Plant Kingdom

Algae:

• They 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.%%
• The form and size of algae are highly variable, ranging from $colonial forms like Volvox,$ $and the$ $filamentous forms like Ulothrix and Spirogyra.$
• The algae reproduce by vegetative, asexual and sexual methods.
    * Vegetative reproduction is by @@fragmentation@@@@.@@
      * Each fragment develops into a @@thallus.@@
    * Asexual reproduction is the production of different types of spores, the most common being the ==zoospores.==
      * They are flagellated (motile) and on germination give rise to new plants.
    * Sexual reproduction takes place through the fusion of two gametes.
      * These gametes can be ^^flagellated and similar in size (as in Ulothrix)^^ or on-flagellated (non-motile) but similar in size (as in Spirogyra). Such reproduction is called ==isogamous.==
      * Fusion of $two gametes dissimilar$ in size, as in species of $Eudorina$ $is termed$ $anisogamous.$
      * Fusion between one large, nonmotile (static) female gamete and a smaller, motile male gamete is termed ==oogamous as in Volvox, Fucus.==

Importance of Algae:

• At least half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis.
• Being photosynthetic they increase the level of dissolved oxygen in their immediate environment.
• They function as primary producers of energy-rich compounds which form the basis of the food cycles of all aquatic animals.
• Certain marine brown and red algae produce large amounts of @@hydrocolloids (water-holding substances)@@ @@like@@ @@algin and carrageen@@@@.@@
    * Porphyra, Laminaria and Sargassum are among the 70 species of marine algae ==used as food.==
      * ^^Agar^^^^,^^ obtained from Gelidium and Gracilaria is used to grow microbes and ^^in preparations of ice creams and jellies.^^
      * %%Chlorella%%, a unicellular alga rich in proteins is used as a food supplement even by space travellers.

Chlorophyceae:

• The members of Chlorophyceae are commonly called ^^green algae.^^
• The plant body may be ^^unicellular, colonial or filamentous.^^
    * They are usually grass green due to the dominance of %%pigments chlorophyll a and b.%% %%The pigments are localised in%% %%definite chloroplasts.%%
      * The chloroplasts may be $discoid, plate-like, reticulate, cup-shaped, spiral or ribbon-shaped$ in different species.
    * Most of the members have one or more @@storage bodies called pyrenoids@@ located in the chloroplasts.
      * @@Pyrenoids contain protein besides starch.@@
    * Some algae may store food in the form of %%oil droplets.%%
• They have a rigid cell wall made of an $inner layer of cellulose and an outer layer of pectose.$
    * Vegetative reproduction usually takes place by %%fragmentation%% or by the formation of different types of %%spores%%%%.%%
    * Asexual reproduction is by ^^flagellated zoospores produced in zoosporangia.^^
    * Sexual reproduction shows considerable variation in the type and formation of sex cells and it may be $isogamous, anisogamous or oogamous.$

Examples: ==Chlamydomonas, Volvox, Ulothrix, Spirogyra and Chara.==

Phaeophyceae:

• They are @@brown algae@@ which are found primarily in $marine habitats.$
• They show great variation in size and form.
    * They range from ^^simple branched, filamentous forms (Ectocarpus) to profusely branched forms as represented by kelps, which may reach a height of 100 metres.^^
• They possess $chlorophyll a, c, carotenoids and xanthophylls.$
    * They vary in colour from olive green to various shades of brown depending upon the amount of the @@xanthophyll pigment, and fucoxanthin@@ present in them.
• Food is stored as %%complex carbohydrates%%%%,%% which may be in the form of %%laminarin or mannitol.%%
    * The vegetative cells have a cellulosic wall usually covered on the outside by a ^^gelatinous coating of algin.^^
• The protoplast contains plastids, a centrally located vacuole and a nucleus.
• The plant body is usually attached to the substratum by a @@holdfast@@ @@and has a@@ @@stalk, the stipe@@ @@and leaf-like@@ @@photosynthetic organ@@@@,@@ @@the frond.@@
    * Vegetative reproduction takes place by @@fragmentation.@@
    * Asexual reproduction in most brown algae is by biflagellate zoospores that are pear-shaped and have two %%unequal laterally attached flagella.%%
    * Sexual reproduction may be ^^isogamous, anisogamous or oogamous.^^
      * The union of gametes may take place in water or within the oogonium.
      * The gametes are $pyriform$ $(pear-shaped) and$ $bear two laterally attached flagella.$

