Algae
* Algae are simple, thalloid, chlorophyll-bearing organisms found in fresh and marine water, moist stones, soils and wood.
* Algae can be unicellular, colonial or filamentous.
* Based on the type of pigment and the type of stored food we can classify algae into Chlorophyceae, Phaeophyceae and Rhodophyceae.
* Chlamydomonas, Volvox, Spirogyra, are some members of the class Chlorophyceae.
* They are commonly called green algae.
* Class Phaeophyceae includes Ectocarpus, Dictyota, and Laminaria.
* These species, commonly termed as brown algae, are mostly found in brackish or salt water.
* Rhodophyceae is another class of algae whose members include Polysiphonia, Gracilaria and Gelidium, often called red algae.
Bryophytes
* Bryophytes are plants that usually grow in damp, humid and shaded localities.
* The plant body is thallus-like and is either prostrate or erect, and is attached to substratum by unicellular or multi-cellular rhizoids.
* Bryophytes have both male and female multi-cellular sex organs.
* The male sex organ is called the antheridia and it produces biflagellate antherozoids.
* The female organ is called the archegonium and produces a single egg.
* Bryophytes are of two types- liverworts and mosses.
* Liverworts grow in moist and shady environments.
* Liverworts reproduce both asexually and sexually.
* Moss is another type of bryophyte.
* Mosses reproduce vegetatively and sexually.
Pteridophytes
* Pteridophytes are plants that:
• Flourish in a damp, cool and shady habitat although some species also grow in sandy-soil conditions.
• Possess vascular tissues- xylem and phloem.
* The main plant body is a sporophyte and is differentiated into organs such as roots, stem and leaves.
* Sporophytes bear sporangia, which germinate into a prothallus- a structure that bears antheridia and archaegonia.
* An antherozoid fuses with the egg in the archegonium to form a zygote.
* The sporophytic phase is the dominant phase in the lifecycle of pteridophytes.
Gymnosperms
* Gymnosperms are plants in which the ovules are not enclosed by an ovary wall and remain exposed both pre and post-fertilisation.
* The seeds that develop from the ovules post-fertilisation too are uncovered or naked.
* Most gymnosperms have tap roots.
* The stems of a gymnosperm can be branched or unbranched and the leaves compound or simple.
* The reproductive structure of a gymnosperms is called a strobilus or a cone.
* Gymnosperms have both male and female strobili.
* Fertilisation occurs when the pollen grains, released from the microsporangium, are carried by the air currents and come in contact with the opening of the ovules.
Angiosperms
* Angiosperms are plants in which the ovules are not naked but present inside the flower.
* Reproduction in angiosperms takes place through stamen and pistil, the male and female sex organs in the plant’s flower.
* The stamen consists of two parts- filament and anther.
* The pistil consists of the stigma at the followed by the style in the middle and an ovary at the base.
* The embryonic-sac has a three-celled egg apparatus consisting of one egg cell, two synergids, three antipodal cells and two polar nuclei.
* During pollination, one male gamete fuses with the egg cell to produce a zygote, while the other gamete fuses with the diploid secondary nucleus to produce the triploid primary endosperm nucleus or PEN.
Plants Life cycles and Alternation of Generations
* The life cycle of any sexually reproducing plant, alternates between a spore producing diploid sporophyte and a gamete producing haploid gametophyte. This is termed as alternation of generations.
* In the haplontic lifecycle:
• Dominant gametophytic generation is represented by a haploid free-living and photosynthetic gametophyte.
• Short-lived sporophytic generation is represented by a diploid single-cell zygote. In the diplontic lifecycle:
• The dominant sporophytic generation is represented by a diploid, free-living and photosynthetic sporophyte.
• Short-lived gametophytic is represented by a haploid, single or few-celled gametophyte.
* In the haplo-diplontic lifecycle, then:
• Dominant gametophytic generation is represented by a haploid, free-living, thalloid, photosynthetic and erect gametophyte.
• Short-lived sporophytic generation is represented y a diploid, multi-cellular sporophyte that is totally or partially dependent on the gametophyte for its anchorage and nutrition.