Reproduction (II) Reproduction in Flowering Plants
S
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P.1
Reproduction (II) Sexual Reproduction in Flowering Plants
exual reproduction confers on species the advantages of increased v
in offspring by :
Recombination of g
due to crossing- over between homologous chromosomes during meiosis.
Independent assortment of ch
during
and
Mixing of p
genetic make-ups especially when cross-fertilization occurs. (even self-fertilization
achieves certain degree of increase in variety due to the process of meiosis).
Plants are incapable of moving to transfer genetic material between individuals, fertilization
requires the assistance of e
agents like insects or the wind. This, however, exposes the
vulnerable gametes to dangers such as drying. To overcome this, seed plants enclose their m___
gametes within a spore, the microspore or p
grain, a protective structure resistant to desiccation.
I)
Floral structure of Angiosperms
A typical flower is made up of four sets of modified leaves: carpels, stamens, petals and sepals, all
attached to the receptacle.
At the centre of the flower, comprise a sticky st
the end of a slender stalk called the st . At
the base is the o
, a hollow structure containing one or more o
, each of which encloses
the f
gamete, the egg nucleus.
Situated around the carpel, each comprising a long f
Anthers have pollen s
which give rise to the pollen g
Large, conspicuous, brightly c
produce n
and be sc
.
, at the end of which are the an
.
which contain the m gametes.
leaf-like structures which at
insects. They may
They form the outer ring. These are usually g
and may photosynthesize, but their main
function is to p
the other floral parts when the flower is a b . Occasionally the sepals are
brightly coloured, e.g. in lilies, and help in insect at_______ or may later assist in dis______.
Reproduction (II) Reproduction in Flowering Plants
A)
P.2
Development of Pollen grains
Each stamen has an anther which
consists of four pollen s
.
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Within the pollen sacs are spore m______
cells which undergoes meiosis to form 4
pollen grains1.
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The haploid nucleus within each pollen
grain divides into two by mitosis into a
g_________ nucleus and a t___
nucleus.
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the generative nucleus will later give rise
to the two m
gametes
B)
Development of the Embryo sac and the Female gamete
Within the ovary are ovule(s), each attach to the ovary wall by the funicle. The ovule is enclosed by
the integ
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. A small pore is left at one end of the ovule, the mic
.
Within the ovule, the megaspore m______ cell divides by meiosis into four haploid megaspores,
three degenerate leaving one which become the haploid m
, which grows into the em
sac.
The nucleus of the embryo sac then divides by mitosis into eight h
nuclei.
One of which is the f
gamete (egg nucleus); two fuse to form a dip
nucleus at the centre.
1Pollen
grain is equivalent to microspore.
Reproduction (II) Reproduction in Flowering Plants
II)
P.3
Pollination
Pollination is the transfer of pollen from anther to stigma.
The design of any individual flower is related
to the precise agent used to transfer pollen.
If the plant is insect-pollinated its bright colour &
scent attract potential pollinating insects.
Wind-pollinated flowers are dull, unattractive and
without scent. With anthers & stigmas exposed
to the action of wind.
Comparison of Wind and Insect-pollinated flowers
Wind-pollinated flowers
e.g. Rye grass
Flowers are often uni______ with an
excess of male flowers
Insect-pollinated flowers
e.g. Buttercup
Mostly bisexual flowers
Petals are d__ and much reduced
in s___
Petals are l____and b______
coloured. Insect-g____ on petals
guide them to n_____
b. Landing platform for
insects?
No
Yes, making it easy for insects to
alight on the flower
b. Scent or nectar produced?
No s_____or n_____is produced
Flowers produce scent and/or
nectar to attract insects
Stigmas often protrude o_____ the
flower on long styles
Stigmas lie deep within the
corolla
Stigmas relatively l_____ &
f_______, giving a large s_____
area to catch pollen from the air
Stigmas are relatively small as the
pollen is deposited accurately by
the pollinating insects
Anthers dangle o_____ the flower
on long filaments. Pollen is easily
rel_____ into the air
Lie inside corolla so pollinating
insect brushes against them
when collecting nectar
b. Quantity of pollen
produced?
