1. Heterospory evolved in plants where sporophytes produce two different sized spores - microspores and megaspores. 2. Microspores develop into male gametophytes while megaspores contain female gametophytes. 3. The evolution of seed habit began with heterospory and included protective integuments forming around megaspores, a single functional megaspore, an embryo sac, and pollen capture - leading to the ovule and seed. 4. Seed habit provided advantages like independence from water for fertilization and effective dispersal, helping plants diversify and dominate land.

1. HETEROSPORY
AND
EVOLUTION OF
SEED HABIT

2. WHAT IS
HETROSPORY?
Heterospory is the production of spores
of two different sizes and sexes by the
sporophytes of land plants.
Tow types of spores include:
1. MICROSPORE
2. MEGASPORE

3. WHAT ARE
MICROSPORES?
Microspores are haploid spores that
germinates to form male gametophyte,
which is carried to the megaspores by
wind, water currents or animal vectors.
The morphology of the microspore consists
of an outer double walled structures.

4. WHAT ARE
MEGASPORES?
Megaspores contain the female
gametophytes in heterosporic plant species.
They develop archegonia that produce egg
cells that are fertilized by sperm of the
male gametophyte originating from the
microspore.
They are significantly larger than their
male counterparts.

5. HISTORY
Heterospory evolved during the Devonian period from isospory.
Pteridophytes are the first true land plants.
Being the first successful colonizers in land habit
pteridophytes show both the homospory and
heterospory.
Most of the pteridophytes are homosporous, produce
iso-spores which are morphologically and
physiologically identical through the phenomenon
called homospory.

6.  However, There are some
Pteridophytes which produces two
different types of spores (different
in size, structure, and function)
 Such Pteridophytes are known as
heterosporous and the phenomenon
is known as heterospory.
 Heterospory has been reported in
nine genera namely, Selaginella,
Isoetes, Stylites, Pilularia,
Regnellidium, Marsilea, Salvinia,
Azolla and Platyzoma.

7. HOMOSPOROUS
LIFE CYCLE
HETEROSPOROUS
LIFE CYCLE

8. INCIPIENCE OF
HETEROSPORY
It has been universally accepted that the
homosporous condition is primitive and hetero-
spory is derived.
Heterospory might have first appeared in plants
where each sporangium contained both
microspores and megaspores. So one might
expect some conditions showing incipient
heterospory.

9. INTRASPORANGIAL
HETEROSPORY
(ANISOSPORY)
Heterospory in the fossil record includes
1. Inetersporangial heterospory
In which there are two different types of sporangia
micro and megasporangia.
2. Intrasporangial heterospory
In which there is a bimodal distribution of spore
sizes with in the same sporangium.

10. In a few fossils example, different sized spores
occur in the same sporangium, with each size class
presumably producing a different type of
gametophyte.
FOR EXAMPLE
Spores of two different sizes have also been
reported in the same sporangium of
Archaeopteris. (A. halliana and A. macilenta)
In Barinophyton citrulliformi , each sporangium
contains thousands of trilete microspores ≤50 μm
in diameter and around 30 trilete megaspores
which are up to 900 in diameter.

11. The Intrasporangial heterospory or anisosproy is interpreted
as an intermediate stage leading to complete heterospry.

12. Biological Advantages of Heterospory
A major leading step towards seed habit.
In heterosporous forms, the endosporic gametophytes are
independent of external conditions.
It creates a proper nutritive environment for the developing
embryo better than an independent green prothallus in
homosporous ferns.
In heterosporous forms the differentiation of sex can be
predicted at the spore stages. Thus the sex differentiation
can be extended from the early stage of gametophyte to the
sporophyte.

13. WHAT IS
SEED?
A seed is an embryonic plant enclosed in a
protective outer covering.
Simply it is called ripened or fertilized
ovule of the plant used for sowing which
are capable of forming a complete new
plant.

14. EVOLUTION OF
SEED HABIT
One of the most significant events in the
history of land plants was the development
of seed habit. It was an important change in
the reproductive system of the vascular
plants which occurred approximately 390
million years ago.
Seed formation is the characteristic of the
seed plants that are called
SPERMATOPHYTES which including, the
gymnosperm and angiosperm plants.

15. First complete seeds appeared approximately 365 million
years ago during late Devonian times. Various steps
involved in the evolution of seed habit are as follows.
1. Evolution of heterospory.
2. Retention and germination of megaspore within the
megasporangium.
3. Development of protective layers around
megasporangium.
4. Reduction to a single functional megaspore per
sporangium.
5. Development of an embryo sac within the sporangium.
6. Modification of distal end of megasporangium for pollen
capture.

