Week 17

Question 1

What are the materials required in this practical and why?

Answer 1

Pollen germination medium - this is made up of 10 g sucrose, 0.01 g boric acid, and 0.03 g calcium nitrate, in 100 ml distilled water.
Pollen from Tradescantia, a Monocotyledon AngiospermCavity slide
A cavity slide and coverslip
Plastic pipette for adding the germination medium to the cavity slide

Question 2

Why is sucrose used?

Answer 2

Sucrose is used as a source of energy for pollen metabolism and maintains osmotic pressure. Boron (provided by boric acid) and calcium both play roles in successful pollen tube development.

Question 3

How do you view pollen germination?

Answer 3

To view the pollen germination you will need to use the x10 and x40 objective lenses of the compound microscope.

Question 4

What are Angiosperms?

Answer 4

This group is known as the flowering plants and is the most diverse group of land plants. The largest group of angiosperms is the eudicotyledons (dicots) and the second largest the monocotyledons (monocots). This grouping refers to the first leaves which appear from an embryo called cotelydons.
In the practical you will examine and compare the main features of the monocotyledon angiosperms, and the eudicotyledon angiosperms.

Question 5

When monocotyledonous plants germinate, how many initial ‘leaves’ would you expect to see?

Answer 5

Monocotyledons have one inital ‘leaf’, i.e. one cotyledon when they germinate
1

Question 6

When dicotyledon seeds germinate, how many inital ‘leaves’ would you expect to see?

Answer 6

Dicotyledons have 2 initial ‘leaves’ when they germinate, i.e. they have two cotyledons

Question 7

What characteristic separates angiosperms from gymnosperms?

Answer 7

Angiosperms produce covered ovules within a carpal or ovary. Gymnosperms produce naked seeds and have separate male and female cones.

Many angiosperms have male and female anatomy within the same structure - a flower.

Angiosperms produce ovules contained in a carpal or ovary.

Question 8

Seeds are very resistant structures, that can ______and nourish the young ____.

Answer 8

Blank 1: protect

Blank 2: embryo or plant

Question 9

Which of the following accurately identifies all plants that produce seeds?

Answer 9

Angiosperms and gymnosperms

Question 10

On of the most important functions of flowers is to ______.

Answer 10

promote genetic diversity

Question 11

Place the following female reproductive structures in order from the fewest cells to the most cells. Start with the structure with the least cells on top.

Answer 11

Gametophyte, integuments, ovule, ovary.

Question 12

Seeds represent a major advance in the evolution of plants because they

Answer 12

provide the embryo with protection and food

Question 13

Select all adaptive features of seeds.

Answer 13

They nourish the embryo.

They protect the embryo.

Question 14

Because seeds can remain dormant, they allow plants to

Answer 14

survive in unpredictable conditions

Question 15

Seeds are produced by ______.

Answer 15

gymnosperms and angiosperms only

Question 16

The male gametophyte in seed plants is called a

Answer 16

pollen grain.

Question 17

Select all functions of fruits.

Answer 17

Seed dispersal

Protection of seeds

Question 18

In seed plants, the embryo

Answer 18

encloses the ovules.

Question 19

In some seed-bearing plants, the need for water during fertilization has been eliminated because the sperm moves to the egg by traveling through a(n) _____ ______.

Answer 19

pollen tube

Question 20

Which of the following are advantages provided by seeds?

Answer 20

Introducing a dormant phase into the life cycle

Providing a food source for the growing embryo

Protecting the embryo

Facilitating dispersal

Question 21

If environmental conditions are unfavorable, seeds can remain inactive in a state of _____ until conditions improve.

Answer 21

Dormancy.

Question 22

Pollen grains are tiny male ____

that carry sperm.

Answer 22

gametophytes

Question 23

The production of a pollen tube eliminates the need for

Answer 23

water during fertilization

Question 24

In most seed plants, the entire _____
gametophyte moves to the _____
gametophyte.

Answer 24

Blank 1: male

Blank 2: female

Question 25

In angiosperms, the ovule and the surrounding sporophyte tissue collectively comprise the

Answer 25

Ovary.

Question 26

Because seeds can remain dormant, they allow plants to

Answer 26

survive in unpredictable conditions

Question 27

The gymnosperm clade includes

Answer 27

ginkgophytes

cycadophytes

coniferophytes

gnetophytes

Question 28

Select a feature that distinguishes gymnosperms from angiosperms.

