Classification within Kingdom Plantae

There are so many different kinds of plants all around you. And when

you are eager to know more about them how do you go about doing

it? Do you see each and every single plant and try to learn about it? Or

can dig into biology books to know how and why plants are classified

in Kingdom Plantae.

The Kingdom of Plants – Kindom Plantae!

The best way to learn about plants is to first understand how the

classification occurs within Kingdom Plantae. Once you know the

basic stuff, it then becomes easier to know in detail about each plant.

Biologist Whittaker gave us the Five Kingdom Classification,

classifying all the living organisms into five kingdoms – Protista,

Monera, Fungi, Plantae, and Animalia. To know more about plants, it

is essential to know more about the Kingdom Plantae or in simple

words the plant kingdom.

When you enter this kingdom, all you will find is plants and plants

and more plants. You will be amazed to see the different kinds of

plants that are included here. You can see microscopic plants like the

algae as well as the tallest trees like the Sequoia.

Some plants have the most beautiful and attractive looking structures

called flowers, while some don’t have any. In some plants, you can

see a proper root system, shoot system, and leaves, while in the

simpler plants, it is just a thalloid structure. So, how did the early

biologists classify plants?

Classification within Kingdom Plantae

Very early on, biologists used the superficial features of plants and

classified them based on these features. Some of the morphological

features included colour, number, the shape of leaves as well as the

habitat, etc.

This system was considered as an artificial system of classification, as

plants were classified based on vegetative characters. These vegetative

characters can always show changes because of the effect of the

environment. Therefore, many closely related species were classified

under different divisions.

As biologists became more aware of the other characteristics of plants,

they slowly began another system of classification, called the natural

system of classification. This system considered the external and

internal features of plants while classifying them.

Read about Plant Life Cycles and Alternation of Generations here

However, currently, the phylogenetic classification is being followed.

This is based on evolutionary relationships between various

organisms. This is more advanced. It is assumed that all organisms

that are classified under one heading will have the same common

ancestor. Thus many difficulties in classification have been overcome,

by following this system.

And so, as organisms evolved various classification systems have

come into being. The current system of classification that is being

followed is widely accepted. According to this, the Kingdom Plantae

has been divided into five major groups. They are:

● Thallophyta

● Bryophyta

● Pteridophyta

● Gymnosperms

● Angiosperms

Each group of plants has special and unique features that belong

exclusively to that group. While the thallophytes are the simplest of

the plants, the angiosperms are plants with a complex structure and a

very well developed vascular system and reproductive system.

The gymnosperms are the group of plants that have an evolutionary

significance. These are the first plants to have developed seeds, which

is an efficient mechanism of continuing the plant generation at places

away from the parent plant.

Solved Questions For You

Q: What are the three types of classification?

Ans. The three types of system of classification are:

● Artificial system

● Natural System

● Phylogenetic System

Algae

When you think of plants, you probably will think of the tall trees and

the bushy shrubs. But have you ever given a thought to the smaller

versions of plants, even the microscopic ones? Yes, that is what algae

are, microscopic plants! Let us learn about algae classification and

features.

What are Algae?

(Source: Britannica)

Generally, many do not bother much about these organisms called

algae which are chlorophyll bearing simple organisms. But as science

students, it is imperative to know more about them. You must be quite

familiar with seaweeds and pond scums. These are nothing but algae!

These are simple living organisms that have chlorophyll. They are the

simplest forms of producers in a food chain. They can be single-celled

or multicellular. Known to be largely aquatic, algae have a thalloid

structure, without much differentiation. You can find algae in a variety

of habitats such as freshwater, marine, moist stones, wood, and even

soil. A mutual association is found in between fungi and algae, leading

to an entirely new organism called the lichens.

What is more interesting about algae is that they are economically

very important. They are a source of crude oil and also for many

pharmaceutical and industrial products that are used by humans.

Unique Features of Algae

(Source: Britannica)

Algae can be microscopic or even as large as 60 meters in length.

They can exist singly or in colonies like the Volvox or may be

unicellular like Chlamydomonas or may even have a filamentous

structure like Spirogyra and Ulothrix.

The unique feature of algae is the ability to perform photosynthesis.

This is what differentiates them from other organisms and is also the

main reason for their inclusion in the plant kingdom. The mode of

nutrition is naturally autotrophic mode.

The mode of reproduction that is found in algae is either by vegetative

mode or asexual mode or sexual mode. Vegetative reproduction

occurs by fragmentation wherein each fragment develops into a

thallus. Asexual reproduction occurs by the production of spores,

called the zoospores. These zoospores have flagella that make them

motile. On germination, they give rise to new plants.

