Plant Kingdom

Written and reviewed by Anjali Sharma, Biology faculty at UniDrill (M.Sc. Botany, Banaras Hindu University · 8 years teaching NCERT Biology). Based on NCERT Biology Class XI, Chapter 3.

Why is a moss always short and a pine always tall?

Walk through a damp wall in the monsoon and you will find a green carpet of moss, a centimetre or two high. It never becomes a tree. A pine on the same hill reaches 30 metres or more. The difference is not size for its own sake. It is the story of how plants conquered land, one problem at a time: first staying wet, then carrying water upwards, then protecting the embryo in a seed, and finally enclosing the seed in a fruit.

This chapter walks that story in order: algae → bryophytes → pteridophytes → gymnosperms → angiosperms. Each group solves one problem the previous group could not. Keep that thread in mind and the facts below stop being a list.

How plants have been classified

Three systems have been used, and you need to know which is which.

Modern work also draws on three tools. Numerical taxonomy codes hundreds of characters, all given equal importance, and processes them by computer. Cytotaxonomy uses chromosome number, structure and behaviour. Chemotaxonomy uses the chemical constituents of the plant.

Plantae in this chapter covers algae, bryophytes, pteridophytes, gymnosperms and angiosperms. Cyanobacteria, once called blue green algae, are not in it: they are prokaryotes and belong to Monera, as you saw in Biological Classification.

Algae: simple, green or not, and mostly aquatic

Algae are chlorophyll bearing, simple, thalloid, autotrophic organisms, mostly aquatic, in both fresh water and the sea. They are also found on moist stones, soil and wood. Some live in partnership with fungi (lichens) and some on animals, such as the sloth bear. Their size ranges from the single celled Chlamydomonas, through the colonial Volvox and the filamentous Ulothrix and Spirogyra, to marine kelps that form massive plant bodies.

How algae reproduce. There are three ways, and the sexual one has three forms.

Why algae matter. Algae fix at least half of all the carbon dioxide fixed on Earth, so they raise the dissolved oxygen of their surroundings and are the primary producers of aquatic food chains. People eat Porphyra, Laminaria and Sargassum. Agar, used to grow microbes and in jellies and ice creams, comes from Gelidium and Gracilaria. Algin (from brown algae) and carrageen (from red algae) are hydrocolloids, substances that hold water, used commercially. Chlorella is protein rich and is used as a food supplement, even by space travellers.

Beyond the textbook. For the exam: Chlorella and Spirulina are listed as unicellular algae used as protein rich food supplements. In reality: Spirulina is a cyanobacterium, which is a prokaryote and not an alga at all. Why it matters: in the exam, treat both as algae, but this is the same "blue green algae are not algae" lesson from the Monera chapter showing up again.

The three classes of algae. Algae are sorted by pigment, stored food, wall and flagella. This table is the most repeated one in the chapter.

Brown algal zoospores are pear shaped (pyriform) with two unequal flagella attached laterally. Red algae reproduce asexually by non motile spores, because they have no flagella at all.

Why it works this way. Light of different colours penetrates water to different depths. Extra pigments such as fucoxanthin and phycoerythrin let brown and red algae use the light that reaches them, which is why red algae can live at great depth where light is poor.

Pause and predict: an alga from the shore has a brown colour from fucoxanthin and stores mannitol. Does it have flagella on its zoospores, and how many?

Check your answer: Yes. It is a brown alga, whose zoospores have two unequal flagella attached to the side. A red alga would have none.

Bryophytes: plants that live on land but still depend on water

Bryophytes are called the amphibians of the plant kingdom. They live in soil but need water for sexual reproduction. They also grow in moist, shady places and are among the first plants to colonise bare rock. Together with lichens they break rock down and make a thin soil that higher plants can use, and mats of moss on the ground reduce the impact of rain and so prevent soil erosion.

The body is thallus like, a flat or leafy structure without true roots, stems or leaves. It is anchored by rhizoids, which may be unicellular or multicellular. The main plant body is the haploid gametophyte. It makes gametes in two kinds of sex organs. The antheridium produces biflagellate antherozoids, and the flask shaped archegonium holds a single egg. The antherozoids swim through a film of water to the archegonium, and the egg is fertilised.

The zygote becomes a multicellular sporophyte, which is not free living. It stays attached to the green gametophyte and takes its food from it. Cells of the sporophyte undergo meiosis to produce haploid spores, and these germinate into new gametophytes.

Why it works this way. Two facts explain why bryophytes are short and why they live in damp places. They have no xylem or phloem, so water cannot be carried far upwards. And their sperms must swim to the egg, so a film of water is essential for fertilisation.

The two classes differ in body plan and in how they multiply.

Sphagnum, a moss, gives peat, which is used as fuel, and because it holds a lot of water it is also used as packing material for sending live plants.

Pause and predict: you find a green plant with a creeping thread like stage that grows from a spore, followed by an upright leafy stage. Liverwort or moss?

Check your answer: Moss. A protonema stage belongs to mosses. Liverworts do not have it. Their asexual buds are gemmae, formed in gemma cups.

