ALTERNATION
OF GENERATION
Life cycle of an organism is a sequence of events that crcur from
birth to death of an organism. In plants, both haploid and diploid
cells can divide by mitosis.
This feature leads to the formation of different plant bodies, i.e.
haploid and diploid. ‘The haploid plant body produces gametes by
mitosis. This plant body represents a gametophyte.
After fertilisation, zygote also divides by mitosis to produce a
diploid sporophytic plant body.
Haploid spores are produced by this plant by meiosis.
These in turn, divide by mitosis to form a haploid plant
once again.
Thus, during the life cycle of any sexually reproducing plant,
there is an alternation of generation between gamete producing
haploid gametophyte and spore producing diploid sporophyte.

GYMNOSPERMS
The gymnosperms (Gymnos = naked; sperma = seeds)
are plants in which the ovules are not enclosed by
any ovary wall and remain exposed, both before and
after fertilisation. These are small groups of seed
plants which are represented by only 900 living species.
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.
Habitat
These plants are mostly found in colder parts of
northern hemisphere, where they form extensive
forests. A number of gymnosperms are now
known as ornamentals, e.g., Cycas, Ginkgo, Thuja,
Araucaria, etc.
Morphology
Gymnosperms include, medium-sized trees or tall
trees and shrubs. The giant red wood tree Sequoia is
one of the tallest tree upto 100 m. Species of Gnetum
are woody climbers. The smallest gymnosperm is
Zamia pygmaea which reaches a height of 25 cm.
Many of the gymnosperms live for more than 4000
years, e.g., pine (Pinus), redwood (Sequoia) trees.
External Features
The plant body is sporophyte and very well-differentiated into
root, stem and leaves.
Plant Body
Tap roots present serve the function of proper anchorage of heavy
plant to the soil. Roots in some genera have fungal association in
the form of mycorrhiza (Pinus), while in some others (Cycas) small
specialised roots called coralloid roots are associated with
N 2-fixing cyanobacteria such as Anabaena, Nostoc, etc.
The stems are branched (Pinus, Cedrus) or unbranched (Cycas).
The leaves may be simple or compound.
In Cycas, the pinnate leaves persist for few years. The leaves in
gymnosperms are well-adapted to withstand extremes of
temperature, humidity and wind. While, in conifers, the
needle-like leaves reduce the surface area to withstand the extremes
of cold temperatures and snowfall. Their thick cuticle and sunken
stomata also help to reduce water loss.
Internal Featuies
Plants possess vascular tissues, i.e., xylem and phloem. They are
arranged in vascular bundles.

Vascular Tissues
The conducting part of xylem consists of tracheid. Vessels occur in
Gnetum and its relatives. Phloem has sieve cells, but are arranged in
rows in Gnetum.
Reproduction
The gymnosperms are heterosporous, i.e., they
produce haploid microspores and megaspores.
Cones
The two kinds of spores are produced within
sporangia that are born on sporophylls, which are
arranged spirally along an axis to form compact
strobili or cones. They are produced in different
types of sporangia called microsporangia or
microsporophylls and ovules or megasporangia,
respectively.
The strobili bearing microsporangia are called
microsporangiate or male strobili and the strobili,
bearing megasporophylls with ovules or
megasporangia are called macrosporangiate or
female strobili.
The plant may be monoecious in Pinus (i.e., male
and female cones or strobili may borne on same tree)
or may be dioecious in Cycas (i.e., they bear on
different trees).
Pollen Grains
The microspores develop into a male
gametophytic generation which is highly reduced
and is confined only to a limited number of cells,
known as pollen grain. The development of pollen
grains occurs within the microsporangia.
Ovules
The megaspore mother cell is differentiated from
one of the cells of the nucellus, which is protected by
envelopes and the composite structure called ovule.
The ovules are borne on the megasporophylls which
may be clustered together forming female cones.
Archegonia
The megaspore mother cell thus, undergoes meiotic
division forming four megaspores. Out of which
one is enclosed within the megasporangium
(nucellus) and develops into a multicellular female
gametophyte, bearing two or more archegonia of
female sex organs.
The multicellular female gametophyte is also
retained within megasporangium.
Fertilisation
Air current is required for transport of male
gametes. The male gametes are carried to the
archegonia, i.e., gamete in the ovule by means of a
tube called pollen tube and discharge their contents
near the mouth of the archegonia. This is called
siphonogamy.
Seeds
Following fertilisation, zygote develops into an embryo and the
ovules into seeds. The seeds contain food laden tissue called
endosperm. It lies naked or exposed. Endosperm provides
nourishment for growth of seedling at the time of seed
germination.
Examples Cycas, Pinus, Ginkgo, Ephedra, Gnetum, Cedrus, Abio, etc.
Classification of Gynno.sperms
Gymnosperms further include three main classes
Cycadopsida (cycads), Coniferopsida (conifers) and Gnetopsi&
(Gnetum).
Economic Importance
of Gymnosperms
(i) Seeds of Pinus germdiana (chilgoza) are used as
food after roasting. Some other edible
gymnosperms plant parts are endosperm of
Ginkgo, seed kernel of some Cyrus and Gnetum,
sago grains from stems of Cycas, etc.
(ii) Gymnosperms provide softwood for
construction, plywood and paper industry.
(iii) Branches of gymnosperm trees are used as fuel.
(iv) Saw dust of conifers is used in making plastics
and linoleum.
(v) Resin is a semifluid secreted by special tubes of
a number of conifers. It helps in sealing female
cones after pollination, scale leaves around leaf
bases and apical buds. Resin is distilled to
obtain turpentine and resin. Turpentine is used
for thinning paints, varnishes, etc., and resin is
used for sealing joints of wheel, preparation of
writing paper, oil, clothes, etc.
(vi) Ephedrine an antibiotic is obtained from
Ephedra.
It is used for curing respiratory problems and
asthma. Taxol is an anti-cancer drug obtained
from Taxus.

Seedless vascular plants are pteridophytes. Economic
importance of pteridophytes are
(i) Pteridophytes are a good source of food for animals.
For example, sporocarps of Marsilea is edible. ,
Angiopteris and Alsophila have starchy pith eaten by I
natives of Australia.
(ii) Ferns protect soil from erosion by providing a good
cover on the hill slopes and other fragile places.
(iii) Azolla a water fern has a symbiotic association with
nitrogen-fixing cyanobacterium Anabaena azollae. It
is cultured in paddy fields to harbour nitrogen-fixing
bacterium. Thus, act as a biofertiliser.
(iv) Rhizomes and petioles of Dryopteris are used to
produce anthelmintic drug. Roots of Adiantum can
cure throat infections. Lycopodium is used in treatment
of rheumatism and disorders of lungs and kidneys.
(v) Ferns are also grown as ornamental plants for their
graceful plant body.