Plant Kingdom
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THË PTËRÏDØPHÝTËS HÅVË 4 ÇLÅSSËS:
1.
Psilopsida: E.g.
Psilotum
2.
Lycopsida: E.g.
Selaginella,
Lycopodium
3.
Sphenopsida:
E.g. Equisetum
4.
Pteropsida:
E.g. Dryopteris,
Pteris, Adiantum
(walking fern)
GÝMÑØSPËRMS
The gymnosperms (gymnos: naked, sperma: seeds) are plants
in which the ovules are not enclosed by any ovarian 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 fall trees and shrubs.
Out of the gymnosperms, the giant redwood tree Sequoia is
one of the tallest tree species.
Roots are generally tap roots.
Stems are unbranched (Cycas) or branched (Pinus and Cedrus).
Roots in some genera have fungal association in the form of
mycorrhiza (E.g. Pinus).
In plants like Cycas, small specialized roots (coralloid roots) are
associated with N2-fixing cyanobacteria.
Leaves are well adapted to with stand extremes of temperatures,
humidity and wind. In conifers, needle-like leaves reduce the
surface area. Their thick cuticle and sunken stomata help to
reduce water loss.
Reproduction: Gymnosperms are heterosporous. They produce
haploid microspores and megaspores.
Some leaves are modified into sporophylls. They are compactly
and spirally arranged along an axis to form strobili or cones.
Sporophylls bear sporangia in which spores are produced.