Answer 28

They do not enclose seeds in fruits

Question 29

Select all functions of fruits.

Answer 29

Protection of seeds

Seed dispersal

Question 30

The male gametophyte of seed plants differs from that of the seedless plants because in seed plants

Answer 30

the entire male gametophyte moves to the female gametophyte, rather than just the sperm.

Question 31

Which of the following is unique to angiosperms?

Answer 31

The production of fruit

Question 32

In angiosperms, the _____

develops into the fruit.

Answer 32

ovary

Question 33

Angiosperms differ from gymnosperms because, at pollination, their ______
are enclosed within diploid tissues, the carpel. The carpel eventually develops into a ______, a unique angiosperm feature.

Answer 33

ovules

fruit

Question 34

The coniferophytes, cycadophytes, gnetophytes, and ginkgophytes are examples of

Answer 34

gymnosperms

Question 35

Although both gymnosperms and angiosperms produce seeds, gymnosperms do not produce ____
or ____.

Answer 35

Blank 1: flowers

Blank 2: fruits

Question 36

The incidence of horizontal gene transfer plants is likely enhanced by associations with

Answer 36

other land plants

Question 37

Unlike gymnosperms, angiosperms

Answer 37

have enclosed ovules

Question 38

Unlike the gymnosperms, the angiosperms produce

Answer 38

seeds within a fruit

Question 39

In the ancient angiosperm Amborella trichopoda, mitochondrial genes have been incorporated from other plant species through the process of

Answer 39

horizontal gene transfer

Question 40

Which of the following groups have microphylls?

Answer 40

Lycophytes

Question 41

Which of the following groups have megaphylls?

Answer 41

Ferns

Question 42

Which of the following groups made up the spore trees of the Carboniferous?

Answer 42

Ferns
Progymnosperms
Lycophytes
Arthrophytes

Question 43

Which of the following times would land you in the Carboniferous period?

Answer 43

300 mya

Question 44

What is Homospory?

Answer 44

Individuals produce one type of spore- one type of gametophyte (antheridia and archegonia) or two types of gametophyte with either antheridia or archegonia.
All Bryophytes
All extant arthrophytes (horsetails)
Most extant lycophytes (club mosses) and ferns.

Question 45

What is anisospory?

Answer 45

Extinct lineages such as Barinophyton (386 mya) large and small spores in same sporangium.

Question 46

What is heterospory?

Answer 46

Individuals produce two types of spore- always two types of gametophyte.

Many extinct lineages of lycophytes, arthrophytes, ferns and progymnosperms.
Some extant lycophytes
Some extant (water) ferns.
All seed plants (gymnosperms and angiosperms)

Question 47

What are megaspores?

Answer 47

Are produced in a megasporangia which sit on modified megasporophylls and they go onto germinate into megagametophytes. in this case are female but are not the same as eggs.

Question 48

What are microspores?

Answer 48

Are produced in a microsporangia which sit on modified microsporophylls and they go onto germinate into microgametophytes. in this case are male and are not the same as sperm.

Question 49

Heterospory has at least __ independent origins.

Answer 49

4

and there are many heterosporous fossils found in the carboniferous.

Question 50

What are the proposed selective advantages of Heterospory?

Answer 50

Resource allocation: more nutrients for fewer, bigger spores –
better chances of survival, but lots of microspores needed for
sperm to find the egg (in the archegonium in the gametophyte)
[analogous to evolution of oogamy in algal ancestors]
• Increased potential for outbreeding (heterosis):
i. aerodynamic sorting of spores – microspores from
same individual blown away from megaspores;
ii. unisexual gametophytes
[contrast with homosporous ferns, horsetails and club mosses
with bisexual gametophytes – highly inbred]
• Protection of gametophytes:

Question 51

• Seed plants __ ___ ___ their megaspores.
• ___ ____ is produced per megasporangium.
• The megaspore _____ _____ the modified
megasporangium (nucellus) to produce a megagametophyte in situ - on the sporophyte
• The nucellus and the megagametophyte are enclosed and protected by 1 or 2 layers of sporophyte tissue (integuments) – the whole structure is called an _____.
• ______ (‘pollen’) contain the _________.
•An OVULE is therefore an ‘______ ______ ______’
•A fertilized OVULE develops into a _____in which the
embryo _____ develops and is protected