 Examples: @@Ectocarpus, Dictyota, Laminaria, Sargassum and Fucus.@@

Rhodophyceae:

• They are red algae because of the predominance of the red pigment, %%r-phycoerythrin%% %%i%%n their body.
• The majority of the red algae are marine with greater concentrations found in the $warmer areas.$
    * They occur in both well-lighted regions close to the surface of the water and also at great depths in oceans where relatively little light penetrates.
      * The red thalli of most of the red algae are ^^multicellular.^^
      * Some of them have complex body organisation.
    * The food is stored as floridean starch which is very similar to @@amylopectin and glycogen@@ in structure.
    * The red algae usually reproduce vegetatively by ==fragmentation====.==
    * They reproduce asexually by non-motile spores.
    * Sexually, they reproduce by non-motile gametes.
      * Sexual reproduction is ==oogamous== and accompanied by complex post-fertilization developments.

Examples: %%Polysiphonia, Porphyra, Gracilaria and Gelidium.%%

Bryophytes:

• They are found commonly growing in ==moist shaded areas== ==in the hills. They usually occur in== ==damp, humid and shaded localities====.==
• They 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 is $thallus-like and prostrate or erect,$ and attached to the substratum by ==unicellular or multicellular rhizoids==.
    * They $lack true roots, stems or leaves$ $but may$ $possess root-like, leaf-like or stem-like$ structures.
      * The main plant body of the bryophyte is haploid. It produces gametes, hence is called a gametophyte.
    * The sex organs in bryophytes are %%multicellular%%.
      * The male sex organ is called ^^antheridium^^ ^^which produces^^ ^^biflagellate antherozoids.^^
      * The female sex organ called $archegonium$ $is flask-shaped and produces a$ $single egg$$.$
        * The antherozoids are released into the water where they come in contact with archegonium.
          * An antherozoid fuses with the egg to produce the zygote.
            * Zygotes do not undergo reduction division immediately.
            * They produce a ==multicellular body called a sporophyte.==
            * The sporophyte is %%not free-living%% but attached to the photosynthetic gametophyte and derives nourishment from it.
• Some cells of the sporophyte undergo $reduction division$ (meiosis) to produce haploid spores. These spores germinate to produce %%gametophytes.%%

Importance of Bryophytes:

• They play an important role in @@plant succession on bare rocks/soil.@@
• Some mosses provide @@food for herbaceous mammals@@, birds and other animals.
• Species of %%Sphagnum%%%%, a moss,%% %%provide peat%% %%that has long been used as%% %%fuel, and as packing material for trans-shipment%% %%of living material because of their%% %%capacity to hold water%%.
• ^^Mosses along with lichens are the first organisms to colonise rocks^^ and hence, are of great ecological importance.
• They decompose rocks making the substrate suitable for the growth of higher plants. Since mosses form dense mats on the soil, they $reduce the impact of falling rain and prevent soil erosion.$

Liverworts:

• They grow usually in ==moist, shady habitats== such as banks of streams, marshy ground, damp soil, the bark of trees and deep in the woods.
• The plant body of a liverwort is $thalloid$. The thallus is dorsiventral and closely appressed to the substrate.
    * The leafy members have tiny %%leaf-like appendages%% in two rows on the stem-like structures.
    * Asexual reproduction in liverworts takes place by ^^fragmentation of thalli,^^ ^^or by the^^ ^^formation of specialised structures called gemmae^^
      * %%Gemmae are green, multicellular, asexual buds%%%%, which develop in small receptacles called%% %%gemma cups%% %%located on the thalli.%%
    * During sexual reproduction, male and female sex organs are produced either on the same or on different thalli.
      * The sporophyte is differentiated into a %%foot, seta and capsule.%% After meiosis, spores are produced within the capsule.
        * These spores germinate to form ^^free-living gametophytes.^^

Mosses:

