Systems of plant classification and the algae
When you check the sea-weed lying on a beach after high tide, you are holding a plant body with no root, stem or leaf. This simple body is what scientists call a thallus, and it is the main feature of algae.
Classification of plants has passed through three stages: artificial systems (based on a few characters such as vegetative features or stamens, like Linnaeus's), natural systems (based on many natural features, like Bentham and Hooker's) and phylogenetic systems (based on evolutionary relationships). Algae are simple, chlorophyll-bearing, thalloid, mostly aquatic autotrophs.
In the figure : The three systems of plant classification, from the oldest to the most recent.
Rule 1: Systems of classification — Artificial systems gave equal weight to vegetative and sexual characters and separated closely related species. Natural systems used natural affinities including internal features. Phylogenetic systems are based on evolutionary relationships, but they lack fossil evidence for many groups, so numerical taxonomy (computer-based, many characters), cytotaxonomy (chromosome data) and chemotaxonomy (plant chemicals) are used as aids.
- Easy : Linnaeus's system is an artificial system.
- Medium : Bentham and Hooker's system is a natural system for flowering plants.
- Exam-type : Q: Which kind of taxonomy uses chromosome number and behaviour? A: Cytotaxonomy.
Rule 2: Algae — Algae occur in fresh water, sea water and moist places. Their body can be unicellular, colonial or filamentous, and they reproduce vegetatively (fragmentation), asexually (zoospores) and sexually (isogamous, anisogamous or oogamous). Algae fix about half of the carbon dioxide on earth and add oxygen. Some are used as food (Porphyra, Laminaria, Sargassum), as agar (Gelidium, Gracilaria) and as single-cell protein (Chlorella, Spirulina).
- Easy : Spirulina is used as a protein supplement.
- Medium : Agar used in laboratory culture plates comes from red algae.
- Exam-type : Q: Name two algae used as food. A: Porphyra and Laminaria (or Sargassum).
The plant body of algae is a thallus: no real root, stem or leaves. Algae are autotrophic because they have chlorophyll.
Thinking the phylogenetic system is always based on fossils. Fossil evidence is often missing, so other aids are used.
Memory clue: A-N-P — Artificial few, Natural many, Phylogenetic ancestry.
Spot the difference
| System | Basis | Example |
|---|---|---|
| Artificial | A few characters | Linnaeus |
| Natural | Many natural affinities | Bentham and Hooker |
| Phylogenetic | Evolutionary relationships | Modern systems |
Practice questions
- 1What is a thallus?
- 2Name the three systems of plant classification.
- 3Name one alga used to make agar.
- 4Which algae are used as single-cell protein?
- 5What does chemotaxonomy use?
- 6What are the three kinds of sexual reproduction in algae?
- 7Why were artificial systems of classification replaced?
- 8Algae are called the 'lungs' of water bodies. Give one reason.
Show answers (try them yourself first)
- 1An undifferentiated plant body with no true root, stem or leaves.
- 2Artificial, natural and phylogenetic.
- 3Gelidium or Gracilaria.
- 4Chlorella and Spirulina.
- 5Chemical constituents of plants.
- 6Isogamous, anisogamous and oogamous.
- 7They used only a few characters and gave equal weight to all of them, so they could separate closely related species.
- 8They photosynthesise, take up carbon dioxide and release oxygen that other aquatic organisms use.
Classes of algae: green, brown and red
Pond scum is green, the sea-weed on rocks is brown and the thin tangle in deeper sea is red. The pigment decides the colour, and the colour decides the class.
Algae are placed in three classes according to their pigments, food reserve and cell wall: Chlorophyceae (green algae), Phaeophyceae (brown algae) and Rhodophyceae (red algae).
In the figure : The three classes of algae.
Rule 1: Chlorophyceae and Phaeophyceae — Green algae have chlorophyll a and b, store starch (pyrenoids help) and have a cellulose wall; examples: Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara. Brown algae are mostly marine, have fucoxanthin, store mannitol and laminarin and have a holdfast, a stalk (stipe) and a leaf-like frond; examples: Ectocarpus, Dictyota, Laminaria, Sargassum and Fucus.
- Easy : Spirogyra is a green filamentous alga of ponds.
- Medium : Laminaria is a big brown alga with a holdfast and a frond.
- Exam-type : Q: Which pigment is responsible for the brown colour? A: Fucoxanthin.