Answer 51

• Do not shed
• One megaspore
• germinates within
• OVULE
• Microspores and microgametophyte
• ‘integumented female gametophyte’
• SEED and sporophyte

Question 52

A pollen grain =

Answer 52

= a microspore with a microgametophyte inside

Question 53

Benefits of retaining megaspores (the seed habit)

Answer 53

1. Reduces significantly the requirement for
external water to fertilize eggs
2. Protection of female gametophyte and
embryo sporophyte
3. Nourishment of female gametophyte and
embryo sporophyte.

Evolution of the seed habit in the sporophyte mirrors
the evolution of the embryophyte habit in the
gametophytes of the first land plants:

Heterospory and the seed habit have transferred plant
sexuality from the gametophyte to the sporophyte.

Question 54

First seed plants appear in

Answer 54

Carboniferous Spore Tree Forests (~350 mya).

Date back to the mid/late Devonian period. As we move into the Carboniferous we see more reminiscent plant forms. We’ve got to reduce sperms and cordaites. Pteridosperms are not ferns but look like them and so hence the name. But they bear ovules and pollen. And Cordaites are more like modern conifers and grow very very tall.

Question 55

What occurs within the megasporangium?

Answer 55

We have the four megaspores within the megasporangium, which we now call the nucellus surrounded by a layer of sporophyte tissue, which we now call the integument. Three of these megaspores abort, leaving just one which can then germinate into the female gametophyte. This is the next half generation from the parental sporophyte. This can then develop archegonia, which are the specialised reproductive structures which house the egg. Gametophytes produce gametes. We then have a microspore pre pollen being caught in the wind- via means of a pollination droplet. The microspore will then germinate into the microgametophyte, produce the sperm, the gamete from the gametophyte fertilises the egg. Then we get the development of the embryo within the structure. So this then becomes the next-generation of sporophyte and the integument layer becomes the seed coat.

Question 56

What occurs in
Early Carboniferous (~350 mya) 
Early Permian (~300 mya)

Answer 56

Early Carboniferous (~350 mya) Pteridosperms and Cordaitales established.
Early Permian (~300 mya) Numbers of gymnosperms increase dramatically. Pteridosperms and Cordaitales expand.

Question 57

Why the transition from spore forest to seed forest?

Answer 57

Changes in climatic conditions. When seed plants appeared in the fossil records, in the permian, this is where they really started to diversify. This is the same time at which the supercontinent Pangaea was forming.

Question 58

What was climate change associated with?

Answer 58

Associated with formation of supercontinent Pangea:
• widespread aridity in continental interiors,
• high seasonal temperature fluctuations,
• equatorial aridity and monsoons.
Conditions were way more unpredictable.

Seed plants are better adapted to reproduction in arid conditions.

Question 59

What are the Ginkgoales (Ginkgos)?

Answer 59

Ginkgos appear in early Permian (~280 mya) – over 16 genera.
Today single species – Ginkgo biloba (the Maidenhair Tree).
All of these species are Dioecious- which means they have separate male and female individuals. A seed will develop after fertilisation of the egg within the megagametophyte and ginkgo is actually produce these ripe seeds which are surrounded by this fleshy tissue. This tissue is rich in sugars in order to attract frugivores, smells really rancid although its not technically a fruit, because this is a gymnosperm, not an angiosperm, but you can see how this is laying the path for the evolution of fruits. And it was basically a precursor to angiosperms.

Question 60

What are the Cycadales (Cycads)?

Answer 60

We also see the emergence of cycads, and these are an example of ancient lineage of seed plants. Massive reproductive structures in the shape of cones, all of these plants are dioecious, meaning that the male and female reproductive plants are present on separate plants. They have individual male and females.

Appear in early Permian (280 mya) – fossil cycads up to 15m.
Today there are 10 genera and 100 species in tropical and warm tropical regions.

Question 61

How do Cycad Reproduce?

