Plant Kingdom: common doubts, answered
The questions students ask most often about Plant Kingdom, each with a short answer. For the full chapter, read the Plant Kingdom notes.
How plant classification changed
Read this section in the notes →What is the difference between artificial, natural and phylogenetic systems of classification?
An artificial system uses a few superficial characters, mostly vegetative ones or, in Linnaeus's system, the androecium, and treats vegetative and sexual characters as equally important. A natural system, like that of Bentham and Hooker, rests on natural affinities and uses external as well as internal features such as anatomy, embryology and phytochemistry. A phylogenetic system groups organisms by evolutionary relationships, assuming members of a taxon share a common ancestor.
Are cyanobacteria algae?
No. Cyanobacteria, once called blue-green algae, are prokaryotes and belong to Kingdom Monera. In the five kingdom scheme the term algae is kept for eukaryotic, chlorophyll-bearing, mostly aquatic organisms of the plant kingdom. So cyanobacteria are photosynthetic bacteria and should not be listed as algae in an answer.
Algae: body and reproduction
Read this section in the notes →What is the difference between isogamy, anisogamy and oogamy in algae?
In isogamy the two fusing gametes are alike in size, as in Ulothrix and Spirogyra. In anisogamy the gametes are dissimilar in size, as in Eudorina. In oogamy a large non-motile female gamete fuses with a smaller motile male gamete, as in Volvox and Fucus. Whether gametes are flagellated does not decide the type.
Why is Spirogyra isogamous even though its gametes do not swim?
Isogamy depends on the gametes being similar in size, and the non-flagellated gametes of Spirogyra are indeed alike in size. Motility is a separate feature. Ulothrix also has isogamy but its gametes are flagellated, so both examples show that isogamous gametes may be motile or non-motile.
Why algae matter
Read this section in the notes →What is the economic importance of algae?
Algae fix at least half of the carbon dioxide taken up on earth through photosynthesis and add oxygen to their surroundings, so they are important primary producers. Some are eaten, such as Porphyra, Laminaria and Sargassum. Brown algae give algin and red algae give agar and carrageen. Chlorella is a protein-rich supplement used by space travellers.
Which hydrocolloids come from which algae?
Algin comes from brown algae, while carrageen comes from red algae. Agar, used to grow microbes and make jellies, is obtained from Gelidium and Gracilaria, which are red algae. These substances hold water, so they serve as thickeners and gelling agents. Students often swap algin and carrageen, so remember brown gives algin.
Green, brown and red algae
Read this section in the notes →How do green, brown and red algae differ in pigments and stored food?
Green algae have chlorophylls a and b and store starch. Brown algae have chlorophylls a and c with fucoxanthin and store laminarin or mannitol. Red algae have chlorophylls a and d with r-phycoerythrin and store floridean starch. The pigment mixture also explains the colour of each group.
Do red algae have flagella?
No. No stage in the life of a red alga carries flagella. They multiply vegetatively by fragmentation, while both their asexual spores and their gametes are non-motile. Sexual reproduction is oogamous, and fertilisation is followed by complex developments. The red pigment r-phycoerythrin lets many red algae live at great depths in the sea, where relatively little light reaches.
Bryophytes: amphibians of the plant kingdom
Read this section in the notes →Why are bryophytes called the amphibians of the plant kingdom?
Bryophytes live in soil but depend on water for sexual reproduction, just as amphibians live on land but depend on water to breed. Their male gametes are flagellated and need a film of water to swim to the egg. They are found in damp, shaded places and lack true roots, stems and leaves.
Liverworts and mosses
Read this section in the notes →What is the main plant body in bryophytes, gametophyte or sporophyte?
The main plant body of a bryophyte is the haploid gametophyte. The sporophyte is a smaller structure that stays attached to the gametophyte and takes nutrition from it. In mosses the leafy plant that we see is therefore haploid, while the foot, seta and capsule make up the diploid sporophyte.