16. 1. Evolution of heterospory.
2. Retention and germination of megaspore within
the megasporangium.
In some plants (e.g. Selaginella) the megaspore is
not allowed to escape from megasporangium
immediately after its formation. In others the
megaspore is permanently retained within the
megasporangium. Here, within the confines of the
megasporangium wall the megaspore germinates to
form egg containing female gametophyte

17. Development of protective
layers around megasporangium
Some branch like structure of sporophyte
surrounding the megasporangium fused
around to megasporangium to form
protective envelope or integument.
This important change led to the evolution
and formation of the ovule, which is
nothing but an integumented indehiscent
megasporangium.

18. Reduction to a single
functional megaspore per
sporangium
Each megaspore mother cell within a
megasporangium used to produce four
gametophytes.
There was a competition for space and food
among the four gametophytes.
Soon the early vascular plants adopted a new
strategy i.e., only one megaspore is selected for
further development into a healthy female
gametophyte while the remaining three are
aborted.

19. Development of an embryo sac
within the sporangium
The single healthy megaspore retained within
the megasporangium germinates to form an egg
containing female gametophyte called an
embryo sac.
EMBYO SAC :
the female gametophyte of a seed plant consisting
of a thin-walled sac within the nucellus that
contains the egg nucleus and other nuclei which
give rise to endosperm on fertilization

21. Modification of distal end of
megasporangium for pollen
capture
When most of the structural and functional
changes leading to the development of seed
habit were completed, another important
modification took place in the
megasporangium. The distal end of the
megasporangium became modified for
capturing pollen (microspore containing male
gametophyte).

22.  Pollen after being trapped in the distal cavity of
the megasporangium produces pollen tube which
carry male gametes deep into the embryo sac to
fertilize the egg, forming a zygote, that forms an
embryo.
 The megasporangium (ovule) after fertilization is
transformed into a seed, the integuments
becoming the seed coats.
 The seed offers maximum degree of protection to
a developing embryo under the unfavorable
terrestrial environment.

23. The
development
and evolution of
seed habit was
a great success
and a giant
leap which
ultimately
enabled plants
to colonize land
permanently.

24. EVOLUTIONARY
IMPORTANCE OF SEED
HABIT
NO WATER FOR FERTILIZATION
The evolution of seeds allowed plants to
decrease their dependency upon water for
reproduction.
CAPABILITY OF DORMANCY
Seeds contain an embryo that can remain
dormant until conditions are favorable when
it grows into a diploid sporophyte.

25. EFFECTIVE DISPERSAL
Seeds are transported by the wind, water, or
by animals to encourage reproduction and
reduce competition with the parent plant.
PROTECTION
Protective seed coat and energy reserves
enable plants to resist harsh
environments.
SEASONAL GROWTH
Seed dormancy allows synchronization of
plant life cycle with seasonality.

26. SPERMATOPHYTES
(SEED PLANTS)
There are two main groups of seed plants
1. Angiosperms, the flowering seed plants
250,000 species.
Reproductive structures are flowers.
Seeds are enclosed in ovaries.
2. Gymnosperms, the non-flowering seed plants
800 species, mostly conifers.
Reproductive structures are cones.
Seeds are “naked”, not enclosed in ovaries.

27. IMPORTANCE OF
SEED PLANTS
Today, seed plants dominate the terrestrial ecosystem and
are found in a wide variety of habitats owing to the selective
advantages that this SEED HABIT has provided.
Basis of human civilization.
 Nearly all crops are seed plants (mostly angiosperms).
 Invention of agriculture transformed human society.
Mostly seed plants are used for food, textiles, medicines,
building materials, and many other products that are
important to humans.

28. WHY SEEDS ARE NOT
FORMED IN SELAGINELLA
the life history of Selaginella approaches towards
seed habit because of the following features:
1. The occurrence of the phenomenon of heterospory.
2. Germination of megaspore inside
megasporangium.
3. Development of only one megaspore per
megasporangium for example, in Selaginella
monospora, S. rupestris etc.

29. Even then the seeds are not formed in Selaginella
because:
1. Megasporangium is not surrounded by
integument.
2. The megaspore is not permanently retained in the
megasporangium. The retention is reported in
Selaginella rupestris, but the fusion of megaspore
and megasporangium wall is absent which is
necessary for permanent retention of megaspore.
3. The is lack of resting period after development of
embryo.