Rule 2: Rhodophyceae — Red algae have the pigment r-phycoerythrin, store floridean starch and live in well-lit shallow waters as well as at great depths of the sea. Examples: Polysiphonia, Porphyra, Gracilaria and Gelidium.
- Easy : Porphyra is an edible red alga.
- Medium : Gelidium and Gracilaria give agar.
- Exam-type : Q: Name the reserve food of red algae. A: Floridean starch.
Green: chlorophyll a, b and starch. Brown: fucoxanthin and mannitol. Red: r-phycoerythrin and floridean starch.
Saying that brown algae have no chlorophyll. They have chlorophyll a and c, but fucoxanthin hides it.
Memory clue: G-B-R = S-M-F — Green = Starch, Brown = Mannitol, Red = Floridean starch.
Spot the difference
| Point | Green | Brown | Red |
|---|---|---|---|
| Main pigment | Chlorophyll a, b | Fucoxanthin | r-phycoerythrin |
| Food | Starch | Mannitol, laminarin | Floridean starch |
| Example | Spirogyra | Laminaria | Porphyra |
Practice questions
- 1Which class do Volvox and Spirogyra belong to?
- 2Name the pigment of red algae.
- 3What is the stored food of brown algae?
- 4Give two examples of brown algae.
- 5What is the cell wall of green algae made of?
- 6What is the holdfast in brown algae?
- 7Why can some red algae live at great depths?
- 8A student finds a brown thalloid sea plant with a stipe and a frond. Which class is it and why?
Show answers (try them yourself first)
- 1Chlorophyceae (green algae).
- 2r-phycoerythrin.
- 3Mannitol and laminarin.
- 4Laminaria and Sargassum (also Fucus, Ectocarpus, Dictyota).
- 5Cellulose (with pectose on the outer layer).
- 6A root-like structure that anchors the alga to rock.
- 7Their pigments absorb the blue-green light that penetrates deep water, so they can photosynthesise there.
- 8Phaeophyceae, since the body is brown due to fucoxanthin and has a holdfast, stipe and frond.
Bryophytes: liverworts and mosses
After the rains, a damp brick wall or a shady rock turns green with a soft carpet. This carpet is often a moss, one of the plants that cannot grow without a film of water.
Bryophytes are small plants of moist, shady places. They are called the amphibians of the plant kingdom because they live on land but need water for fertilisation. They have no true roots, stem or leaves and no vascular tissue.
In the figure : Bryophytes are divided into liverworts and mosses.
Rule 1: Plant body and life cycle — The main plant body is the haploid gametophyte. It bears antheridia (male) and archegonia (female). The male gamete swims in water to the egg, forms a zygote, which grows into a sporophyte that stays attached to and depends on the gametophyte. Spores formed after meiosis grow into new gametophytes.
- Easy : A moss plant that you see on a wall is the gametophyte.
- Medium : The sporophyte of a moss is a stalk with a capsule that sits on the green plant.
- Exam-type : Q: Why do bryophytes need water for reproduction? A: The male gametes swim to the egg through water.
Rule 2: Liverworts, mosses and their uses — Liverworts (Marchantia) have a flat, dorsiventral thallus and reproduce asexually by gemmae formed in gemma cups. Mosses have a creeping protonema stage followed by a leafy stage with rhizoids, as in Funaria, Polytrichum and Sphagnum. Sphagnum gives peat used as fuel and as packing material, as it holds a lot of water. Mosses and lichens are early colonisers of bare rocks and prevent soil erosion.
- Easy : Gardeners use Sphagnum to keep plant roots moist during transport.
- Medium : A bare rock becomes soil bit by bit because lichens and mosses colonise it first.
- Exam-type : Q: What are gemmae? A: Green, multicellular asexual buds of Marchantia formed in gemma cups.
Dominant stage in bryophytes is the gametophyte; the sporophyte is dependent on it.
Calling a moss rhizoid a root. Rhizoids are unicellular or multicellular threads, not true roots.
Memory clue: BAG — Bryophytes, Amphibians of the plant kingdom, Gametophyte dominant.
Spot the difference
| Point | Liverworts | Mosses |
|---|---|---|
| Body | Flat thalloid, dorsiventral | Protonema and leafy stage |
| Asexual means | Gemmae in gemma cups | Fragmentation, budding in protonema |
| Example | Marchantia | Funaria, Sphagnum |
Practice questions
- 1Why are bryophytes called the amphibians of the plant kingdom?