Enor___ amounts of pollen are
produced to offset the high degree
of wastage during dispersal
Less pollen is produced as pollen
transfer is more precise and so
entails less w______
c. Powdery or does it form
clumps?
Smooth or rough surface?
Sm____, l____, p______ and small
and sometimes has 'wing-like'
extensions to aid w___-transport
Larger and form clumps, often
rough-surfaced which help it
adhere to the insect
Flower unisexual or bisexual?
Petals
a. Large & attractive?
Stigmas
a. Hanging outside the flower ?
b. Feathery or sticky?
Anthers and pollen grains
a. Anthers hanging outside the
flower ?
Reproduction (II) Reproduction in Flowering Plants
III)
P.4
Fertilization
On landing on the stigma the pollen
grains absorb water and ger_____ to
give the pollen tube.
The tube2 pushes between the
loosely packed cells of the s___, the
t___ nucleus preceding the male
nuclei.
On entering the embryo sac the tube
nucleus degenerates and the two
male n_____ enter.
One male nucleus fuses with the egg
cell to give a diploid z
.
The other fuses with the two polar
nuclei to form the primary
e_________ nucleus, which is
therefore t________.
This d
fertilization is unique
to flowering plants.
2The
tube shows a positive chemotropic response to some substance produced in the embryo sac. The secretion of
pectinases by the pollen tube may soften the middle lamellae of the cells in the style and so assist its growth towards the
micropyle. The role of the tube nucleus is to control the growth of the pollen tube and it plays no part in
fertilization.
Reproduction (II) Reproduction in Flowering Plants
P.5
Methods to prevent self-pollination / Inbreeding
There can be no doubt that in some plants self-pollination occurs more or less regularly. However,
there appears to be a general tendency to avoid self-pollination since this is a form of inbreeding and
would very quickly reduce the variability of a population.
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The stamens and stigma mature at different times. If the stamens ripen before the stigma, the flower is
protandrous. If the stigma and ovule ripen before the stamens, the flower is protogynous.
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If a plant has separate male and female flowers it is said to be monoecious, e.g. maize (Zea). This
condition clearly limits the possibility of self-pollination.
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A dioecious species is one in which some individual plants have either all male or all female flowers.
Completely dioecious plants are rare.
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The structure of the flower itself makes self-pollination unlikely. eg. Heterostyly in primrose.
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In many plants self-pollination occurs but there is a mechanism to prevent this leading to successful
fertilization of the ovule and production of a seed. e.g. genetically determined incompatibility.
Q. The figure shows two types of primrose flower. They
occur naturally in roughly equal numbers and differ in
length of style (heterostyly) and position of anthers.
a) Given that bees collect nectar from the base of the
corolla tube, explain how cross-pollination between
pin-eyed and thrum-eyed flowers rather than
between flowers of the same type, is favoured.
b) What is the advantage of such a system?
IV)
Development of Fruits and Seeds
Following fertilization, the zygote divides rapidly by m____ and develops into the e
, which
consists of
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a young shoot, called the pl______,
a young root, the ra______
and seed leaves known as c_________.
The primary endosperm nucleus also divides mitotically to give a mass of cells, the en
. This
forms the food source for the growing embryo. In some species, e.g. maize (Zea mays), the
endosperm remains while in others, e.g. peas, it is quickly absorbed by and stored in the cotyledons.
The most common food stores in seeds is c__________. This is usually in the form of starch but
some seeds, e.g. maize and peas, store quantities of sugar. L____ may form a high percentage of the
dry weight, e.g. 60% in walnuts and coconuts; 40% in sunflowers. P_____ are found to a lesser extent
in seeds but wheat has an aleurone layer and protein is stored in the cotyledons of legumes.