• The predominant stage of the life cycle of a moss is the gametophyte which consists of two stages.
    * The first stage is the $protonema stage$$,$ which develops directly from a spore which is a $creeping, green, branched and frequently filamentous stage$$.$
    * The second stage is the @@leafy stage@@, which develops from the @@secondary protonema@@ as a lateral bud.
      * They consist of upright, slender axes bearing %%spirally arranged leaves%%%%.%%
      * They are attached to the soil through ^^multicellular^^ ^^and^^ ^^branched rhizoids.^^
    * Vegetative reproduction in mosses is by ^^fragmentation and budding^^ in the secondary protonema.
    * In sexual reproduction, the sex organs %%antheridia and archegonia%% %%are produced at the%% %%apex of the leafy shoots.%%
      * After fertilization, the zygote develops into a sporophyte, consisting of a foot, seta and capsule.
        * The ^^sporophyte in mosses is more elaborate than that in liverworts.^^
          * The capsule contains spores which are formed after meiosis.
• The mosses have an elaborate mechanism of spore dispersal.

Examples: %%Funaria, Polytrichum and Sphagnum.%%

Pteridophytes:

@@They are the first terrestrial plants to possess vascular tissues – xylem and phloem.@@

• They are found in cool, damp, shady places though some may @@flourish well in sandy-soil conditions.@@
• Their main plant body is a sporophyte which is $differentiated into true roots, stems and leaves$ $and possesses$ $well-differentiated vascular tissues.$
    * The ^^leaves are small^^ ^^(microphylls) as in^^ ^^Selaginella^^ ^^or^^ ^^large^^ ^^(macrophylls) as in^^ ^^ferns.^^
      * The sporophytes bear sporangia that are subtended by %%leaf-like appendages called sporophylls.%%
      * In some cases, sporophylls may form distinct @@compact structures called strobili or cones (Selaginella, Equisetum).@@
    * The sporangia produce spores by meiosis in spore mother cells which germinate to give rise to inconspicuous, small but multicellular, free-living, mostly photosynthetic thalloid gametophytes called ^^prothallus.^^
• These gametophytes require cool, damp, shady places to grow because of which the spread of living pteridophytes is @@limited and restricted@@ to narrow geographical regions.
    * The gametophytes bear male and female sex organs called antheridia and archegonia, respectively.
    * %%Water is required for the transfer of%% %%antherozoids.%%
      * Fusion of male gamete with the egg present in the archegonium results in the formation of zygote.
        * Zygote thereafter produces a multicellular well-differentiated sporophyte which is the ^^dominant phase^^ ^^of the pteridophytes.^^
      * In the majority of the pteridophytes, all the spores are of similar kinds; such plants are called %%homosporous.%%
      * Genera like ^^Selaginella and Salvinia^^ which produce two kinds of spores, macro (large) and micro (small) spores, are known as %%heterosporous.%%
    * The megaspores and microspores germinate and give rise to female and male gametophytes, respectively.
      * The female gametophytes in these plants are retained on the parent sporophytes for @@variable periods.@@
        * The %%development of the zygotes%% %%into young embryos takes place within the%% %%female gametophytes.%%
      * This event is a precursor to the %%seed habit%% considered an important step in evolution.
• The pteridophytes are further classified into four classes:
    * ^^Psilopsida (Psilotum);^^
    * ^^Lycopsida (Selaginella, Lycopodium)^^
    * ^^Sphenopsida (Equisetum)^^
    * ^^Pteropsida (Dryopteris, Pteris, Adiantum).^^

Gymnosperms:

• These are plants in which the %%ovules are not enclosed by any ovary wall and remain exposed, both before and after fertilisation.%%
• ^^The^^ ^^seeds^^ ^^that develop post-fertilisation, are not covered, i.e., are^^ ^^naked^^^^.^^
    * They include medium-sized trees or tall trees and shrubs, one of which is the ==giant redwood tree Sequoia== ==is one of the tallest tree species.==
    * The roots are generally @@tap roots.@@
      * Roots in some genera have a fungal association in the form of ^^mycorrhiza (Pinus)^^^^, while in some others^^ ^^(Cycas) small specialised roots called coralloid roots are associated with N2 - fixing cyanobacteria.^^
      * The stems are %%unbranched (Cycas) or branched (Pinus, Cedrus).%%
    * The leaves may be ==simple or compound.==
      * In Cycas, the pinnate leaves persist for a few years.
      * The leaves in gymnosperms are well-adapted to withstand extremes of temperature, humidity and wind.
      * In conifers, $the needle-like leaves$ reduce the surface area. Their thick cuticle and sunken stomata also help to reduce water loss.

