CDS (II) 2020 General Knowledge · Question 91 of 120

Correct — B, Algae. In the school classification of the plant kingdom the four divisions run Thallophyta (algae), Bryophyta, Pteridophyta and the seed plants, and only the first of them lives mainly in water. Algae have no roots, no stems, no leaves and no vascular tissue, so nothing in their body is built to lift water against gravity or to stop it escaping; the whole thallus simply sits in the medium and absorbs from it. That works only where the medium is water, and it is why algae fill ponds, lakes, rock pools, the rice-field film and the whole sunlit layer of the sea, from single-celled Chlamydomonas through filamentous Spirogyra and Ulothrix to the large marine kelps. Each of the other three divisions has already moved on to land: bryophytes and pteridophytes grow on damp soil, rock and bark, and gymnosperms are woody land trees.

• (a)Bryophytes — Mosses and liverworts are called the amphibians of the plant kingdom, which is the source of the confusion. They are land plants — they grow on damp soil, walls and tree trunks — and need water only as a film for the sperm to swim to the egg during fertilisation. Needing water for one stage is not the same as living in it.
• (c)Pteridophyta — Ferns, horsetails and club mosses are the first land plants with proper vascular tissue, roots and true leaves. They favour shaded, humid ground, and a few genera such as Azolla and Salvinia do float on water, but the division as a whole is terrestrial.
• (d)Gymnosperms — The naked-seeded plants — pines, cycads, deodar, Ginkgo — are woody perennials of dry land and cold mountains. They are the least aquatic group on the list; several are adapted to conserve water with needle leaves and thick cuticles.

The plant kingdom is arranged in an order that tracks the conquest of land. Thallophyta, whose plant body is an undifferentiated thallus, contains the algae and stays in water. Bryophyta adds a land habit but no vascular tissue and no true roots. Pteridophyta adds xylem and phloem, roots and leaves, and so can stand upright away from water. The seed plants — gymnosperms with naked seeds and angiosperms with seeds enclosed in fruit — free reproduction from an external film of water altogether.

The word to fix on in the stem is 'predominantly'. Almost every one of these groups touches water somewhere in its life cycle, so a student who tests each option against 'does it ever need water?' will find several passing. Test them instead against 'where does the mature plant live?' and only algae answer 'in water'. Two useful cross-checks: the group has no vascular tissue at all, which is only tolerable if water surrounds the whole body, and it is the group that gives us pond scum, seaweed and the algal bloom. Worth flagging that the traditional textbook placement of algae inside Plantae is a simplification the stem inherits — under the five-kingdom system many algae are placed in Protista, and the blue-green algae are not algae at all but cyanobacteria, which are bacteria. The item is asking the school classification, and on that classification the answer is algae.

• Algae have a thallus body — no true root, stem, leaf or vascular tissue — so they absorb directly from the surrounding water.
• Bryophytes are the amphibians of the plant kingdom: land plants that still need a film of water for the sperm to reach the egg.
• Pteridophytes were the first plants with vascular tissue, which is what let plants grow tall on dry land.
• Gymnosperms bear naked seeds not enclosed in a fruit; pine, cycad, deodar and Ginkgo are the standard examples.
• Blue-green algae (Anabaena, Nostoc) are cyanobacteria — prokaryotes — and are not part of Kingdom Plantae at all despite the name.

• Reading 'needs water for fertilisation' as 'lives in water' and choosing bryophytes.
• Assuming every alga is microscopic — kelps are algae several metres long.
• Treating blue-green algae as plants; they are prokaryotic cyanobacteria.

Usually as a one-line group-to-property item — which division is aquatic, which lacks vascular tissue, which is called the amphibian of the plant kingdom, or which bears naked seeds.

• practice — not a real PYQ

  Which one of the following groups of plants lacks vascular tissue as well as true roots?

  • (a)Bryophytes
  • (b)Pteridophytes
  • (c)Gymnosperms
  • (d)Angiosperms

  Answer(a) Bryophytes — mosses and liverworts have neither xylem and phloem nor true roots; they anchor themselves with rhizoids.

• practice — not a real PYQ

  Which one of the following is a gymnosperm?

  • (a)Fern
  • (b)Cycas
  • (c)Moss
  • (d)Mango

  Answer(b) Cycas — a gymnosperm, bearing naked seeds. Fern is a pteridophyte, moss a bryophyte and mango an angiosperm.