XII BIOLOGY
CH-1 Reproduction in lower and higher plants
PART-PART- Seed
and Fruit Development
Seed
and Fruit Development:
The goal
of reproduction, in
every living organisms
including plants, is to create
offsprings for the next
generation.
One of the ways
that’s plants can produce offpsrings is by forming (making) seeds.
The flowers must be pollinated in order to produce seeds and fruit.
Seed
development –
1) Seed
development is initiated
by fertilization. The integuments
of the fertilized ovule persist and get transformed into the seed coat of
mature seed .
2) Seed sometimes consists of two distinct coverings
i.e. outer testa and the
inner thin tegmen.
3) In addition to these two the nucellus in the
ovule may remain as a thin, papery layer, the perisperm, e.g. some genera like black
pepper and beet.
4) In some seeds, the food reserves in the endosperm
are partially used up in the development of an embryo , in such seeds
the endosperm remain conspicuous and fills a greater part of the
seed. Thus, the resultant seed is called
endospermic or albuminous.
e.g. Castor, Coconut, Maize, etc
5) In other seeds, embryo absorbs food reserve from
the endosperm completely during its developmental stages.
Thus, endosperm disappears (disorganizes) in
mature seeds such
seed is called non-endospermic or ex-albuminous . e.g. Pea,
bean, etc.
6)The cotyledons in some non-endospermic seeds act
as a food storage and endospermic seeds they are the first photosynthetic organs.
7) Micropyle
persists as a small pore in seed coat to allow the entry of water and
oxygen during soaking.
Fruit development :-
Fruit development is triggered by
hormones produced by developing seeds.
After fertilization the zygote is
formed and the ovary begins to differentiate into the fruit and ovary wall
develops into pericarp.
Pericarp is basically three layered
like epicarp, mesocarp & endocarp which get differentiated in the fleshy
fruit like mango, coconut, etc.
Significance
of seed and fruit formation:
Fruits provide nourishment to the
developing seeds.
Fruits protect the seeds in
immature condition.
Seeds serve as important
propagating organs (units) of plant.
Seeds and fruits develop special
devices for their dispersal and thus help in the distribution of the species.
Structural or physiological
adaptive mechanism for survival is called Dormancy.
Mature and viable seeds will not
germinate even in the presence of favorable conditions . Seeds get dispersed at
different places during dormancy.
Viable seeds germinate only after
completion of dormancy period.
Apomixis:
It is phenomenon of formation of embryo(s) through asexual method
of reproduction without
Formation of gamets and the act of fertilization.
Formation of seed without fertilization is called as
apomixes, and the seeds are called as apomictic
seeds.
It is the type of asexual reproduction where their
no meoisis and syngamy. Embryo
develops in the ovule and
ovule developes to form seed .
In apomixis, when a gametophyte organ or cell
produces embryo like structure without fertilization, it is termed as apogamy.
When diploid sporophyte cell produces a diploid
gametophyte without undergoing meiosis is called apospory. e.g. Orange,
Mango .
The
main categories of apomixis are:
1)The diploid embryo sac is produced either from an
archesporial cell or from some other part of the nucellus.
2)Diplospory- the embryo sac is produced from the
diploid megaspore mother cell e.g. Taraxacum.
3)Apospory- the nucellar cells give rise to
apomictic embryo sac.
1)Megaspore mother cell undergoes usual meiotic
division and a haploid embryo sac is formed.
2)The embryo arises either from the egg by
parthenogenesis or from some other haploid cells of gametophyte through
apogamy. .
3)Plants produced by this method are generally
sterile i.e. do not reproduce sexually, e.g. Nicotiana .
1)Embryos may develop from somatic nucellus or integuments along with normal zygotic
embryo.
2)It common in
Mango ,Orange, Lemon, etc.
3)It gives rise to a condition called polyembryony Genetically identical plants can be produced effectively and rapidly by apomixis.
1.
Development of fruit without fertilization is called as
parthenocarpy.
2.
In these plants
the placental tissue in the unfertilized ovary produces auxin IAA (Indole-3 Acetic Acid) which convert ovary
into fruit.
