Sorting a Mixed Collection from a Pond Bank and Laterite Wall
During a first-year field trip to a pond and the adjoining laterite wall, a student collects four specimens and records these notes. P: a filamentous green alga with a spiral chloroplast; it is free-living and no multicellular sex organs were seen. Q: a thalloid plant growing on the damp wall; it has rhizoids, multicellular sex organs, and a small sporophyte that remains attached to and dependent on the plant body. R: a small vascular plant with microphyllous leaves; it produces spores of two sizes inside strobili and no seeds. S: a vascular plant with seeds borne on the surface of cone scales; there is no ovary wall. The student labels P and Q together as Bryophyta because neither shows vascular tissue, and labels R as a gymnosperm because it bears strobili. Both labels are marked wrong. The teacher asks the student to defend a fresh classification using only the recorded characters.
1. Assign P, Q, R and S to the correct group and give the single most decisive character for each assignment.
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P is a green alga (Chlorophyta). The decisive character is the free-living thallus with no multicellular sex organs and no embryo; the spiral chloroplast supports green-algal affinity.
Q is a bryophyte. The decisive character is the multicellular sex organ together with a sporophyte that stays attached to and dependent on the gametophyte. In other words, fertilisation produces an embryo that is retained and nourished by the parent plant.
R is a pteridophyte, specifically a heterosporous fern ally such as Selaginella. The decisive character is vascular tissue plus spores of two sizes produced in strobili, with no seeds.
S is a gymnosperm. The decisive character is the seed borne naked on a cone scale, with no ovary wall enclosing it.
The reasoning is that a classification must rest on a set of fundamental characters. The presence of an embryo, the position of the sporangia and the seed condition carry more weight than the single negative character of vascular tissue being absent.
2. Explain why the absence of vascular tissue is not a valid reason to put P and Q in the same group. Name the feature that separates them and explain what it implies about their life cycle.
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Absence of vascular tissue is a negative character. It can result from reduction or from retention of an ancestral condition, and it does not by itself unite two plants. P is not an embryophyte: it has no multicellular sex organs and no embryo. Q is an embryophyte: after fertilisation an embryo is retained and nourished by the gametophyte.
That difference in life cycle is decisive. P completes its life cycle free-living, with no dependent embryo stage. Q shows the bryophyte pattern: a dominant gametophyte and a sporophyte that remains attached to it. Grouping P with Q on the basis of a shared absence would also wrongly merge liverworts with algae and would ignore the jacketed multicellular sex organs of Q. The positive character, the retained embryo, is what places Q among the embryophytes.
3. R is often described as showing a step towards the seed habit. Using the recorded characters, justify this statement and state what R still lacks that S has.
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R shows heterospory: it produces spores of two sizes. The larger megaspore is retained on the parent plant, and the female gametophyte develops inside the spore wall, so the egg is protected and supplied by the sporophyte. This retention and endosporic development is the essential pre-condition for the seed habit.
However, R still lacks the seed itself. There is no integument enclosing the megasporangium, and there is no embryo resting inside a seed with a food store. S has that seed, borne on a scale, with the embryo enclosed. R therefore represents the precursor stage: heterospory and retention of the megaspore, but not the fully enclosed seed.