Through the fusion of gametes, sexual mode of reproduction occurs in

some algae. Depending on the size of the gametes fusing, sexual

reproduction could be of isogamous( similar size), or anisogamous (

dissimilar size) or oogamous, when a large female gamete and a

smaller male gamete fuse.

(Source – Microbiology online)

Algae Classification

There is three main Algae classification:

● Chlorophyceae – These are called green algae, due to the

presence of pigments chlorophyll a and b. Examples are

Chlamydomonas, Spirogyra, and Chara

● Phaeophyceae – Also called as brown algae, they are

predominantly marine. They have chlorophyll a, c, carotenoids

and xanthophyll pigments. Examples are Dictyota, Laminaria,

and Sargassum

● Rhodophyceae – They are the red algae because of the presence

of the red pigment, r-phycoerythrin. Examples are Porphyra,

Gracilaria, and Gelidium.

Importance of Algae

According to some scientists, algae produce half of the earth’s

oxygen. And this production of oxygen is by far the most important

contribution of algae. As mentioned in the beginning they are a source

of crude oil. These algal biofuels could well be a replacement for the

fossil fuels.

Algae also play an effective role in keeping the atmospheric carbon

dioxide stable and also using it efficiently. The food industry also uses

some algae. Agar is obtained from Gelidium and Gracilaria and is

making ice-creams and jellies. The other food supplements that are

algae and which are widely used are Chlorella and Spirulina.

Solved Questions For You

Q: State whether True or False – “The body of an algal plant can be

differentiated into root, stem, and leaves.”

Ans: This statement is False. Algae are simple plants with a thalloid

structure. They lack body differentiation.

Bryophytes

Who is not familiar with moss, the thick green mat that grows on the

surface, seen mostly in the lawns? Well, these plant-like structures are

the bryophytes. To know about the plant kingdom, step out into your

gardens and start observing! There is a lot more to discover and learn

practically!

What are Bryophytes?

Bryophytes are plants that are found growing in moist and shady

places. Something unique about these plants is that they can survive

on bare rocks and soil. They play an important role in plant succession

on bare rocks. They show alternation of generations and have a unique

nickname. So they are called the amphibians of the plant kingdom.

Though they grow in a terrestrial environment, they are dependent on

water for the reproduction process.

Unique features of Bryophytes

(Source: Wikipedia)

Though not as complex as other plants, bryophytes live in humid and

shaded places. But, they can be also found in arid forests, rainforests,

apart from the alpine habitats. They are widely distributed throughout

the world. You can see them growing on varied surfaces like rocks,

soil, tree trunks, bones, rotting wood etc.

The tissue organization is not complex. But, they do display some

amount of diversity. Some bryophytes maybe just over a millimetre

long and some are a meter long too. They contain chlorophyll and can

synthesize food through the process of photosynthesis. The plant body

is slightly differentiated, though they do not have true roots, stems,

and leaves. They generally have something similar to roots called the

rhizoids. The main body of the bryophytes is more thallus like and

haploid.

Life Cycle of Moss ( Source – Encyclopedia Britannica)

Bryophytes are spore-producing plants that do not have a vascular

tissue. They reproduce through gametes and hence have earned the

name gametophytes. The sex organs of bryophytes are multicellular.

Anthredium is the male sex organs while the female sex organ is

called archegonium. The archegonium is a flask-shaped organ and

produces a single egg. The anthredium produces antherozoids, which

have two flagella.

When the antherozoids are released into the water and when they

come in contact with the archegonium, a fusion of the gametes occurs

and a zygote is produced. The zygote remains in the archegonium for

some time. It then undergoes many cell divisions that are mitotic in

nature and produces an embryonic sporophyte. This growing embryo

is covered by a protective structure, called the calyptra.

The sporophyte is dependent on the gametophyte for the nutrients and

the gametophore for water and minerals. The cells of the sporophytes

undergo meiosis and give spores that are haploid. When these

germinate, they produce the gametophyte.

Classification of Bryophytes

Bryophytes are classified into two main divisions. They are:

● Liverworts – Example – Marchantia

● Mosses – Example – Sphagnum

Importance of Bryophytes

As bryophytes can survive on bare rock, they can initiate soil

formation in any barren terrain of land. They can maintain the soil

moisture and also help in the recycling of the nutrients in forest

vegetation.

Apart from this, there a few economically important bryophytes such

as the peat moss. It is harvested, processed and sold. It is used in

horticulture as well as a fuel. Because of their capacity to hold water,

they are sometimes used as packing material for shipment of living

material.

