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.