Plant Kingdom — Algae to Angiosperms, and the Conquest of Land
The five plant groups are not a random list; they are five successive answers to one problem — how to live away from water. Algae never solved it and stayed aquatic. Bryophytes solved the body but not the sperm, so they still need a film of water to fertilise. Pteridophytes invented vascular tissue and stood upright, but kept the swimming sperm. Gymnosperms replaced the swimming sperm with a pollen grain and the spore with a seed, cutting the last dependence on water. Angiosperms added the flower, the fruit and double fertilisation. Read the chapter as that sequence and two examinable patterns fall out for free: which generation dominates at each stage, and what still ties each group to water.
Bryophytes and Pteridophytes — Halfway onto Land
| Feature | Bryophytes | Pteridophytes |
|---|---|---|
| Nickname | Amphibians of the plant kingdom | First vascular land plants |
| Dominant generation | Gametophyte (haploid); sporophyte is dependent on it | Sporophyte (diploid); the plant body you see |
| Vascular tissue | Absent — no true roots, stem or leaves | Present — xylem and phloem, true roots and leaves |
| Fertilisation | Needs a water film for the flagellated sperm | Also needs water — the sperm still swims to the archegonium |
| Gametophyte form | The main thalloid or leafy plant; liverworts make gemmae | A small, free-living prothallus |
| Examples | Liverworts: Marchantia, Riccia. Mosses: Funaria, Sphagnum, Polytrichum | Psilotum, Selaginella, Lycopodium, Equisetum, Dryopteris, Adiantum, Salvinia |
🎯 Why heterospory in Selaginella and Salvinia matters out of all proportion to its size
Most pteridophytes are homosporous — one kind of spore, which germinates into a gametophyte bearing both sex organs. A few, notably Selaginella and Salvinia, are heterosporous: they make a large megaspore and a small microspore. That difference sounds minor and is in fact the beginning of the seed. Because the megaspore is retained on the parent plant, the female gametophyte develops inside it rather than being released, and a young embryo therefore begins life protected and supplied by the parent. That is precisely the arrangement a seed formalises, which is why the textbook calls this the precursor to the seed habit and why every question on these two genera is really a question about seed evolution. Note also that in pteridophytes the spores are produced in sporangia borne on sporophylls, which in Selaginella and Equisetum are compacted into a strobilus or cone — another structure the seed plants inherit.Gymnosperms, Angiosperms and Alternation of Generations
| Feature | Gymnosperms | Angiosperms |
|---|---|---|
| The ovule and seed | Naked — not enclosed in an ovary, so no fruit | Enclosed in an ovary, which ripens into a fruit |
| Reproductive structure | Male and female cones (strobili) | Flowers |
| Xylem | Vessels absent (except Gnetum) | Vessels present |
| Fertilisation | Single fertilisation; endosperm is haploid and formed before fertilisation | Double fertilisation; endosperm triploid, formed after triple fusion |
| Notable members | Cycas (coralloid roots with cyanobacteria), Pinus (mycorrhiza), Ginkgo, Sequoia — the tallest tree | Smallest is Wolffia; among the tallest is Eucalyptus; divided into dicots and monocots |
🧠 Alternation of generations: three life-cycle patterns, and how to name them from a diagram
Every plant alternates a haploid gametophyte with a diploid sporophyte; what differs is which one is free-living and dominant. Haplontic: the gametophyte dominates and the only diploid cell in the whole life cycle is the zygote, which undergoes meiosis immediately — most algae, such as Spirogyra and Volvox. Diplontic: the sporophyte dominates and the gametophyte is reduced to a few cells that never live independently — gymnosperms and angiosperms. Haplo-diplontic: both generations are multicellular, which covers bryophytes (gametophyte dominant, sporophyte attached and dependent) and pteridophytes (sporophyte dominant, gametophyte a small free prothallus). The exam usually gives a diagram or a description and asks for the name, so use this test: if the zygote divides by meiosis at once, it is haplontic; if the gametophyte never lives alone, diplontic; if both stages are multicellular plants, haplo-diplontic.⚠️ The endosperm question: haploid in gymnosperms, triploid in angiosperms
This is one of the most reliable single marks in the whole plant kingdom, and it turns on a difference in timing. In a gymnosperm the female gametophyte tissue that will nourish the embryo is produced before fertilisation by mitosis from the haploid megaspore, so it is haploid (n). In an angiosperm the endosperm is produced after fertilisation, when the second male gamete fuses with the two polar nuclei — triple fusion — giving triploid (3n) tissue. Two further contrasts follow from the same event: only angiosperms show double fertilisation, and only angiosperms enclose the seed in a fruit. Candidates who remember "endosperm is 3n" as a universal rule lose the gymnosperm version of this question every time; the safe habit is to ask before or after fertilisation? rather than to recall a number.Key takeaways
- Algae classes: green = chl a+b, starch; brown = chl a+c, fucoxanthin, laminarin; red = phycoerythrin, floridean starch.