Pteridophytes: the first vascular land plants

Pteridophytes include horsetails and ferns. They are the first plants to have vascular tissues, xylem and phloem, which carry water and food over long distances. They prefer cool, damp and shady places, and they are used for medicine, as soil binders and as ornamentals.

The main plant body is the sporophyte, differentiated into a true root, stem and leaves. This is a big change from bryophytes, where the gametophyte dominates. Leaves may be small, microphylls (as in Selaginella), or large, macrophylls (as in ferns). Spore producing sporangia sit on leaf like sporophylls, which in some plants form compact strobili or cones, as in Selaginella and Equisetum.

Spores germinate into a small, multicellular, free living, mostly photosynthetic gametophyte called the prothallus. It needs cool, damp, shady conditions, and water is again needed for the male gametes to reach the egg. The zygote grows into the new sporophyte.

Homospory and heterospory. Most pteridophytes make one kind of spore and are homosporous. A few, including Selaginella and Salvinia, make two kinds: small microspores and large megaspores. These grow into male and female gametophytes respectively. In these plants the female gametophyte is kept on the parent sporophyte for some time, and the zygote develops into a young embryo inside it. This is called a precursor to the seed habit.

Why it matters. This is the missing link in the story. Once the female gametophyte is retained on the parent and the embryo grows inside it, the only steps left to a seed are adding protective layers and food. Gymnosperms take those steps.

Gymnosperms: seeds with no fruit around them

Gymnos means naked and sperma means seed. In gymnosperms the ovules are not enclosed by an ovary wall, and so the seeds stay naked after fertilisation. The group includes medium and tall trees and shrubs. The giant redwood Sequoia is one of the tallest trees.

Their roots are tap roots. In Pinus the roots carry a mycorrhiza (fungus partner), and in Cycas special coralloid roots associate with nitrogen fixing cyanobacteria. The stems may be unbranched (Cycas) or branched (Pinus, Cedrus). Leaves may be simple or compound. Cycas has pinnate leaves that stay on for a few years. In conifers the needle like leaves have a small surface area, a thick cuticle and sunken stomata, which together reduce water loss.

All gymnosperms are heterosporous. Microsporangia on microsporophylls cluster into male cones, and megasporangia (ovules) on megasporophylls form female cones. The microspore grows into a highly reduced male gametophyte, the pollen grain, which develops inside the microsporangium. Pollen is carried by air currents to the ovule, and a pollen tube delivers the male gametes towards the archegonia. The male and female gametophytes are not free living: they stay on the sporophyte. After fertilisation the zygote becomes an embryo and the ovule a seed.

Other examples are Ginkgo and Cedrus.

Beyond the textbook. For the exam: gymnosperms do not need water for fertilisation, because the pollen tube carries the male gametes. In reality: in Cycas and Ginkgo the male gametes are still flagellated and swim for a short distance in fluid inside the ovule This is an ancestral trait. Why it matters: it shows that the shift from water dependent to pollen tube fertilisation happened step by step, not in one jump.

Angiosperms: seeds inside fruits

Angiosperms are the flowering plants. Their pollen grains and ovules develop in flowers, and the seeds are enclosed in fruits. They range from the tiny Wolffia to Eucalyptus trees over 100 metres tall. They are divided into dicotyledons (two seed leaves) and monocotyledons (one seed leaf), compared in Morphology of Flowering Plants.

The stamen produces pollen grains, the reduced male gametophyte, and the pistil holds the ovule, which produces the reduced female gametophyte, the embryo sac. After pollination, double fertilisation takes place: one male gamete fuses with the egg and the other fuses with the two polar nuclei in the embryo sac. The zygote becomes the embryo, and the second fusion gives the endosperm. The sporophyte is the dominant phase and the gametophytes are tiny. Sexual Reproduction in Flowering Plants covers this in detail.

Three patterns of life cycle

Every plant alternates between a haploid gamete producing phase (gametophyte) and a diploid spore producing phase (sporophyte). What differs is which phase is the dominant, visible one.

The trend across groups in one line: algae and bryophytes (haploid body dominant) → pteridophytes (diploid body dominant, gametophyte free living) → gymnosperms and angiosperms (diploid body dominant, gametophyte hidden on the sporophyte).

Beyond the textbook. For the exam: algae come first in the plant kingdom, and the groups form a ladder from simple to complex. In reality: green algae, not red or brown, are the closest relatives of land plants, and land plants arose from a group of freshwater green algae. Red and brown algae are only distant cousins. Why it matters: it explains why green algae share chlorophyll a and b and cellulose walls with plants, while brown and red algae use quite different pigments and storage foods.

Exam traps

60-second revision

Where this chapter connects

• Previous: Biological Classification introduced the five kingdoms and the one paragraph on Plantae that this chapter expands, and explains why cyanobacteria are not algae.
• Next: Animal Kingdom does the same job for animals.
• Later in this unit: Morphology of Flowering Plants takes the angiosperm paragraph here and studies roots, stems, leaves, flowers, fruits and seeds in detail.
• Class XII link: Sexual Reproduction in Flowering Plants takes the pollen grain and embryo sac mentioned here through pollination, double fertilisation and seed formation.