EVOLUTION OF SEED HABIT Birbal Sahni Institute of Palaeosciences lucknow

1. EVOLUTION OF SEED HABIT
SANGRAM SAHOO

2. What is a seed ?
A true seed is ripened
ovule consisting of an
embryo covered by a
coat. It contains nutrition
to sustain the embryo till
it germinates and starts
photosynthesis
Do all plant produce seeds ?
No only the spermatophytes
(Gymnosperm and Angiosperms)
bears seeds.
Archegoniates
Tracheophytes
Embryophytes
Cryptogames
Phanerogames
Which of these
groups
exclusively
belongs to
seed plants ?

3. Which of these groups exclusively belongs to seed plants ?
Archegoniates: Bryophtes, Pteridophytes and Gymnosperms (Only
Gymnosperms produce seeds).
Tracheophytes : Pteridophytes, Gymnosperms and Angiosperm (i.e. Vascular
plants) (Pteridophytes do not produce seeds).
Embryophytes : All land plants starting from Bryophytes to (Bryo- and
Pteridophytes do not produce seeds).
Cryptogames : Non-seed plants from Algae to Pteridophytes (No seeds
produced).
Phanerogames : Gymnosperms and Angiosperm (Both produce seeds).

4. Evolutionary trends in plants
Primitive plants are exclusively aquatic (Algae)
Next group becomes amphibious (Bryophytes)
Next advancement lead to more terrestrial habitat (Pteridophytes), along with vascular
system
Next advancement leads to production of seeds (Gymnosperms), but they are naked
In the highest evolved group of plants (Angiosperms), seeds are produced and are
covered.
1. AQUATIC
2. SEEDLESS
TERRESTRIAL
SEEDED

5. EVOLUTIONARY TRENDS
Most important change in the evolutionary process is :
 REDUCTION OF GAMETOPHYTES
 INDEPENDENCE OF SPOROPHYTES
(a) Sporophytes depend
on gametophyte (e.g.,
bryophytes)
(a) Large sporophytes and
small independent
gametophyte (e.g., ferns)
(a) Reduced gametophyte
dependent on sporophytes
(e.g., seed plant)
PRIMITIVE ADVANCED

6. “”SEED” How it all started
Pteridophytes – Non seed bearing
Gymnosperms – Seed bearing
Is this a sudden jump ?
Or some connecting link/process is present in between ??

7. Major advancement towards Seed Habit
1. Development of heterospory.
2. Protection to the gametophyte and the young
developing sporophytes from drying and injury.
3. Provision of nourishment for their proper
development.
4. Freedom from external water required during
fertilisation for transfer of gametes. All these
conditions have been achieved in the seed of
gymnosperms and angiosperm.

9. Homospory & Heterospory
Homosporous plants
Sporophyte Single type of
spores
Bisexual Gametophyte
(with both kinds of
Gametangia)
Eggs & Sperms
Heterosporous plants
Sporophyte
Microspore Male gametophyte Sperm
Megaspore
Female gametophyte
(inside the spore wall)
Eggs

10. Heterospory
Two different types of spores:
The smaller Microspores
The larger Megaspores
Microspores produce Male gametophyte and
Megaspores produce Female gametophyte
E.g. Selaginella

11. “”SEED” How it all started
Heterospory started in Pteridophytes
Reached to climax in Selaginella
It produced: Microspores (Many in number) in Male strobilus
Megaspores (only four) in Female strobilus
Most of the Pteridophytes are Homosporous
Selaginella
Isoetes
Azolla
Regnelidium
Marsilea
Pilularia
Salvinia
Selaginella Isoetes Azolla Marsilea Salvinia

12. Significance of the heterospory
• Heterospory expresses sex determining capability of the plant
• Freedom from external water required during fertilisation for transfer of
gametes.

13. Palaeobotanical evidences of heterospory
Cooksonia
Rhynia Horneophyton Zosterophyllum
Potobariophyton
Chaleuria
Middle Devonian
Barinophyton Upper Devonian

14. Important Evolutionary steps towards ovule formation
1. Reduction in the number of megaspore in the megasporangium from many
to single tetrad.
2. Retention of the single functional megaspore in megasporangium.
Archaeopteris
Bothrodendrostrobus
Bensonites Archaeosperma Cycas
Cardiocarpus

16. Important Evolutionary steps towards ovule formation
3. Additional protection to megaspore is provided by formation of one or more
protective coats, the integument that delimit the micropyle.

17. Important Evolutionary steps towards ovule formation
4. Germination of micro- and megaspore begins in situ.
5. Formation of an endosporic megagametophyte within an indehiscent
megasporangium (nucellus) and dependent on parent sporophyte for its
nutrition
6. Formation of a pollen tube or pollen tube-like structure from
microgametophyte.
7. The multicellular megagametophyte has archegonia in the distal end which
have no neck canal cells and often the ventral canal cell too is absent.
8. Elaboration of the apex of indehiscent nucellus to form pollen chamber for
pollen reception.

18. Important Evolutionary steps towards ovule formation
9. Pollination where light weight microspores are passively carried to the
attached megasporangia by pollinating agents like wind or other agencies
10. Fertilization is effected by means of a pollen tube (siphonogamy).
11. Movement of sperm to archegonia.
12. Product of fertilization - the zygote is divided to form an embryo inside the
seed before the seed was shed by the parent plant.

19. Important Evolutionary steps towards Seed Habit in
Selaginella
Some species of Selaginella clearly show “Seed Habit” and they resemble seed
plants in the facts that:
1. The ripe megaspore (=embryo sac) is not shed from megasporangium
(=ovule) and remains permanently within it.
2. It is fertilized while still in megasporangium and after fertilization a simple or
advanced embryo is also produced.
3. In Selaginella rupestris, this embryo even germinates to produce future
sporophyte.
However, the “seed-like structure” of Selaginella differs from typical seed in not
having seed coat or testa, because ovule is not covered by integument, thus the
ripe structure cannot be called a true seed.

20. True seed of Gymnosperm
One megaspore survive
here like Selaginella. It is
retained in archegonia and
eggs develop and they are
fertilized like Selaginella.
The integument is not
present in Selaginella.

21. Advantages of seed Habit
Seed plants have these advantages over non-seed plants:
 Multicellular embryo gets a “head start” at the germination stage, due to
stored food.
 The storage material helps in passing required dormancy period.
 Seed are larger, more complex and much resistant to harsh conditions
than a spore.
 Seeds have increased capacity to develop dispersal adaptations.

22. EVOLUTION OF SEED HABIT
Origin and Development of Seed Habit
Three major advances made by seed plants
i. Development of Heterospory
ii. Development of Seed
iii. Development of non-swimming male gametes

23. DEVELOPMENT OF HETEROSPORY AND ITS ORIGIN
Origin: Upper Devonian Period of Palaeozoic era

24. DEVELOPMENT OF HETEROSPORY
1. J. Pattitte (1970) - Heterospory is developed from Homospory*
(*Homosporous articulates & Lycopods).
Either homospores
developed
heterosporous
developed in same
sporangia
In last stage of
development both
sporous developed in
different sporangia
2. Thomas & Spicer (1987) - Heterospory is developed due to mutations.
3. Most of the scientist believe - Heterospory is developed due to degeneration
of spores.