The Plant Kingdom
An Evolutionary Journey from Algae to Angiosperms
1. Systems of Classification
Plant classification has evolved from simple artificial systems to complex phylogenetic ones.
Artificial System
Based on a few superficial morphological characters. Linnaeus's system is a prime example. It was simple but lacked evolutionary context.
Natural System
Based on natural affinities, considering both external and internal features. Bentham and Hooker's system is a classic example.
Phylogenetic System
The modern approach, based on evolutionary relationships (phylogeny). It uses data from fossils, cytotaxonomy, and chemotaxonomy.
2. Algae (Thallophytes)
Algae are simple, chlorophyll-bearing, thalloid (undifferentiated body), and largely aquatic autotrophs. They show great diversity in form and size.
Classes of Algae
| Feature | Chlorophyceae (Green Algae) | Phaeophyceae (Brown Algae) | Rhodophyceae (Red Algae) |
|---|---|---|---|
| Major Pigments | Chlorophyll a, b | Chlorophyll a, c, Fucoxanthin | Chlorophyll a, d, r-Phycoerythrin |
| Stored Food | Starch | Mannitol, Laminarin | Floridean Starch |
| Cell Wall | Cellulose | Cellulose & Algin | Cellulose, Pectin |
| Examples | Chlamydomonas, Spirogyra | Laminaria, Sargassum | Porphyra, Gelidium |
3. Bryophytes (Amphibians of the Plant Kingdom)
Bryophytes (mosses and liverworts) are called amphibians of the plant kingdom as they require water for sexual reproduction. Their main plant body is a haploid gametophyte, and the diploid sporophyte is dependent on it.
Liverworts
Have a thalloid body (Marchantia). Asexual reproduction can occur via gemmae in gemma cups.
Mosses
Have a two-stage gametophyte: a creeping protonema and an upright leafy stage. Example: Funaria, Sphagnum.
4. Pteridophytes (First Terrestrial Vascular Plants)
Pteridophytes (ferns and horsetails) are the first terrestrial plants with vascular tissues (xylem and phloem). Here, the main plant body is the diploid sporophyte, which is differentiated into true root, stem, and leaves.
A significant evolutionary step seen in Selaginella and Salvinia is heterospory (producing two types of spores), which is a precursor to the seed habit.
5. Gymnosperms (Naked Seed Plants)
Gymnosperms have ovules that are not enclosed within an ovary wall. Their seeds are "naked." The main plant body is a sporophyte. They are heterosporous, producing spores in compact structures called strobili or cones.
Examples include Pinus, Cycas, and the giant redwood tree Sequoia.
6. Angiosperms (Flowering Plants)
The most advanced and diverse plants. They bear flowers, and their seeds are enclosed within a fruit. They exhibit a unique phenomenon called double fertilization.
Double Fertilization
One male gamete fuses with the egg cell (Syngamy) to form the zygote (2n). The other male gamete fuses with the two polar nuclei (Triple Fusion) to form the primary endosperm nucleus (3n), which develops into the nutritive endosperm.
7. Plant Life Cycles and Alternation of Generations
Plant life cycles involve an alternation between a haploid gametophyte (n) and a diploid sporophyte (2n). There are three main patterns.
Haplontic Life Cycle
The dominant phase is the gametophyte (n). The sporophyte is represented only by the zygote (2n). Common in most algae like Volvox and Spirogyra.
Diplontic Life Cycle
The dominant phase is the sporophyte (2n). The gametophyte is highly reduced. Found in all seed plants (Gymnosperms and Angiosperms).
Haplo-diplontic Life Cycle
Both gametophyte and sporophyte phases are multicellular. In Bryophytes, the gametophyte is dominant. In Pteridophytes, the sporophyte is dominant.
