4. INTRODUCTION
DEFINATION OF EVOLUTION
The process by which different kinds of
living organism are believed to have
developed from earlier forms during the
history of the earth.
The gradual development of something.
5. • The male gametophytes of modern gymnosperms
differ appreciably from those of the homosporous
pteridophytes in being endosporous, much reduced and
in possessing a siphonogamous outgrowth called the
pollen tube.
• The male gametophyte is an outcome of the
germination of the microspore or the pollen grains.
• The germination is partially precocious and partially it
takes place on the nucellus of the ovule.
6. • It is an extremely reduced structure as compared to
the massive gametophytes of the homosporous
bryophytes and pteridophytes.
• It is devoid of any orderly arrangement of cells as
compared to the antheridia of mosses and
pteridophytes.
• Some sort of comparison can be made with the male
gametophytes of heterosporous pteridophytes like
selaginella and isoetes.
7. • Both these pteridophytes have completely endosporic
male gametophytes with one prothallial cell and an
antheridium with 256 antheriozoids in selaginella and
only four in isoetes.
• Liebig (1931) pointed out a sort of similarity between
the male gametophytes of isoetes and some fossil
gametophytes like cordainthus.
• In cordainthus , Florin (1936) described a row like
disposition of spermatozoids and a tendency towards
a similar disposition is evident in isoetes.
8. • Sewards and ford (1906) also pointed out towards
a resemblance between the organisation of male
gametophytes of such heterosporous pteridophytes
and some living and fossil Araucariaceae.
• Structure of male gametophytes of fossil
gymnosperms is not completely known for all
fossil groups.
• No data are available regarding the fossil remains
of fossil microgametophytes of coniferophytes of
carboniferous and permain periods.
9. • Well preserved male gametophytes of
cycadofilicales and cordaitales are available and
all such specimens show multicellular nature of
the microgametophytes.
• Germinated pollen grains have been described
in the nucellus of ovules of many genera of
cycadofilicales.
• Oliver (1904) and Renault (1885) described the
pollen grains and microgametophytes of the
fossil genus stephanospermum.
10. • In some specimens these cells have been seen to
form a parietal layer enclosing a central cavity.
• In S.caryoides he reported two cells lying in this
cavity and a single peripheral cell enclosing them
partially.
• The peripheral cell was regarded by oliver to be
comparable to a tube cell of the modern
gymnosperms.
• The central two cells perhaps divided further to
produce antherozoid mother cells.
11. • Hoskins and Cross and Stewart studied the male
gametophytes of pachytesta vera, P. hexangulata, P.
illionensis and P. composita.
• The male gametophytes have many peripheral
cells enclosing a number of internal cells lying in
the central cavity of the microspore.
• Renault (1902) described a small protuberance on
one side of the microspore of Aetheotesta and
Stephanospermum.
• He interpreted it as the base of the pollen tube.
12. • Florin (1936) described the germinated pollen
grains of Cordainthus and reported the presence
of definite peripheral cells enclosing a central
space containing a row of about five nuclei
along the polar axis of the microspore.
• Florin compared the peripheral cells to the male
prothallial cells of living gymnosperms and the
five nuclei to the gamete nuclei.
• These germinated pollen grains were found in
the microsporangia.
13. • In Cycadeoidea Wieland (1906) found germinated
pollen grains in microsporangia, they also contained
1-5 peripheral cells that were interpreted as
prothallial cells.
• The fossil genera completely lacked a pollen tube.
• A careful study of the microgametophytes of modern
gymnosperms reveals that there has been a gradual
reduction in the size and content of the male
gametophyte.
• In Ginkgo and Welwitschia the male prothallial cells
are altogether wanting.