Answer 61

Within cycads we see the formation of two archegonia so that means they can produce two seeds with every process of fertilisation. The male gametophytes emerge from the microspore grain into a cluster of cells. The generative cell unevenly divides to produce the stalk and body cell. The tube cell becomes a structure that’s very similar to a pollen tube in angiosperms. The body cell then divides to produce two male gametes. So we see the formation of the male gametes which are still motile. So they have cilia which propel it towards the ovule. The tube cell elongates towards the archegonia and the male motile gametes are able to swim through this tube cell and fertilise the eggs within the archegonia.

Question 62

What were the first conifers?

Answer 62

Reproductive structures are ‘cones’

First fossil record from Carboniferous (~310 mya Swillingtonia), numbers increased through Permian and underwent major radiation in Triassic (~245-208 mya) – All 8 families appear almost together (7 alive today):

1. Utrechtiaceae (Utrechia - extinct)
2. Podocarpaceae
3. Taxaceae (Yew family)
4. Araucariaceae (Monkey Puzzles) -This araucariaceae existed from the time of the dinosaurs, so in 1994 living specimen was found in a Gorge in Australia and its essentially a living fossil.
5. Cupressaceae (Cypresses)
6. Taxodiaceae (Giant Redwoods [102m!])
7. Cephalotaxaceae (Plum Yews [4 sp.])
8. Pinaceae (Pines)

Question 63

What are characteristics of conifers?

Answer 63

1. All arborescent (trees)
2. Pyramidal growth form- keeps snow off the branches and lets more light through.
3. Small simple ‘needle’ leaves- are resilient to cold temperatures.
4. At maturity stems mostly composed of secondary wood
5. Tracheids frequently arranged into distinct annual rings with resin canals – characteristic of true ‘wood’
6. Roots simple branching tap-root
7. Reproductive structures are cones
8. Mostly monoecious (male and female cones on different parts of tree)
9. Pollen with air bladders
10. Pollen produces a tube to deliver non-motile sperms to ovule.

Question 64

What do some conifers exhibit?

Answer 64

Conifers such as this pine tree exhibit resin canals that secrete resin, which is a form of defence against parasites and diseases. We also see secondary thickening, which is lateral expansion of a concentric stem. New xylem is deposited towards the outside of the stem and water mainly travels in these peripheral areas. That’s why we see this annular ring formation in trees such as these, because the growth rate is faster when its lighter and during the summer, slower during the winter, so we see these intense areas which basically forms these rings. When you cut open a pine tree, for example, you can count the number of years, how old it is, by the number of rings shown and thats because of this secondary thickening. We have a cork cambium, which is separate to a vascular cambium, is involved in depositing new epidermal tissue. So in this particular instance, it produces cork, which is a strong material.

Question 65

How do conifers reproduce?

Answer 65

When we look at reproduction in conifers we see separate male and female cones. They’re synchronised that they develop at different stages to avoid self-fertilisation. If we look at the female cone, we can see sporophylls which are leaves containing ovules beneath. The pollen which will then be generated in a male cone, will the fly through the air on the wind because this saccate pollen, the leaves will produce a secretion of water to catch the pollen grains in the female cone.

Question 66

Outline the process of fertilisation in the pine cone.

Answer 66

The process of fertilisation in the pine cone takes a very long time. We see within the male and female cones, so within the female cone we see its ovulate seed bearing cones- this is where the female reproductive structures are. And in the male cone we see it bears pollen. Within one of the scales of this cone we see the megaspore mother’s cell- this is all within the sporophyte tissue so it’s still diploid. And as this undergoes meiosis to produce four daughter cells, we see three of these abort one of them remaining, which is then able to grow into the new megagametophyte, which is the female gametophyte. And in this instance it produces two archegonia. For the male, we see the male colonists scale the male cone producing a microspore mother cell. So again this will divide by meiosis to produce haploid microspores. These then look like pollen and in this instance we see the formation of air bladders, which help them to carry on the wind and spread far and wide. These will then be caught on a watery secretion from the scale and they will then be absorbed into this whole structure. So then as the male gametophytes germinate from within the pollen green, we see formation of pollen tube and various divisions of cells, which will then ultimately result in sperm being produced. The male gametophyte is made up of three cell types. We have a tube stalk and body cells and these will, because there’s few cells you can actually map the destination of each of these cell lineages to what they end up producing. But the point is once this pollen grain is absorbed by the ovule, it kind of sticks around and doesn’t really do anything until next spring. Only after many months do we see the pollen tube that starts to grow through towards the megasporangium. Only do we get fertilisation a whole year and a bit later after pollination. And eventually we see the male sperm fertilising the eggs within the archegonia and the formation of a zygote. This fertilisation produces a diploid embryo. Cones wil start to harden in process and during autumn, cones will close to protect the seeds and they will open during the spring in order to be dispersed by the wind. In this particular species, we see the formation of two seeds because we have two archegonia, which then once the pine cone opens, the seeds can disperse and carry on the wind and land and then germinate into a brand new sporophyte.

