Four Wet-Notebook Specimens from Kolli Hills
A team of four Second Year students spends two days surveying plants at Kolli Hills (about 1,100 m) for their practical record. On the last afternoon rain soaks the field notebook, and only these notes survive. Specimen 11: flat green ribbon-like thallus branching into two, no veins, fine hair-like rhizoids underneath, growing in a wet rock crevice, with small flask-shaped structures embedded in the upper surface. Specimen 12: creeping over jackfruit bark, a slender stem-like axis with tiny overlapping leaf-like flaps, and a stalked oval capsule with a lid at the tip. Specimen 13: on the forest floor, fronds that unroll from the tip, brown dotted patches on the lower surface, and a blackish wiry underground stem. Specimen 14: a bright green mat on the pond margin, unbranched threads with spiral green bands inside, no rhizoids and no conducting tissue. Each specimen needs a division, a one-line justification and a herbarium label by Monday. The teacher has already warned that "it looks like a moss" will not be accepted as a justification.
1. Assign each of the four specimens to a division and name the single character that settles the assignment. Where two characters seem to conflict, state which one you trust and why.
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The key must be applied in order of rank, not by overall appearance.
Rank 1 - is there a multicellular embryo/archegonium? Algae lack it; all land plants have it. Rank 2 - is there vascular tissue (tracheids and sieve elements)? Bryophytes lack it; pteridophytes and above have it. Rank 3 - are seeds produced? Pteridophytes are seedless; gymnosperms and angiosperms bear seeds.
- Specimen 11 - Bryophyta (thallose liverwort, Marchantia type). Decisive: no vascular strands, hair-like rhizoids instead of roots, gametangia in flask-shaped archegoniophores/cushions embedded on the dorsal surface. The plant body is the gametophyte.
- Specimen 12 - Bryophyta (moss). Decisive: the capsule with a lid (operculum) is a single sporangium-bearing sporophyte growing out of a leafy gametophyte; the leaf-like flaps are one cell thick and have no vein.
- Specimen 13 - Pteridophyta. Decisive: vascular tissue present (dark wiry rhizome) with no seeds; circinate vernation and sori of many sporangia are confirmatory, not decisive.
- Specimen 14 - Thallophyta (Chlorophyta). Decisive: no vascular tissue and no embryo; a filament of single cells with spiral chloroplasts is an undifferentiated thallus.
Where characters conflict, the higher-ranked one wins: a leaf-like flap does not make a plant a pteridophyte if it has no vascular supply.
2. A classmate insists Specimen 12 must be a pteridophyte because it has a stem, leaves and a spore case. Give the observations that decide against this, and say what section you would cut to prove your case.
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Three independent observations defeat the pteridophyte claim:
- Anatomy. Cut a transverse section of the stem-like axis and stain with safranin–fast green. A pteridophyte axis shows a stele with xylem tracheids and phloem. Specimen 12 shows only parenchyma with no lignified tracheids - the axis is a conducting strand at best, not a vascular cylinder.
- Which generation is conspicuous. In pteridophytes the leafy plant is the diploid sporophyte and the gametophyte is a small independent prothallus. Here the leafy plant is haploid and the spore-bearing capsule is attached to and dependent on it - the sporophyte is a parasite on the gametophyte. That is the bryophyte condition.
- The spore case itself. A pteridophyte sorus is a cluster of many separate sporangia on the underside of a frond. Specimen 12 has one capsule with a lid and (in most mosses) peristome teeth at the mouth - a single sporangium on a seta.
Add the absence of true roots (rhizoids only) as supporting evidence. The decisive argument remains the lack of vascular tissue, because it is a character of the whole plant body rather than of one organ.
3. The team collected no gymnosperm. Where and how should they have searched, and which single feature would have separated a gymnosperm from Specimen 13 at a glance?
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Gymnosperms in Tamil Nadu are not crevice or forest-floor plants; the team was looking in the wrong habitat. They should have searched the drier, higher, more exposed slopes and shola margins (for example the upper reaches of Kolli Hills, Nilgiris or Palani hills, 1,500–2,000 m) for woody trees or shrubs with needle-like or scale-like leaves - Pinus, Podocarpus, cycads such as Cycas in the plains and foothills, and Gnetum in the low country.
Search tactic: look for cones, not for sori or capsules. Mature female cones with winged seeds lying on the scale give the identification immediately, and a hand lens shows the ovule/seed sitting naked on the cone scale.
The decisive contrast with Specimen 13: a gymnosperm bears seeds - the ovule is exposed on a cone scale and is not enclosed in an ovary - whereas Specimen 13 is seedless and reproduces only by spores released from sori. Habit (tree versus herb) is only a supporting clue; seed versus no seed is the character that settles the division.