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PinkMonkey Online Study Guide-Biology
The neck of the mature archegonium opens in presence
of water. The venter canal cell and the neck canal cell disintegrate.
Their contents form a substance similar to mucilage which comes out through
the open neck. Mucilage contains malic acid which attracts the
antherozoids for fertilization. This phenomenon is called chemotaxis.
Fertilization: Many antherozoids may enter the open archegonium. However, only one fertilizes the egg. This results in the formation of a diploid zygote (fertilized egg) in the venter cavity. Haploid gametophytic generation ends with fertilization (Fig. 15.6B). The diploid zygote (2n) starts a new sporophyte generation and develops into a fern plant (adult sporophyte).
f) Alternation of generations
The sporophyte (diploid) and the gametophyte (haploid)
generations in fern are produced in alternate succession one after the
other. This phenomenon is called alternation of generations.
Meiosis and gamete fertilization are the two important events
which regulate this phenomenon. As the sporophyte (fern plant) and gametophyte
(prothalths) are morphologically dissimilar, it is described as a heteromorphic
alternation of generations.
Figure 15.7 Schematic representation of the life cycle of a fern showing alternation of generations
The sporophyte generation ends when diploid spore mother cells divide
by meiosis and produce haploid spores. The spore starts the gametophyte
generation. This generation ends when the haploid gametes fuse during
fertilization and form a diploid zygote. The zygote again starts a new
diploid generation. Alternation of generations is represented schematically
in the life cycle of a fern (Fig. 15.7).
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SUMMARY - PTERIDOPHYTA
Pteridophytes are vascular cryptogams with a sporophyte-dominated
life cycle. The diploid plant body is differentiated into true roots,
stem and leaves. Nephrolepis is a common fern. Sporangia
are organized into distinct sori. It is homosporous and produces
haploid spores as a result of meiosis in diploid spore mother cells.
The gametophyte is a totally independent prothallus. Sex organs
are multicellular. Antherozoids are multiflagellate. Fertilization
is water-dependent and takes place inside the archegonium. Meiosis
and fertilization are responsible for alternation of diploid and
haploid generations in the life cycle.
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