3.
The fruit appear
normal, but it is seedless.
4.
E.g.- Pineapple,
Banana, Papaya, etc. Polyem bryony:
Polyembryony-
1.
Development of
more than one embryo, inside a seed is called as polyembryony.
2.
It was first
noticed by leeuwenhoek (1719) in the seeds of Citrus genus.
3.
It will results
in the germination of multiple seedlings.
4.
The additional
embryos are produced because of the differentiation and development of various
maternal and zygotic tissues associated with the ovule of seed.
5.
Polyembryony may
be true or false depending upon whether many embryos arise in the same embryo
sac or in different embryo sacs in the same ovule.
Adventive
polyembryony- An embryo
develops directly from the diploid cell of nucellus and integuments as in Citrus.
Cleavage polyembryony- Zygote proembryo sometimes divides (cleaves) into
many parts or units.
Each unit then develops into an embryo.
Polyembryony increases the chances of survival of
the new plants.
Nucellar
adventive polyembryony is of great significance in
horticulture.
Parthenogenesis is the development of embryo
directly from egg cell or a male gamete.
It is a kind
of apogamy .
1.
Here plants
produce seeds. But embryo, inside it} is produced without (omitting) meiosis
and syngamy.
2.
Parthenocarpy can be induced artificially by - spraying of
gibberellins, delaying pollination, use of foreign pollens, etc.
3.
Genetically
uniform parental type seedlings are obtained from nucellar embryos.
Development of a Seed
1.
The
mature ovule develops into the seed. A typical seed contains a seed coat,
cotyledons, endosperm, and a single embryo
2. The storage of food reserves in angiosperm
seeds differs between monocots and dicots. In monocots, such as corn and wheat,
the single cotyledon is called a scutellum; the scutellum is
connected directly to the embryo via vascular tissue (xylem and phloem). Food
reserves are stored in the large endosperm.
3. Upon germination, enzymes are secreted by
the aleurone, a single layer of cells just inside the seed coat
that surrounds the endosperm and embryo.
4. The enzymes degrade the stored
carbohydrates, proteins and lipids, the products of which are absorbed by the
scutellum and transported via a vasculature strand to the developing embryo.
5. Therefore, the scutellum can be seen to be
an absorptive organ, not a storage organ.
6. The two cotyledons in the dicot seed also
have vascular connections to the embryo. In endospermic dicots, the
food reserves are stored in the endosperm.
7. During germination, the two cotyledons
therefore act as absorptive organs to take up the enzymatically released food
reserves, much like in monocots (monocots, by definition, also have endospermic
seeds).
8. Tobacco (Nicotiana tabaccum), tomato (Solanum lycopersicum), and pepper (Capsicum annuum) are examples of endospermic dicots.
9. In non-endospermic dicots, the
triploid endosperm develops normally following double fertilization, but the
endosperm food reserves are quickly remobilized and moved into the developing
cotyledon for storage.
10. The two halves of a peanut seed (Arachis hypogaea) and the split peas (Pisum sativum)
of split pea soup are individual cotyledons loaded with food reserves.
11.
The
seed, along with the ovule, is protected by a seed coat that is formed from the
integuments of the ovule sac.
12. In dicots, the seed coat is further
divided into an outer coat known as the testa and inner coat
known as the tegmen.
The
embryonic axis consists of three parts:
1. The plumule, the radicle, and the
hypocotyl. The portion of the embryo between the cotyledon attachment point and
the radicle is known as the hypocotyl (hypocotyl means “below
the cotyledons”).
2. The embryonic axis terminates in a radicle (the
embryonic root), which is the region from which the root will develop.
3. In dicots, the hypocotyls extend above
ground, giving rise to the stem of the plant. In monocots, the hypocotyl does
not show above ground because monocots do not exhibit stem elongation.
4. The part of the embryonic axis that
projects above the cotyledons is known as the epicotyl.
5. The plumule is composed
of the epicotyl, young leaves, and the shoot apical meristem.
6.
Upon
germination in dicot seeds, the epicotyl is shaped like a hook with the plumule
pointing downwards.