Question 67

Pyriscence-

Answer 67

Pyriscence- is a form of serotonin when environmental factors triggers the release of seeds rather then spontaneous seed maturation. Some cones and indeed many angiosperms release seeds as a result of forest fires. The high temperatures, the heat will melt the resin surrounding a cone.

Question 68

What are the Gnetales?

Answer 68

Gnetum, Ephedra and Welwitschia
• Very derived group of advanced Gymnosperms
• Fossil record poor - molecular data places them close to Pines
• All dioecious
• Possess many features of flowering plants - convergent evolution:
1. Xylem contains vessels
2. Reproductive structures (strobili) resemble primitive flowers
3. Insect pollinated
4. Female gametophyte reduced to an ‘embryo sac’
5. Gnetum and Ephedra have a form of double fertilisation but no endosperm formed.

Question 69

1. What is the name of the group that includes all seed plants?
2. What was the name of the supercontinent formed in the Permian?
3. What did the megasporangium become known as in seed plants?
4. If you were to look at a typical tree that belongs to the angiosperms, what are you most likely looking at?
5. True or false: you can tell the likely age of a tree by counting the number of concentric rings through the trunk
6. Why is this true or false?

Answer 69

1. gymnosperms
2. Pangea
3. seed
4. sporophytes
5. true.
6. The concentric rings show the diameter growth and indicate the amount of wood added during one growing season.

Question 70

What were the major innovations in angiosperms?

Answer 70

Flowers & pollination
• Effective fertilisation (symbioses with pollinators)

Fruits
• Effective seed dispersal (symbioses)

Double fertilisation & endosperm formation
• Resource management
• Nourishment for embryo

Question 71

The Cretaceous Radiation of Angiosperms-

Answer 71

Dramatic appearance of flowering plants in the early Cretaceous and beyond. The number of species of gymnosperms starts to decline somewhat as angiosperms really start to take over the landscape. This is of course based on fossil diversity, so you’re naturally more likely to get more in recent times. But this paints a fairly good picture of what the landscape looked like so much earlier on we saw the appearance of some gymnosperms. But really in this picture the pteridophytes are the vascular plants which includes ferns and allies but we also saw lycophytes around this time as well. We essentially saw major radiation of angiosperms around the Cretaceous period.

Question 72

How did angiosperms evolve?

Answer 72

1. Ovules enclosed within a carpel/pistil
2. Presence of a ‘flower’ – unique hermaphrodite reproductive structure consisting of sepals, petals, stamens and carpel(s)
3. Double fertilisation
4. Presence of double integument- seed and ovules are even more protected by the parental sporophyte.
5. Presence of complex pollen wall
6. Presence of ‘vessels’ in xylem
7. Phloem companion cells
8. Net-veined leaves (not monocotyledons)
9. Fruits developing from flower parts

These features enable them to colonise and be more resilient to different climatic features.

Question 73

What are Bennettitales?

Answer 73

-Very abundant from early Triassic (~248 mya)
-Extinct by end of Cretaceous (~140mya)
Related to cycads- but some had bisexual cones.

The bisexual cone may of been some kind of a precursor to the flower. Then we have unisexual cones which are closer to something perhaps like a cycad. But they do show net-veining which is another characteristic of angiosperms.

Question 74

What are Glossopterids?

Answer 74

• Dominated flora of Southern Hemisphere during Permian (~290-248 mya)
• All had very advanced ‘angiosperm-like’ net-veined leaves but unisexual cones.