- 2Name the dominant phase of a bryophyte.
- 3Name a liverwort and a moss.
- 4What are antheridia and archegonia?
- 5What is peat and which moss gives it?
- 6What are gemma cups?
- 7Why does the sporophyte of a moss depend on the gametophyte?
- 8How do mosses help in making soil?
Show answers (try them yourself first)
- 1They live on land but need water for fertilisation.
- 2The gametophyte.
- 3Liverwort: Marchantia; moss: Funaria (or Polytrichum, Sphagnum).
- 4The male and female sex organs of bryophytes.
- 5A fuel formed from the moss Sphagnum.
- 6Small cups on the thallus of Marchantia in which gemmae form.
- 7It remains attached to the gametophyte and gets water and nutrients from it.
- 8With lichens they colonise bare rocks, and their dead parts add to the soil, which also reduces soil erosion.
Pteridophytes: the first vascular land plants
Ferns grow wild in the monsoon on moist slopes of the Western Ghats and in many gardens. Unlike moss, a fern has real stems and leaves with tubes inside that carry water.
Pteridophytes include ferns and their relatives. They are the first terrestrial plants to have vascular tissue (xylem and phloem). The main plant is the diploid sporophyte, which has true roots, stems and leaves.
In the figure : The four classes of pteridophytes.
Rule 1: Life cycle — Sporangia are borne on leaf-like sporophylls, which in some form compact cones called strobili (Selaginella, Equisetum). Spores germinate into a small, multicellular, free-living, photosynthetic gametophyte called the prothallus. It bears antheridia and archegonia; water is needed for the male gamete to swim to the egg. The zygote grows into the sporophyte.
- Easy : The fern plant that we see is the sporophyte.
- Medium : The tiny heart-shaped prothallus of a fern is the gametophyte.
- Exam-type : Q: What is a prothallus? A: The small free-living gametophyte of a pteridophyte.
Rule 2: Homospory and heterospory — Most pteridophytes are homosporous: one kind of spore. Selaginella and Salvinia are heterosporous with larger megaspores and smaller microspores. Megaspores develop into female gametophytes that remain on the parent sporophyte; the zygote develops into the young embryo there. This is a precursor to the seed habit.
- Easy : Dryopteris is homosporous.
- Medium : Selaginella makes two kinds of spores, so it is heterosporous.
- Exam-type : Q: Why is heterospory important? A: It is a step towards the seed habit.
Pteridophytes have vascular tissue but no seeds; the sporophyte is dominant; fertilisation still needs water.
Thinking the fern leaf bears the seeds. Ferns have spores in sporangia, not seeds.
Memory clue: PSLP — Psilotum, Selaginella/Lycopodium, Equisetum, Pteris: four classes.
Spot the difference
| Point | Bryophytes | Pteridophytes |
|---|---|---|
| Dominant phase | Gametophyte | Sporophyte |
| Vascular tissue | Absent | Present |
| True roots, stem, leaves | Absent | Present |
Practice questions
- 1Name the vascular tissues in pteridophytes.
- 2What is the dominant phase in pteridophytes?
- 3What is a strobilus?
- 4Give one example each of Sphenopsida and Pteropsida.
- 5Name two heterosporous pteridophytes.
- 6What is the gametophyte of a pteridophyte called?
- 7Why do pteridophytes still need water for fertilisation?
- 8Explain how heterospory leads towards seed formation.
Show answers (try them yourself first)
- 1Xylem and phloem.
- 2The sporophyte.
- 3A compact cone of sporophylls bearing sporangia.
- 4Sphenopsida: Equisetum; Pteropsida: Dryopteris, Pteris or Adiantum.
- 5Selaginella and Salvinia.
- 6Prothallus.
- 7The male gametes are motile and swim to the egg.
- 8The megaspore develops into a female gametophyte inside the sporangium on the parent plant and the zygote forms the embryo there, as in seeds.
Gymnosperms: plants with naked seeds
A pine cone on the ground in a hill station is a woody, scaly structure. The scales hold seeds that are not covered by any fruit.
Gymnosperms (gymnos = naked, sperma = seed) are plants whose ovules are not enclosed by an ovary wall, so the seeds remain naked after fertilisation. They include Cycas, Pinus, Cedrus (deodar), Ginkgo and the very tall Sequoia.
