Chapter 3 : Plant Kingdom
Algae comprise several different groups of organisms which produce energy through photosynthesis.
Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic (both fresh water and marine) organisms.
They occur in a variety of other habitats: moist stones, soils and wood.
Some of them also occur in association with fungi (lichen) and animals (e.g., on sloth bear).
The form and size of algae is highly variable.
The size ranges from the microscopic unicellular forms like Chlamydomonas, to colonial forms like Volvox.
The algae reproduce by vegetative, asexual and sexual methods.
Vegetative reproduction is by fragmentation. Each fragment develops into a thallus.
Algae are useful to man in a variety of ways.
At least a half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis.
The algae are divided into three main classes:
Topics covered in this snack-sized chapter:
- Chlorophyceae: The members of chlorophyceae are commonly called green algae.
- Phaeophyceae: The members of phaeophyceae or brown algae
Bryophytes include the various mosses and liverworts that are found commonly growing in moist shaded areas in the hills
Bryophytes are also called amphibians of the plant kingdom because these plants can live in soil but are dependent on water for sexual reproduction
The plant body of bryophytes is more differentiated than that of algae.
It is thallus-like and prostrate or erect, and attached to the substratum by unicellular or multicellular rhizoids.
They lack true roots, stem or leaves. They may possess root-like, leaf-like or stem-like structures.
The main plant body of the bryophyte is haploid. It produces gametes, hence is called a gametophyte.
The bryophytes are divided into following categories:
- Rhodophyceae: Rhodophyceae are commonly called red algae because of the predominance of the red pigment, r-phycoerythrin in their body.
The liverworts grow usually in moist, shady habitats such as banks of streams, marshy ground, damp soil, bark of trees, and deep in the woods.
They consist of upright, slender axes bearing spirally arranged leaves. They are attached to the soil through multicellular and branched rhizoids.
The pteridophytes are vascular plants (plants with xylem and phloem) that produce neither flowers nor seeds, and are hence called vascular cryptogams.
The Pteridophytes include horsetails and ferns. Pteridophytes are used for medicinal purposes and as soil-binders.
They are also frequently grown as ornamentals.
Evolutionarily, they are the first terrestrial plants to possess vascular tissues – xylem and phloem.
In pteridophytes, the main plant body is a sporophyte which is differentiated into true root, stem and leaves.
The sporophytes bear sporangia that are subtended by leaf-like appendages called sporophylls.
The gymnosperms are plants in which the ovules are not enclosed by any ovary wall and remain exposed both before and after fertilisation.
Gymnosperms include medium-sized trees or tall trees and shrubs
The roots are generally tap roots.
The gymnosperms are heterosporous; they produce haploid microspores and megaspores.
The two kinds of spores are produced within sporangia that are borne on sporophylls which are arranged spirally along an axis to form lax or compact strobili or cones.
Angiosperms are flowering plants.
They are the most widespread and diverse group of land plants.
Their seeds are found in a closed ovary.
Unlike the gymnosperms where the ovules are naked, in the angiosperms or flowering plants, the pollen grains and ovules are developed in specialized structures called flowers.
In angiosperms, the seeds are enclosed by fruits.
The angiosperms are an exceptionally large group of plants occurring in wide range of habitat
The life cycle of an angiosperm is shown below:
In plants, both haploid and diploid cells can divide by mitosis.
This ability leads to the formation of different plant bodies - haploid and diploid.
The haploid plant body produces gametes by mitosis. This plant body represents a gametophyte.
The plant life cycle, unlike that of animals, consists of alternating generations of individual organisms that are haploid (the gametophyte) and diploid (the sporophyte).
Specialized diploid cells in the sporophyte undergo meiosis to produce haploid spores (hence the name "sporophyte").
Each spore grows mitotically to become the new gametophyte, which then produces gametes (hence, the name "gametophyte") which fuse to form a zygote.
3.6 Plant Life Cycles and Alternation of Generations arrow_upward
In haplontic life cycle, haploid individual forms haploid gamete by mitosis during gametogenesis.
Then, this gamete (N) fuses and forms zygote that is diploid.
This zygote undergoes meiosis to form haploid adult. Hence, adult is haploid and exhibits zygotic meiosis.
It takes place in Spirogyra, Chlamydomonas, and Volvox.
3.6.1 Haplontic Life Cycle
In diplontic life cycle, diploid individual forms haploid gamete by meiosis during gametogenesis.
Then, these gametes (N) fuse and form zygote that is diploid, which by mitosis forms diploid adult.
Hence, adult is diploid and exhibits gametic meiosis.
It takes place in all seed-bearing plants such as gymnosperms and angiosperms.
3.6.2 Diplontic Life Cycle
3.6.3 Haplo-diplontic Life Cycle
Both haplontic and diplontic type of life cycle alternate with each other.
It takes place in bryophytes and pteridophytes.