Fate of parts of ovule after fertilization
PARTS
Zygote
OF
Primary endosperm
nucleus
OVULE
Integuments
 embryo (plumule, radicle, one or two cotyledons--may
or may not store food)
 endosperm (may or may not persist as a food store)
 testa (seed coat)
Reproduction (II) Reproduction in Flowering Plants
P.6
Distinguishing seeds from fruits
Seed
Fruit
Origin
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a fertilised ovule
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Scar
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one scar - funicle stalk
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an ovary containing one or more fertilized
ovule / seeds
two scars - receptacle and styles
Wall
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seed coat / testa develop from
integument
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ovary wall becomes the pericarp (fruit wall)
V)
Advantages and Disadvantages of Reproduction by Seed
The seed is the product of sexual reproduction in seed plants, it is a fertilised ovule. It contains an embryo
plant with one or more cotyledons, sometimes an endosperm, and is surrounded by a protective testa.
Advantages
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The seed p______ the embryo.
The seed contains f____ for the embryo (either in cotyledons or in the endosperm).
The seed is usually adapted for d________.
The seed can remain d_______ and survive adverse conditions.
The seed is physiologically s________ to favourable conditions and sometimes must undergo a period
of after-ripening so that it will not germinate immediately.
As a product of sexual reproduction, seeds have the attending advantages of genetic v_________.
Disadvantages
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Seeds are relatively large structures because of the extensive food reserves. This makes d________
more difficult than by spores.
Seeds are often e_____ by animals for their food reserves.
There is a reliance on external a_____ such as wind, insects and water for pollination. This makes
pollination (and hence fertilisation) risky, particularly wind pollination.
There is a large w______ of seeds because the chances of survival of a given seed are limited. Plants
must therefore invest large quantities of material and energy in seed production to ensure success.
The food supply in a seed is l______, whereas in vegetative propagation food is available from the parent
plant until the daughter plant is fully established.
Two individuals are required in dioecious species making the process more r___ than reproduction in
which only one parent is involved. However, dioecious plants are relatively rare.
Reproduction (II) Reproduction in Flowering Plants
V)
P.7
Fruit and seed dispersal
After seed development either the entire fruit or the seed(s) contained within it are dispersed from the parent
sporophyte. If the peri____ (wall) of the fruit becomes hard and dry it is called a dry fruit and if it becomes
fleshy, a succulent fruit.
Why is dispersal necessary ?
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Sexual reproduction generally ensures var
. New genotypes are produced with the potential to
survive in d
habitats. If this potential is to be realized it is essential that fruits and seeds are
dispersed to new areas with different environmental conditions.
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A degree of isolation also reduces the chances of backcrossing with the parents and eliminates all the
problems that in_________ incurs.
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Dispersal prevents over
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and makes species less vulnerable to epidemic att
a)
and competition
by viruses, fungi and insects.
Wind Dispersal
These include modifications that increase the fruit’s or seed’s s
area and so present more
re_________ to the wind. This delays its descent and the longer it is airborne the further it is
carried. eg pericarp is extended to form a wing; in dandelion a pappus of hair forms a 'p_________',
b)
Animal Dispersal
The fruit or seed may develop hooks which catch in the fur of animals. These hooks may be
extensions of the pericarp, e.g. goosegrass. Alternatively the fruit may be eaten by an animal and the
seeds, which are resistant to digestion, later pass out in faeces . These fruits, e.g. cherry and
blackberry, are usually brightly coloured.
Reproduction (II) Reproduction in Flowering Plants
c)
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Water Dispersal
This occurs in a few species, e.g. water-lily and
coconut, living in or near water.
The seed is covered in a spongy or fibrous layer
which traps air, making the seed or fruit buoyant.
d)
Mechanical Dispersal
The fruit may remain attached to the plant but open violently to expel the seeds. This is usually
caused by the unequal drying of the pericarp and occurs in all legumes, e.g. pea
END