In the figure : Pollen from the male cone reaches the ovule and a naked seed forms on the cone scale.
Rule 1: Body and reproduction — They are woody shrubs or trees with taproots. Cycas has coralloid roots associated with nitrogen-fixing cyanobacteria and Pinus has roots with fungal mycorrhiza. Leaves are needle-like in conifers, which cuts water loss, and the stem is unbranched in Cycas and branched in Pinus. Gymnosperms are heterosporous; microspores form pollen grains in male cones and megaspores form in the female cones.
- Easy : A needle-like leaf of pine loses less water in cold dry conditions.
- Medium : Cycas roots that grow upward in a coral shape host cyanobacteria.
- Exam-type : Q: What are coralloid roots? A: Special roots of Cycas associated with N2-fixing cyanobacteria.
Rule 2: Fertilisation — Pollen grains are carried by wind to the ovule. The pollen tube carries the male gametes to the egg, so water is not needed for fertilisation. The ovules become seeds that are not enclosed in a fruit.
- Easy : Wind carries pine pollen to the female cone.
- Medium : No water is needed, which is why gymnosperms do well on dry land and in cold regions.
- Exam-type : Q: Why are the seeds of gymnosperms called naked? A: They are not covered by a fruit wall or ovary.
Gymnosperms: seeds without fruit, cones as reproductive structures, pollen tube present.
Calling a pine cone a flower or a fruit. It is a cone, a compact group of sporophylls.
Memory clue: GNS — Gymnosperm = naked seed.
Spot the difference
| Point | Cycas | Pinus |
|---|---|---|
| Special roots | Coralloid roots | Mycorrhizal roots |
| Stem | Unbranched | Branched |
| Leaves | Pinnate | Needle-like |
Practice questions
- 1What does gymnosperm mean?
- 2Name two gymnosperms.
- 3What are the reproductive structures of gymnosperms called?
- 4Which gymnosperm has coralloid roots?
- 5Which organism lives in the coralloid roots?
- 6How does the needle-like leaf help conifers?
- 7How is the fertilisation of gymnosperms different from that of bryophytes?
- 8Why is Sequoia mentioned among gymnosperms?
Show answers (try them yourself first)
- 1Plant with naked seeds.
- 2Cycas and Pinus (also Cedrus, Ginkgo, Sequoia).
- 3Cones.
- 4Cycas.
- 5Nitrogen-fixing cyanobacteria.
- 6It reduces water loss and lets snow slide off.
- 7It happens through a pollen tube, so water is not needed.
- 8It is a redwood tree and one of the tallest tree species.
Angiosperms: the flowering plants
Every vegetable, fruit, cereal, pulse and spice on your plate comes from the flowering plants. Their flowers become fruits that cover and protect the seeds.
Angiosperms are seed plants whose seeds are enclosed in fruits. Pollen grains are produced in anthers and ovules lie inside ovaries. They are divided into dicotyledons (two cotyledons) and monocotyledons (one cotyledon).
In the figure : Angiosperms are divided into dicots and monocots.
Rule 1: Range and double fertilisation — Angiosperms range from the tiny Wolffia to the tall Eucalyptus (over 100 metres). They give food, fodder, fuel, medicines and many other products. After a double fertilisation, one male gamete fuses with the egg to form the zygote and another fuses with two polar nuclei to form a triploid primary endosperm nucleus. The ovule becomes the seed and the ovary becomes the fruit.
- Easy : A mango is formed from the ovary of the flower.
- Medium : The endosperm of a seed is formed by the fusion of a male gamete with two polar nuclei, so it is triploid (3n).
- Exam-type : Q: What is the ploidy of the endosperm? A: Triploid (3n).
Rule 2: Dicots and monocots — Dicots have two cotyledons, reticulate venation and tap roots; monocots have one cotyledon, parallel venation and fibrous roots. Pea, mango and sunflower are dicots; wheat, maize and rice are monocots.
- Easy : A pea seed splits into two halves, so it is a dicot.
- Medium : A leaf of grass with parallel veins belongs to a monocot.
- Exam-type : Q: Give a feature that separates a monocot leaf from a dicot leaf. A: Parallel venation in monocots; reticulate venation in dicots.
Angiosperm = seed inside a fruit; flowers are the reproductive organs; double fertilisation and triploid endosperm are unique.
Confusing endosperm with embryo. The embryo comes from the zygote (2n); the endosperm is triploid.