What is the difference between a protonema and a leafy stage in mosses?
The protonema is the creeping, branched, often green filamentous stage that develops directly from a spore. The leafy stage develops from a lateral bud on the secondary protonema and bears the sex organs. Both belong to the haploid gametophyte generation, so neither of them is part of the sporophyte.
Why is Sphagnum important?
Sphagnum, a moss, provides peat, which is used as fuel and, because it holds water well, as packing material for trans-shipment of living material. Mosses together with lichens are also among the first organisms to colonise bare rocks, and they slowly make the substratum suitable for higher plants.
Pteridophytes
Read this section in the notes →Why are pteridophytes considered the first terrestrial vascular plants?
They are the first land plants to have xylem and phloem, which carry water and food through the plant body. Their main plant is the sporophyte with a true root, stem and leaves. Their gametophyte, the prothallus, is small and free-living, and water is still needed for fertilisation, so their distribution remains limited.
What is the difference between microphylls and macrophylls?
Microphylls are small leaves with one vein, as in Selaginella, while macrophylls are large leaves with branching venation, as in ferns. This is one of the features used to separate pteridophyte groups. Selaginella and Equisetum bear their sporangia in cone-like structures called strobili, whereas ferns carry them on leaves.
Which pteridophyte examples belong to which class?
Psilopsida includes Psilotum, Lycopsida includes Selaginella and Lycopodium, Sphenopsida includes Equisetum, and Pteropsida includes Dryopteris, Pteris and Adiantum. A frequent mistake is putting Equisetum with the Lycopsida, but it belongs to Sphenopsida, with its jointed stem and whorled scale-like leaves.
Homospory, heterospory and the seed habit
Read this section in the notes →What is the difference between homospory and heterospory?
Homosporous plants produce spores of a single kind, as in most pteridophytes. Heterosporous plants produce two kinds, small microspores and large megaspores, which germinate into male and female gametophytes respectively. Selaginella and Salvinia are heterosporous, and this condition, with the female gametophyte kept on the parent plant, is regarded as a step towards the seed habit.
How is heterospory linked to the evolution of the seed habit?
In heterosporous plants such as Selaginella and Salvinia, the female gametophyte is retained on the parent sporophyte for a variable period, and the zygote develops into a young embryo inside that female gametophyte. Keeping the female gametophyte and the developing embryo on the parent plant in this way is a precursor to the seed habit, regarded as an important step in plant evolution.
Gymnosperms: naked seeds
Read this section in the notes →What does the term gymnosperm mean?
Gymnosperm means naked seed. In these plants the ovules are not enclosed by an ovary wall, so after fertilisation the seeds are uncovered as well. They include Cycas, Pinus and Cedrus. In contrast, angiosperms have their ovules enclosed within the ovary, and their seeds develop inside fruits.
Which roots in Pinus and Cycas form partnerships with other organisms?
Pinus roots form a mycorrhizal association with fungi, while the coralloid roots of Cycas harbour nitrogen-fixing cyanobacteria. These two examples are often swapped in exams. In both cases the root partner helps the plant, with the fungus assisting absorption and the cyanobacteria supplying fixed nitrogen.
Gymnosperm cones, pollen and ovules
Read this section in the notes →Are male and female cones borne on the same plant in gymnosperms?
It depends on the genus. Pinus bears both male and female cones on the same tree, whereas in Cycas the male cones and the megasporophylls are borne on different plants. Gymnosperms are heterosporous, and the pollen grain is a much reduced male gametophyte, while the female gametophyte is retained within the ovule.
Angiosperms
Read this section in the notes →What is the range of size among angiosperms?
Angiosperms range from the tiny Wolffia, the smallest flowering plant, to tall trees of Eucalyptus, which can exceed a hundred metres. They bear pollen grains and ovules in flowers, and their seeds are enclosed in fruits. Two classes are recognised within them: dicots and monocots.
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