7. This shape is called the plumule hook, and
it persists as long as germination proceeds in the dark.
8. Therefore, as the epicotyl pushes through
the tough and abrasive soil, the plumule is protected from damage.
9. Upon exposure to light, the hypocotyl hook
straightens out, the young foliage leaves face the sun and expand, and the
epicotyl continues to elongate.
10. During this time, the radicle is also
growing and producing the primary root.
11. As it grows downward to form the tap root,
lateral roots branch off to all sides, producing the typical dicot tap root
system.
Seed
Germination
1. Many mature seeds enter a period of
inactivity, or extremely low metabolic activity: a process known as dormancy,
which may last for months, years or even centuries.
2.
Dormancy
helps keep seeds viable during unfavorable conditions. Upon a return to
favorable conditions, seed germination takes place.
3.
Favorable
conditions could be as diverse as moisture, light, cold, fire, or chemical
treatments. After heavy rains, many new seedlings emerge.
4.
Forest
fires also lead to the emergence of new seedlings. Some seeds require vernalization (cold
treatment) before they can germinate.
5.
This
guarantees that seeds produced by plants in temperate climates will not
germinate until the spring.
6.
Plants
growing in hot climates may have seeds that need a heat treatment in order to
germinate, to avoid germination in the hot, dry summers.
7.
In
many seeds, the presence of a thick seed coat retards the ability to germinate.
8.
Scarification, which includes mechanical or chemical
processes to soften the seed coat, is often employed before germination.
Presoaking in hot water, or passing through an acid environment, such as an
animal’s digestive tract, may also be employed.
9. Depending on seed size, the time taken for
a seedling to emerge may vary.
10.
Species
with large seeds have enough food reserves to germinate deep below ground, and
still extend their epicotyl all the way to the soil surface. Seeds of
small-seeded species usually require light as a germination cue.
11.
This
ensures the seeds only germinate at or near the soil surface .
12.
If
they were to germinate too far underneath the surface, the developing seedling
would not have enough food reserves to reach the sunlight.
Development of Fruit and Fruit Types
1. After fertilization, the ovary of the
flower usually develops into the fruit.
2.
Fruits
are usually associated with having a sweet taste; however, not all fruits are
sweet. Botanically, the term “fruit” is used for a ripened ovary.
3.
In
most cases, flowers in which fertilization has taken place will develop into
fruits, and flowers in which fertilization has not taken place will not.
4.
Some
fruits develop from the ovary and are known as true fruits, whereas others
develop from other parts of the female gametophyte and are known as accessory
fruits.
5.
The
fruit encloses the seeds and the developing embryo, thereby providing it with
protection.
6.
Fruits
are of many types, depending on their origin and texture.
7.
The
sweet tissue of the blackberry, the red flesh of the tomato, the shell of the
peanut, and the hull of corn (the tough, thin part that gets stuck in your
teeth when you eat popcorn) are all fruits.
8.
Fruits
may be classified as simple, aggregate, multiple, or accessory, depending on
their origin.
9.
If
the fruit develops from a single carpel or fused carpels of a single ovary, it
is known as a simple fruit, as seen in nuts and beans.
10.
An aggregate
fruit is one that develops from more than one carpel, but all are in
the same flower: the mature carpels fuse together to form the entire fruit, as
seen in the raspberry.
11.
Multiple
fruit develops from
an inflorescence or a cluster of flowers. An example is the pineapple, where
the flowers fuse together to form the fruit. Accessory fruits (sometimes
called false fruits) are not derived from the ovary, but from another part of
the flower, such as the receptacle (strawberry) or the hypanthium (apples and
pears).
12. Fruits generally have three parts: the exocarp (the outermost skin or covering), the mesocarp (middle part of the fruit), and the endocarp (the inner part of the fruit).
13. Together, all three are known as the pericarp.
14. The mesocarp is usually the fleshy, edible part of the fruit;
however, in some fruits, such as the almond, the endocarp is the edible part.
15. In many fruits, two or all three of the layers are fused, and are indistinguishable at maturity. Fruits can be dry or fleshy.