Memory clue: DD-MM — Dicot has Two (D), Monocot has one (M) cotyledon.
Spot the difference
| Point | Dicots | Monocots |
|---|---|---|
| Cotyledons | Two | One |
| Venation | Reticulate | Parallel |
| Roots | Tap root | Fibrous roots |
| Example | Mango, pea | Wheat, maize |
Practice questions
- 1What is meant by an angiosperm?
- 2Name the two classes of angiosperms.
- 3Give two examples of monocots.
- 4Which part of the flower forms the fruit?
- 5What is the smallest angiosperm?
- 6What is double fertilisation?
- 7What is the ploidy of the primary endosperm nucleus and why?
- 8A plant has fibrous roots, parallel venation and seeds with one cotyledon. Which class does it belong to?
Show answers (try them yourself first)
- 1A plant that bears seeds enclosed in a fruit.
- 2Dicotyledons and monocotyledons.
- 3Wheat and maize (or rice).
- 4The ovary.
- 5Wolffia.
- 6Fusion of one male gamete with the egg and the other with the polar nuclei in the same embryo sac.
- 7Triploid, as it is formed from one male gamete and two polar nuclei.
- 8Monocotyledons.
Life cycles and alternation of generations
A relay race has two runners passing the baton. In a plant's life cycle two stages, one with single sets of chromosomes and one with double sets, pass on the story.
Sexual reproduction has two phases: a haploid (n) phase and a diploid (2n) phase. Plants show alternation of generations between the haploid gametophyte and the diploid sporophyte. Based on which phase is dominant, there are three patterns: haplontic, diplontic and haplo-diplontic.
In the figure : The haploid and diploid phases alternate in the life cycle of a plant.
Rule 1: Three life-cycle patterns — Haplontic: the dominant phase is the free-living haploid gametophyte; the zygote is the only diploid cell (Volvox, Spirogyra and some other algae). Diplontic: the diploid sporophyte dominates, and the gametophyte is only a few cells (gymnosperms, angiosperms, and the alga Fucus). Haplo-diplontic: both phases are multicellular and free living (bryophytes, pteridophytes, and some algae like Ectocarpus and kelps).
- Easy : Spirogyra is haplontic.
- Medium : A mango tree is the diploid sporophyte, so its life cycle is diplontic.
- Exam-type : Q: Which plants show haplo-diplontic life cycle? A: Bryophytes and pteridophytes (and some algae).
Rule 2: Which phase is dominant — In bryophytes the gametophyte is dominant; in pteridophytes, gymnosperms and angiosperms the sporophyte is dominant and the gametophyte becomes smaller and smaller.
- Easy : A moss plant is a gametophyte.
- Medium : The fern is a sporophyte, and its gametophyte (prothallus) is tiny.
- Exam-type : Q: In which group does the gametophyte become reduced? A: In pteridophytes, gymnosperms and angiosperms.
Meiosis converts diploid to haploid; fertilisation converts haploid to diploid.
Saying that the zygote is haploid. The zygote is diploid (2n).
Memory clue: H-D-HD — Haplontic: Haploid dominant; Diplontic: Diploid dominant; Haplo-diplontic: both.
Spot the difference
| Pattern | Dominant phase | Example |
|---|---|---|
| Haplontic | Haploid gametophyte | Volvox, Spirogyra |
| Diplontic | Diploid sporophyte | Pinus, mango, Fucus |
| Haplo-diplontic | Both are free-living | Moss, fern |
Practice questions
- 1What is alternation of generations?
- 2Name the ploidy of a zygote.
- 3What type of life cycle does Spirogyra show?
- 4Which life cycle do gymnosperms show?
- 5Which division makes spores in the sporophyte?
- 6Give one example of a haplo-diplontic plant.
- 7In which group is the gametophyte dominant, and in which is it reduced?
- 8A plant's body is haploid and only its zygote is diploid. Name the pattern and give a reason.
Show answers (try them yourself first)
- 1The regular alternation of a haploid gametophyte phase and a diploid sporophyte phase.
- 2Diploid (2n).
- 3Haplontic.
- 4Diplontic.
- 5Meiosis.
- 6Funaria (moss) or a fern.
- 7Dominant in bryophytes; reduced in pteridophytes, gymnosperms and angiosperms.
- 8Haplontic, since meiosis occurs in the zygote just after fertilisation and the haploid body is dominant.