PLANT KINGDOM
The kingdom plantae includes algae, bryophytes, pteridophytes, gymnosperms and angiosperms. There
are different types of classifications for plants. In artificial system only external characters is taken into
consideration. Natural system of classification considers natural affinities among organisms. It takes into account
the external and internal characters as well. Benthem and Hooker’s classification is an example for natural system
of classification. Phylogentic system is based on the evolutionary relationships of the organisms. Numerical
taxonomy uses computer data for classification. Cytological information is the basis for Cytotaxonomy and
chemical constituent of plants is the basis of Chemotaxonomy.
ALGAE
Algae are chlorophyll bearing, simple, thalloid, autotrophic and largely aquatic. Occur in variety of
habitats. The form and size is highly variable. Microscopic unicellular, colonial, filamentous and macroscopic kelps
are seen as members. Reproduction involves vegetative, asexual and sexual methods. Vegetative reproduction is
through fragmentation. Asexual reproduction is through spores like zoospores. Sexual reproduction is brought
about by similar gametes ( isogamy), dissimilar gametes ( anisogamy) or by large stationary female gamete and
small motile male gamete ( oogamy).
Economically algae are useful in many ways. They are the chief primary producers of the water, being
food for other organism and increase the level of dissolved oxygen. They are used as food. Algin and carrageen are
hydrocolloids obtained from algae. Agar is obtained from algae like Gelidium and Gracilaria. Unicellular algae like
chlorella and spirulina are used as SCPs ( single cell protein).
Algae are classified into chlorophyceae, phaeophyceae and rhodophyceae.
Chlorophyceae:
Commonly called green algae. They may be unicellular, filamentous or colonial. Grass green in color due
to chlorophyll a and b. The chloroplasts are different shaped. They contain pyrenoids where food is stored in the
form of starch and protein. Cell wall contains cellulose. Reproduction is through vegetative, asexual and sexual
means. Chlamydemonas, Volvox, Ulothrix are examples.
Phaeophyceae:
Commonly called brown algae. Primarily found in marine habitats. Members are simple to giant kelps. The
pigments are chlorophyll a , c, carotenoids and xanthophylls. Fucoxanthin pigment determines the colour of algae.
Food is stored as mannitol or laminarin. Cell wall is cellulosic with an outer covering of algin. Plant body has a
holdfast, stipe and frond. The asexual spores are pear shaped. Sexual reproduction involves isogamy, aniosogamy
and oogamy. The gametes are pyreform. Ectocarpus, Dictyota and Laminaria are examples.
Rhodophyceae:
Commonly called red algae, due to the pigment r-phycoerythrin. Marine and present in both surface
water and deep water. Mostly multicellular. Food is stored in the form of floridean starch. Cell wall is cellulosic
with pectin. Reproduce vegetatively asexually and sexually. Sexual reproduction is oogamous. Common members
are Gracilaria and Gelidium.
BRYOPHYTES
Bryophytes are called ambphibians of the plant kingdom, since they need water for fertilization. They
include mosses and liverworts. Present in damp, humid places. They play a major role in plant succession. The
main plant body is gametophytic thallus attached to substratum by rhizoids. The sex organs antheridium and
archegonium are formed in gametophyte. Antherzoids produced from antheridia fuses with egg to form zygote
which is diploid. The zygote produce sporophyte. The sporophyte is attached to gametophyte. After reduction
division sporophyte produce haploid spores. These spores on germination form gametophyte.
Economically bryophytes are of not great importance. They are used as food for herbaceous animals, as
packing material, as fuel etc. They are ecologically important as they play vital role in succession.
Liverworts:
The thallus is dorsiventral and closely appressed to substratum and found in moist, shady habitats.
Asexual reproduction is by fragmentation or through special bodies called gemmae. Sexual reproduction results in
sporophyte which has foot, seta and capsule. Eg. Marchantia
Mosses:
The gametophyte has two stages, protonema (creeping stage) and a leafy stage. Leafy stage has upright,
slender axes bearing spirally arranged leaves. Vegetative reproduction is by fragmentation or budding. The
sporophyte of moss is elaborate having foot, seta and capsule. Ex. Funaria, Sphagnum
PTERIDOPHYTES
They are the first terrestrial plants with vascular system. They include horsetails and ferns. Found in cool
damp places. They have medicinal value and play role in preventing soil erosion. The main plant body is
sporophytic with true root, stem and leaves. The leaves are called microphylls or macrophylls. Sporophylls are
leaves subtending sporangia. Sometimes sporophylls form compact bodies called cones or strobili. Haploid spores
formed in sporangian on germination form inconspicuous gametophyte called prothallus. The gametophyte
requires cool damp places to grow which leads to the restriction of pteridophytes in narrow geographical regions.
The gametes are formed in archegonia and antheridia in prothallus. The fusion of gametes results in sporophyte.
Certain pteridophytes produce two types of spores called heterosporous as in Selaginella. Homosporous condition
seen in most cases where only type of sprore is produced. Heterospory is a precursor of seed habit.
The pteridophytes are further classified into Psilopsida, Lycopsida Sphenopsida and Pteropsida.
GYMNOSPERMS
Gymnosperms are naked seeded plants, where the seeds are not enclosed in fruit. They are flowerless
plants. The habit varies from shrubs, medium trees to the largest Sequoia tree. Roots genraly tap roots. In Pinus
fungal association ( mycorrhiza) is seen in roots. The corolloid roots are seen in Cycas which has symbiotic
association of cyanobacteria. The leaves are well adapted to extreme condition with thick cuticle, sunken stomata
and needle shape.
They are heterosporous produce haploid microspores and megaspores. The spores are produced in
sporangia which are in the form of cones or strobili. The male strobili is microsporangia formed in
microsporophylls. The microspore form pollen grain having male gamete. The megasporangia are female cones.
Ovules are formed in megasporophylls. Inside the ovule the female gametophyte or archegoniums is seen. The
pollen grains are transferred to archegonium by air currents. Following fertilization zygote develops into embryo
and the ovules into seeds.
ANGIOSPERMS
They are flowering plants. They range from microscopic Wolfia to tall trees. They are classified into
dicotyledons and monocotyledons. The male sex organ is anther produces pollen grains. The female sex organ is
pistil produces egg. The egg is enclosed in female gametophyte called embryosac. The embryosac is seen is ovule
which is enclosed in ovary. The embryosac is eight celled, three form egg apparatus, two form polar nuclei and
three form synergids. Pollen grain after shedding on pistil grow towards ovary and releases two gametes. On
gamete fuses with egg and form zygote and the other one fuses with polar nuclei form primary endosperm
nucleus. The zygote form embryo and PEN for the endosperm.
LIFE CYCEL AND ALTERNATION OF GENERATION
Every individual has two phases in their life cycle, haploid gametophytic stage and diploid sporophytic
stage. Zygote is the first stage of diploid phase. It undergoes meiosis or reduction division to form haploid spores.
On germination they form gametophyte. These two phases alternate with each other in the life cycles called
alternation of generation. According to the dominance of the phase the life cycles of organism are classified into
haplonic, diplontic and haplodiplontic.
In haplontic life cycle the dominant phase is haploid as in algae. In angiosperms the dominant phase is
diploid called diplontic life cycle. In certain organisms like pteridophytes and bryophytes the two phases are
dominant called haplodiplontic life cycle. There are exception in certain members of each group.