• 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.@@
      * The strobili-bearing microsporophylls and microsporangia are called ^^microsporangiate or male strobili.^^
        * The microspores develop into a male gametophytic generation which is @@highly reduced and is confined to only a limited number of cells.@@ This reduced gametophyte is called a @@pollen grain.@@
        * $The development of pollen grains takes place within the microsporangia.$
        * The cones bearing megasporophylls with ovules or megasporangia are called %%macrosporangiate or female strobili.%%
      * The male or female cones or strobili may be borne on the ^^same tree (Pinus).^^
    * %%In%% %%cycas male cones and megasporophylls are borne on different trees.%%
      * The megaspore mother cell is differentiated from one of the cells of the nucellus.
      * The %%nucellus%% %%i%%s protected by envelopes and the composite structure is called an @@ovule@@.
        * The ovules are borne on megasporophylls which may be clustered to form the female cones.
        * The megaspore mother cell divides ==meiotically to form four megaspores.==
      * One of the megaspores enclosed within the megasporangium develops into a multicellular female gametophyte that bears two or more archegonia or female sex organs.
      * ^^The multicellular female gametophyte is also retained within the megasporangium^^.
      * Unlike bryophytes and pteridophytes, in gymnosperms, the @@male and the female gametophytes do not have an independent free-living existence. They remain within the sporangia retained on the sporophytes.@@
        * The pollen grain is released from the microsporangium and are carried in air currents and come in contact with the opening of the ovules borne on megasporophylls.
        * The pollen tube carrying the male gametes grows towards archegonia in the ovules and discharges its contents ==near the mouth of the archegonia.==
• Following fertilization, the zygote develops into an embryo and the ovules into seeds which are not covered.

Angiosperms:

• The $pollen grains and ovules are developed in specialized structures called flowers$ $and the$ $seeds$ $are enclosed in$ $fruits$
• They are an exceptionally large group of plants occurring in a wide range of habitats.
    * They range in size from the smallest %%Wolffia to tall trees of Eucalyptus (over 100 metres).%%
• The dicotyledons are characterised by seeds having ^^two cotyledons, reticulate venations in leaves, and tetramerous or pentamerous flowers^^, i.e., having four or five members in each floral whorls.
    * The male sex organ in a flower is the %%stamen.%%
      * Each stamen consists of a slender ^^filament with an anther at the tip.^^
      * Within the anthers, the pollen mother cell divide by meiosis to produce microspores which mature into pollen grains.
    * The female sex organ in a flower is the pistil which consists of a swollen ovary at its %%base, a long slender style and a stigma.%%
      * Inside the ovary, ovules are present.
      * Generally, each ovule has a ^^megaspore mother cell that undergoes meiosis to form four haploid megaspores.^^
      * Three of them degenerate and %%one divides to form the embryo sac.%%
      * Each embryo sac has a %%three-celled egg apparatus%% – one egg cell and two synergids, three antipodal cells and two polar nuclei.
      * The polar nuclei eventually fuse to produce a %%diploid secondary nucleus.%%
    * ^^Pollen grains, after dispersal from the anthers, are carried by wind or various other agencies to the stigma of a pistil. This is termed^^ ^^pollination.^^
    * The pollen grains germinate on the stigma and the resulting %%pollen tubes grow through the tissues of the stigma and style and reach the ovule.%%
      * The pollen tubes enter the embryo sac where two male gametes are discharged.
        * One of the male gametes fuses with the egg cell (syngamy) to form a ^^zygote^^. The other male gamete fuses with the diploid secondary nucleus to produce the ^^triploid primary endosperm nucleus.^^
        * This occurrence of two fusions i.e., syngamy and triple fusion, this event is termed %%double fertilisation, an event unique to angiosperms.%%
      * The ^^zygote develops into an embryo^^ ^^(with one or two cotyledons) and the^^ ^^PEN develops into endosperm which provides nourishment to the developing embryo.^^
      * The %%synergids and antipodals degenerate after fertilisation.%%
• During these events, the ovules develop into seeds and the ovaries develop into a fruit.