Solved Questions For You

Q: In bryophytes, where is the sporophyte generally located and where

does it obtain its nutrition?

Ans: The sporophyte is a long and tiny stem which grows on the

gametophyte. It is covered by a protective structure, called the

calyptra. The sporophyte is dependent on the gametophyte for the

nutrients

Pteridophytes

When you think of forests and woods, you can picture trees, abundant

lush green ferns, and other plants. But trust me the first thing that you

will notice is the abundant foliage of fern plants. These ferns, in

biological terms, are nothing but the pteridophytes, belonging to the

division Pteridophyta of the plant kingdom.

What are Pteridophytes?

These plants are the ones that do not have any flowers or seeds. Hence

they are also called as Cryptogams. They include ferns and horsetails.

In fact, they can be considered as the first terrestrial vascular plants,

showing the presence of the vascular tissue, xylem, and phloem. They

can be found in mostly in damp and shady places. Most ferns are

grown as ornamental plants.

Features of Pteridophytes

Pteridophytes display differentiation. The plant body can be divided

into a true root, stem, and leaves. A saprophyte is the main plant body

here. Some of the species belonging to this division have small leaves

called as the microphylls. For example, Selaginella. Megaphylls are

the large leaves that some pteridophytes have. For Example Fern

plants. The main plant bears the sporangia. These bear some leaf-like

appendages called the sporophylls. In a few species such as

Selaginella and Equisetum, the sporophylls form compact structures

called cones or strobili.

Reproduction in Pteridophytes

Pteridophytes show a true alternation of generations. Here, the

dominant sporophyte produces spores through meiosis. The

gametophyte generation forms gametes by mitosis. The spores are

produced by the sporangia in the spore mother cells. These spores

germinate and give rise to gametophytes.

These gametophytes are free-living, multicellular and photosynthetic.

They are called as the prothallus. Generally, the gametophytes require

damp and cool places to grow, due to their dependence on water. For

this very reason, the growth of pteridophytes is confined to certain

geographical areas.

The male sex organs are called the antheridia and the female sex

organs are called the archegonia. The male gametes are called the

antherozoids, which are released by the antheridia. Antherozoids can

get transferred to the archegonia which are the female sex organs, only

in the presence of water. Once the fusion of the gametes occurs, a

zygote is formed. This zygote produces the sporophyte, after division.

When the spores of the plants are similar then these plants are called

homosporous plants. Heterosporous plants are the ones that have two

different kinds of spores. They are the megaspores and the

microspores. In these heterosporous plants, the megaspores and

microspores germinate and give female and male gametophytes

respectively.

Classification of Pteridophytes

There are four classes, namely:

● Psilopsida

● Lycopsida

● Sphenopsida

● Pteropsida

Learn more about the Life Cycle of Plant here in detail.

Solved Question For You

Q: Why are pteridophytes common in moist and damp areas?

Ans: Pteridophytes have a dependence on water. Reproduction and

fusion of gametes can happen in the presence of water. In humid and

damp conditions, fertilization occurs effectively and the reproduction

is more. Hence they can proliferate more.

Gymnosperms

Did you know that plants also underwent an evolutionary process just

like all other living organisms? The group of plants that we call

gymnosperms is a major evidence for this. These plants were the first

to develop seeds. Seeds are an efficient system of dispersal and this

mechanism ensures that new plants can grow almost anywhere, not

necessarily next to the parent plant. Let us learn more about these

gymnosperms.

What are Gymnosperms?

The term Gymnosperms means naked seeds. These plants do not have

flowers and hence the seeds do not form inside any specialized

structure like the ovary seen in the other group of plants i.e.

angiosperms. Rather, the seeds develop in on the surface of the

reproductive structures of the plants and are visible as cones on

maturity. The seeds can sometimes be found on short stalks too.

Features of Gymnosperms

Gymnosperms have an evolutionary significance and show some

unique features. Their characteristic feature is the absence of flowers

and presence of naked, open seeds. As they do not have flowers,

consequently, fruits are also absent in these group of plants.

The main source of pollination and dispersal is wind. These are

generally medium to tall trees, with a few shrub species also present.

Sequoia is a gymnosperm which is one of the tallest tree species.

The plant body shows differentiation and is divided into leaves, stem,

and roots. The leaves are generally needle-like with a thick cuticle and

sunken stomata, generally seen in conifers. This feature helps in the

reduction of water loss due to transpiration.