- Agar comes from red algae (Gelidium, Gracilaria); algin from brown algae. Chlorella is a protein-rich green alga.
- Bryophytes are the amphibians of the plant kingdom: gametophyte dominant, no vascular tissue, water needed for fertilisation.
- Pteridophytes are the first vascular plants; sporophyte dominant, gametophyte a free prothallus, sperm still swims.
- Heterospory in Selaginella and Salvinia is the precursor of the seed habit.
- Gymnosperm endosperm is haploid and pre-fertilisation; angiosperm endosperm is triploid and post-triple-fusion.
- Gymnosperm xylem lacks vessels except in Gnetum; Cycas has coralloid roots with cyanobacteria, Pinus has mycorrhiza.
- Life cycles: haplontic (most algae), diplontic (seed plants), haplo-diplontic (bryophytes and pteridophytes).
Practice questions (8)
Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.
Bryophytes are called the amphibians of the plant kingdom because they:
Show answer
Answer: A — Live on land but need water for fertilisation
The body has moved onto land but the flagellated sperm has not: it still has to swim through a film of water to reach the archegonium. The name is about the dependence for reproduction, not about the plant living in two habitats, which is why the second and fourth options miss it.The endosperm of a gymnosperm seed is:
Show answer
Answer: A — Haploid, and formed before fertilisation
In gymnosperms the nutritive tissue is female gametophyte formed by mitosis from the haploid megaspore, so it is n and exists before the sperm arrives. The triploid answer is the angiosperm case, and treating "endosperm = 3n" as universal is the single commonest error on this topic.Vessels are absent in the xylem of gymnosperms, with the notable exception of:
Show answer
Answer: A — Gnetum
Gnetum is the standard exception, which is why it appears in almost every question that mentions gymnosperm xylem. Cycas and Pinus are the ones to associate with coralloid roots and mycorrhiza respectively, and Ginkgo with being a living fossil — each genus carries its own single hook.Heterospory in Selaginella and Salvinia is significant because it is regarded as:
Show answer
Answer: A — The precursor of the seed habit
Because the megaspore is retained on the parent, the female gametophyte and then the young embryo develop while protected and nourished by the parent plant — the arrangement a seed formalises. The second option reverses the evolutionary direction, and the flower is an angiosperm innovation far later in the sequence.Agar, used to culture microbes and to make jellies, is obtained from:
Show answer
Answer: A — Gelidium and Gracilaria — red algae
Agar is a red-alga product; the brown algae give algin, and the two are the pair most often swapped in this topic. Green algae are associated with Chlorella as a protein supplement, and diatoms with diatomaceous earth — none of which is agar.A life cycle in which the gametophyte is the dominant, free-living generation and the zygote is the only diploid cell is called:
Show answer
Answer: A — Haplontic
That is the haplontic pattern of most algae: the zygote divides by meiosis straight away, so no multicellular diploid body ever forms. Diplontic is the reverse and describes seed plants, while haplo-diplontic requires both generations to be multicellular, as in bryophytes and pteridophytes.In brown algae the stored food and the flagellar arrangement are respectively:
Show answer
Answer: A — Laminarin or mannitol; two unequal lateral flagella
Brown algae are identified by laminarin or mannitol as reserve food and by two unequal, laterally attached flagella. The second option describes green algae and the third red algae, so all three distractors are correct answers to a different part of the same table.Which feature is present in angiosperms but absent in gymnosperms?
Show answer
Answer: A — Double fertilisation and a fruit enclosing the seed
Both groups have seeds, vascular tissue and pollen — those were the innovations that freed seed plants from water in the first place. What is exclusively angiosperm is double fertilisation, and the ovary that ripens into a fruit; a gymnosperm ovule is naked, which is what the name means.