries.
Types of simple fruits:
There are three types of simple fruits:
Dry indehiscent fruits: These are fruits which dry but do not split to release the seeds.
Each fruit contains one seed.
Example include:
A caryopsis where the pericarp is fused with the testa of a seed for example maize, wheat and other grains.
A cypsela which develops from an inferior ovary and has hairy hooks or silky hairs for example black jack (Bidens pilosa) Tradax.
An achene which has a tough membranous pericarp for example sunflower.
A nut where the pericarp is hard and woody for example cashew nut. (N.B: Groundnuts and coconuts are not true nuts).
A samara which has winged pericarp that aids in dispersal for example jacaranda.
Dry dehiscent fruits: These are fruits which spilt to release the seeds when dry but the fruit remains attached to the plant. Each fruit contains many seeds.
Examples include:
Legumes which split along two lines of weakness (sutures) like the beans, peas.
Follicles which split along one line like the rose periwinkle.
Capsules which split along several slits, like dutchman's pipe.
Schizocarps which split in several one-seeded parts like the desmodium.
Succulent fruits: These are fruits that have a thick pericarp; are fleshy and their outer surfaces are brightly coloured when mature.
Examples of succulent fruits include:
Drupes which have one large seed and a hard woody endocarp which surrounds the seed for example mango, ovacado.
Berries which have many seeds which are not surrounded by an endocarp for example tomatoes, oranges.
Parthenocarpy:
This is the development of fruits from the ovary without fertilization like in plants where pollination is unnecessary for example bananas.
In fact parthenocarpy applies to all seedless fruits for example pineapples.
Fruit and seed dispersal:
This is the scattering of mature seeds and fruits away from the parent plant.
Advantages of dispersal:
- It reduces overcrowding among members of the same species.
- It reduces competition for light, air, water and mineral salts.
- It enables plants to adapt to different conditions.
- It results in colonization of new and better areas.
One disadvantage of dispersal is that seeds may end up in unfavourable conditions where they die.
Agents of dispersal include wind, water, animals and self-explosive mechanisms.
Characteristics of seeds dispersed by wind:
They are small and light for example grass seeds.
They have hairy structures for example silk cotton and tridax seeds.
They have wing-like structure for example jacaranda and dutchman's pipe seeds.
They have a censer mechanisms where dry seeds can be shaken out of the fruits for example poppy.
Water dispersal:
This is not a common method of dispersal because seeds usually lose viability (ability to germinate) when they are left in water for a long time.
Characteristics of fruits dispersed by water:
They have air spaces in them to increase buoyancy so that they can float.
They have little weight.
They have a tough fibrous coat which does not allow water to penetrate for example water lily.
Another example is the coconut which can be carried for hundreds of miles by water and can still germinate even after a hundred days.
Characteristics of seeds and fruits dispersed by animals:
They have hooks so that they can be attached to the hairy coats of mammals or feathers of birds for example black jack.
They are sticky so that they can cling to the bodies of animals for example desmodium.
They are succulent so that only part of the fruit is eaten and the seeds are thrown away for example mango, orange, avocado.
They have woody seed coats for example guavas or slimy seed coats for example tomatoes so that the seeds pass through the alimentary canal of animals undigested.
Their epicarps become brightly coloured when ripen to attract pollinating agents like man, monkeys and birds.
Self-explosive dispersal: Seeds dispersed by this method:
Have unequal drying of the pericarp such that a tension is set up that leads to the explosion that ejects the seeds.
Have a pericarp that increases in turgidity during ripening causing a tension which also explodes the fruit to release the seeds.
Examples include the dry dehiscent fruits like the beans, peas.
Summary
In order to describe the structure of any flowering plant, these must be considered:
1. The structure of the leaves.
2. The structure of the stem.
3. The structure of the roots.
4. The parts of the flower (floral formula).
5. Its placentation.
6. The method of pollination.
7. Its fruits and how they are dispersed.