PLANT KINGDOM

Economic Importance :

• Food for herbaceous animals.
• Sphagnum in form of peat is used as fuel and also used for trans-shipment of living material as it has water holding capacity, prevent soil erosion, along with lichens are first colonizers on barren rocks.
• They decompose rocks making substrate for the growth of higher plant.

PTERIDOPHYTES :

• First terrestrial plant possesses vascular tissue like xylem and phloem.
• The plant body differentiated into true root, stem and leaf.
• The main plant body is sporophytic.
• Leaves may be small (microphyll) as in Selaginella or large (macrophyll) as in ferns.
• Sporangia having spores are subtended by leaf-like appendages called sporophylls. (Sporophylls may be arranged to form strobili or cones.)
• In Sporangia, the spore mother cells give rise to spores after meiosis.
• Spores germinate to form haploid gametophytic structure called prothallus which is free living, small, multicellular and photosynthetic.
• Prothallus bears antheridia and archegonia which bear antherozoids and egg cell respectively which on fertilization form zygote.
• Zygote developed into multicellular, well differentiated sporophyte.
• Most of pteridophytes produce similar kinds of spores hence called homosporous.
• Genera like Selaginella and Salvinia which produce two kind of spores, macro (large) and small (micro) spores are known as heterosporous. Microspore and macrospore germinate and gives rise to male and female gametophyte respectively.
• The female gametophytes in these plants are retained on the parent sporophyte for variable period. The development of zygote into young embryo takes place within the female gametophytes. This events is a precursor to the seed habits considered an important steps in evolution
• Pteridophytes further classified into four classes: Psilopsida (Psilotum), Lycopsida (Selaginella), Sphenopsida (Equisetum) and Pteropsida (Pteris).

GYMNOSPERMS :

• Have naked seeds as the ovules are not enclosed by any ovary wall and remain exposed both before and after fertilization.
• Gymnosperm includes medium-sized trees or tall trees and shrub.
• Root is generally tap root. May be associated with myorrhiza.
• Stem is branched (Pinus), or unbranched (Cycas).
• Leaves may be simple or compound.

REPRODUCTIONS :

• Gymnosperms are heterosporous. They produce haploid microspores and megaspores.
• Male strobili or cone has microsporophylls which bear microsporangia having microspores which develop into reduced gametophyte called pollen grain.
• Female cone or strobili has megasporophylls which bear megasporongia having megaspores which are enclosed within the megasporangium (Nucellus). One megaspore develops into female gametophyte bearing two or more archegonia.
• Pollen grains carried in air currents reach ovules, form pollen tube which reach archegonia and release male gametes which fertilize egg cell and form zygote which produce embryos. Ovules develop into seeds which are not covered.

ANGIOSPERMS :

• Called flowering plants and have seeds enclosed in fruits.
• Divided into two classes − Dicotyledons (have two cotyledons) and Monocotyledons (have one cotyledon).
• Smallest angiosperm: Wolfia (microscopic).
• Large tree: Eucalyptus over 100 meters.
• Reproductive organs developed in flowers.
• Male sex organs in a flower are called stamens.
• Stamen has filament and anther. Anthers on meiosis produce pollen grains. Pollen grains have two male gametes.
• Female sex organs are pistil.
• Pistil has stigma, style and ovary. Ovary has one or many ovule in which female gametophyte (embryo sac) develops by meiosis.
• Embryo sac has 7 cells and 8 nuclei. One egg cell, 2 synergids, 3 antipodal and one central cell having two polar nuclei.
• Pollen grain is carried by wind; water etc. reaches to stigma and produces pollen tube which enters embryo sac.

Double fertilization:

• Syngamy: One male gamete fuses with egg cell to form zygote which develops into embryo.
• Triple fusion: Other male gamete fuses with secondary nucleus (formed by fusion of two polar nuclei) which forms triploid primary endosperm nucleus (PEN). PEN develops into endosperm which nourishes the developing embryo.
• Ovules develop into seeds and ovaries into fruits.

Alternation of generation: Haploid gametophytic and spore producing sporophytic generation alternate with each other in this process.

• Haplontic: Gametophytic phase dominant. e.g., Chlamydomonas
• Diplontic: Sporophytic phase dominant. e.g., Angiosperms and Gymnosperms
• Haplo-Diplontic: Intermediate like stage where gametophytic and sporophytic stage partially dominates at different stages. e.g., Bryophytes and Pteridophytes.

Exceptions: Ectocarpus, Polysiphonia are Haplo-diplontic algae. Fucus is diplontic alga.

 

CBSE Biology (Chapter Wise) Class XI ( By Mr. Hare Krushna Giri ) 
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