The root system present in the gymnosperms is taproot system. In

some plants, these roots have an association with fungi and form

mycorrhiza, e.g. Pinus. In a few other species like the Cycas plant, the

roots are in the form of specialized roots called coralloid roots and are

associated with nitrogen-fixing cyanobacteria. These plants are also

vascular, with both xylem and phloem being present.

Reproduction and Fertilization

In gymnosperms, the reproductive structure is called a strobilus or a

cone. Both male and female strobili can be present on the same tree

(seen in Pinus) or on different trees (seen in Cycas). Gymnosperm

plants are heterosporous. They produce different spores, which are the

haploid microspores and megaspores.

Male Cones – the male strobili or male cones have microsporophylls,

which have the microsporangia that produce the haploid microspores.

Some of these microspores develop into male gametes called the

pollen grains, while the rest degenerate.

Female Cones – the megasporophylls form a cluster and are called as

the female strobili or cones. They bear the ovules having the

megasporangium. Thus produces the haploid megaspores and a

megaspore mother cell.

The megaspore mother cell undergoes meiotic division to produces

four megaspores. One of these megaspores develops into the female

gametophyte which is multicellular. The female gametophyte also has

two or more archegonia, which are the female sex organs.

Fertilization – When the pollen grains are released from the

microsporangium, they get dispersed through wind and reach the

female cones. The pollen grain develops a pollen tube, which grows

towards the archegonium.

The male gametes get discharged near the mouth of the archegonium.

Fusion of the male and female gametes occurs. After fertilization, a

zygote develops to form the embryo and the ovule forms the seed. The

following figure shows the life cycle of a gymnosperm.

Read about the Unique Features of Bryophytes here.

Classification of Gymnosperms

Gymnosperms can be divided into four divisions. They are:

● Pinophyta

● Cycadophyta

● Ginkgophyta

● Gnetophyta

Learn more about Angiosperms here in detail.

Solved Question For You

Q. How do you classify oak trees and pine trees?

Ans. Oak trees and Pine trees belong to two different divisions of the

plant kingdom. Oak trees have seeds that lie inside their acorns. Here

seeds are present inside and are not naked. Therefore Oak trees will be

classified under Angiosperms.

Pine trees, on the other hand, have naked seeds, with needle-like

leaves. These are the characteristic features of Gymnosperms and will

be classified under the same.

Angiosperms

I bet you have few angiosperms in your house. Whatever be the

environment, angiosperms can develop well and give you beautiful

flowers. These plants are the most common plants and also the largest

group of plants on earth. It is estimated that there are around 270,000

known species present. They account for 80% of all the living plants

that are known! They have great economic importance, as they are an

important source of food and a very important ecological component.

What are Angiosperms?

Flowering plants are called as angiosperms. The flowering plants are

the most dominant vascular plants that are found in the fauna all

around the world. The pleasing and attractive colours of their flowers

certainly add much more colour and brighten the landscape of any

place.

Due to the presence of flowers and enclosed seeds, they are called the

phanerogams. Scientifically speaking, in these plants, the seeds are

enclosed, with the ovules present in a hollow ovary.

Browse more Topics under Plant Kingdom

● Classification within Kingdom Plantae

● Algae

● Bryophytes

● Pteridophytes

● Gymnosperms

● Plant Life Cycles and Alternation of Generations

Features of Angiosperms

All angiosperm plants have the characteristic vascular bundle with the

xylem and phloem tissues for conduction of water, minerals, and

nutrients. The plant body is well differentiated with a well-developed

root system, shoot system and leaves. Specialised structures called as

the flowers are present. Within these flowers, the male and female

gametes develop. After fertilization, when these flowers mature, fruits

are formed which have the seeds within them.

Angiosperms can be found in varied habitats and can come in a

different range of sizes. Wolfie is an angiosperm that is microscopic

whereas the Australian mountain ash tree is about 100 meters tall. The

diversity that the angiosperms display is very wide. There are many

plants that are tall woody trees, shrubs, and even herbaceous plants.

These plants also have many adaptations in the roots, stems and leaves

depending on the habitat that they grow in.

Reproduction and Fertilization

In Angiosperms the flower is the reproductive organ. One of its basic

functions is to produce seeds through sexual reproduction. The

lifecycle of angiosperms shows the alternation of generations. The

haploid gametophyte alternates with the diploid sporophyte.

The characteristic double fertilization occurs here, wherein one of the

sperm fuses with the egg and forms the zygote (syngamy). The other

sperm fuses with polar nuclei and forms the endosperm (Triple

fusion). As the two processes of syngamy and triple fusion occur, the

whole process is called double fertilization. The following figure

shows the double fertilization.

Classification of Angiosperms

Based on the types of cotyledon present, angiosperms are divided into

two classes. They are monocotyledons and dicotyledons. The

dicotyledonous angiosperms have two cotyledons in their seeds and

the monocotyledonous angiosperms have one cotyledon.

Monocotyledonous plants

The monocots have some distinct features that include the presence of

adventitious roots, simple leaves with parallel venation, and trimerous

flowers. The number of vascular bundles is more and is closed. Some

of the examples include bamboos, sugarcane plants, banana, cereals,

lilies etc.

Dicotyledonous plants

The dicotyledonous plants have two cotyledons with a tap root system.

The venation seen in the leaves is reticulate. Flowers can be

tetramerous or pentamerous. Vascular bundles are generally arranged

in a ring and number between two to six. Examples include Grapes,

Dandelions, Sunflower, Tomatoes, and Potatoes etc.

(Source: Britannica)

Solved Question For You

Q: Angiosperms and Gymnosperms are both seed-bearing. Justify the

reason for their separate classification

Ans: Even though both gymnosperms and angiosperms are

seed-bearing plants, the difference lies in the location of these seeds.

In angiosperms, the seeds are located within a fruit and are enclosed.

However, in gymnosperms, the seeds are naked. In both these

divisions of plants, there is a difference in the structure of the male

and female reproductive parts of the plant. These are a few reasons

for the separate classification.

Plant Life Cycles and Alternation of Generations

When you see a parent plant and its baby plant, do you think both

resemble each other? Yes, of course! Did you know the reason? The

reason lies in the fact that plants also pass on genetic information to

their plant offspring. Every plant has a unique way in which this

occurs. Let us understand the life cycle of a plant and what alternation

of generations means. 

Alternation of Generations

( Source – Science matters)

In plants, there are two systems of reproduction. Each is called one

generation. There is a rotation between these generations. So, one

complete life cycle of a plant includes two generations that alternate

with each other. Hence the whole mechanism is called alternation of

generations.

The two generations or life cycles that occur are called the sporophyte

generation and the gametophyte generation. When you speak in

genetic terms, it is the rotation in between the haploid and diploid

stages. This refers to the chromosomes within the plant cells.

A diploid cell has two sets of chromosomes (one each from the male

parent and female parent). A haploid cell has only one set of

chromosomes. The haploid generation gives birth to plants with

diploid cells. This then creates a generation of haploid plants, which

again give rise to a generation of diploid plants. And thus the life

cycle of a plant will go on.

The whole life cycle of plants is very complex. But the greatest benefit

it gives is the greater genetic control. And how does this happen? In

the haploid stage, bad genes can be removed and the diploid stage

allows for greater genetic diversity.

Browse more Topics under Plant Kingdom

● Classification within Kingdom Plantae

● Algae

● Bryophytes

● Pteridophytes

● Gymnosperms

● Angiosperms

Know some terms

● Gametophyte – A Haploid plant

● Sporophyte – A Diploid plant

● Mitosis – The process by which gametophytes produce haploid

gametes.

● Fertilization – Fusion of male and female gametes to form a

diploid zygote.

● Meiosis – The process by which sporophyte produces spores

that are haploid.

Life Cycle of a Plant

The life cycle of plants shows some great variation. This is due to the

fact that all plants are not similar. Right from simple algae and

bryophytes to the complex vascular angiosperms, there is a great

amount of diversity in the structure and features. Therefore, different

generations are dominant in different plants. The following figure

gives you an idea of the dominant and reduced stages in the life cycle

of different plants.

● The haplontic life cycle is generally exhibited by algae. The

sporophyte generation is represented only by a one-celled

zygote. There are no free-living sporophytes. The gametophyte

is the dominant phase.

● The diplontic life cycle is commonly seen in gymnosperms and

angiosperms with some variations. However, an Alga called

Fucus also shows this kind of life cycle. In a diplontic life

cycle, the diploid sporophyte is dominant, photosynthetic and is

the independent phase of the plant. The gametophyte stage is

characterized by a single to a few-celled haploid gametophyte.

Bryophytes and Pteridophytes show an intermediate condition called

the haplo-diplontic life cycle, where phases are multicellular. But the

dominant phase differs. In Bryophytes, the dominant phase is the

gametophyte and in Pteridophytes, it is the sporophyte.

Solved Questions For You

Q. What generation does the flowering plants in your garden

represent- Sporophyte or gametophyte?

Ans. Flowering plants represent the sporophyte generation.

Q. In Bryophytes which stage is dominant?

Ans. Gametophyte stage is dominant in Bryophytes.