Question 75

Angiosperm Key Innovations: The Carpel

Answer 75

The flower is formed of 4 structures- the Gynoecium which are the carpals or pistol, the stamens around them which are otherwise called the Androecium, which are the male reproductive structures. Then we have the petals which are also called the Corolla. We also have the sepals which is also called the calyx. We can see the distinct carpels here in a buttercup. A carpel is essentially a theoretical structure containing the ovary, which may contain one or more ovules, the style and the stigma. In some species we see the formation of the pustil, which is basically the combination of different carpals together. Within the gynoecium we see the ovaries, which will contain possibly multiple ovules. Each ovule then becomes a new seed. So you can see already that we have many more potential seeds within the same sort of area as what we would have had in a cone. Not all flowers have distinct structures of the carpal as the buttercup does.

Question 76

Flower together is called the

Answer 76

Perianth.

Question 77

What happens when the ovary becomes fertilsed?

Answer 77

The ovary which contains the ovules, once these are fertilised, these will become the seeds and the fruit will form around the ovary. The remnants of the style and stigma are seen at the bottom of the fruit. Also the sepals are seen as the little stalk bits.

Question 78

How did the carpel evolve?

Answer 78

Within the ovule we have the multicellular female gametophyte which exhibits its archegonia. And in gymnosperms, this is surrounded by a single integument. Pollen grain would land and form a pollen tube, which will then deliver the sperm to the archegonia. In gymnosperms, this female gametophyte can be many thousands of cells big, however, in angiosperms, this has really reduced down to just seven or eight particular cells. So it’s still multicellular. But with angiosperms, it’s highly reduced. Female gametophyte is now being surrounded by two integuments rather than just one. We have the central haploid nuclei, but we don’t have an archegonium. Instead we have an egg cell just sort of within this whole structure because it’s so few cells you can see why it would be pointless to waste even more cells producing a structure as it becomes highly reduced. These two central nuclei get fertilised as the endosperm is created. Typically when we get the formation of a pollen tube, we get the delivery of two sperm cells. One of them would fertilise the egg cell and the other would fertilise the central cell nuclei. The sperm cell that fertilises the egg cell becomes a zygote and develops into the new embryo. The other sperm nuclei will fertilise these two central nuclei and instead will become the endosperm. The synergid cells have no real established function other than guiding the pollen tube. The antipodal cells have no real function at all.

Question 79

What are aspects of the Angiosperm male gametophyte?

Answer 79

Pollen shows even further reduction of the gametophyte phase- now its just either two or three cells down from more like 5 or 7. So we have a large vegetative cell and generative cell which will divide mitotically to produce the sperm. This occurs before the gametophyte is released from the pollen, unlike gymnosperms where the sperm is generated after the gametophyte has left the pollen grain. You can see this advancement in the cell structure of the microspore, the pollen grain itself. This will relate to whether they are aiding the pollen grain to blow along with the wind, or whether they stick onto pollinators that visit the flowers.

Male gametophyte – 2 or 3 cells within the pollen grain (microspore)

Question 80

Outline aspects of the stamen.

Answer 80

Stamen are microspores pollen is formed by meiosis,

The origin of the stamen

Stamens evolved from leaf- or petal-like structures bearing microsporangia.
Stamens of primitive living angiosperms showing leaflike morphology.

Question 81

What happens when a pollen grain lands on the female reproductive structure?

Answer 81

Pollen grain can be dispersed either by animals, by pollinators or by the wind. As it lands on the stigma if it is a correct species, it will then grow a pollen tube. The pollen grain itself will germinate, producing this pollen tube. This pollen tube is just a single vegetative cell that massively elongates. They are actually the longest living cells in the world, and they deliver the sperm to the egg. The sperm cells are not motile at all- they are non-motiles. This formation of the pollen tube + delivery of the sperm cells right into the ovule. Now finally releases the land plants from the requirement of water for the process of fertilisation. We have chemical signalling involved in coordinating the path of the pollen tube down the stigma. One sperm cell will fuse with the egg to make the zygote and the subsequent embryo. The other fuses the two haploid nuclei in the central cell of the ovule. This will then lay down the endosperm which provides the nourishment for the embryo. Callose plugs help the tube grow more quickly as they keep the cytoplasm near the elongating end of the tube.

Question 82

Summary of fertilisation events in angiosperms

Answer 82

Process does not require water.

1. Pollen tube penetrates a synergid and releases sperm nuclei; tube nucleus degenerates
2. One sperm fuses with the egg to form a diploid zygote
3. Second sperm fuses with the two polar nuclei to form a triploid endosperm which nourishes the developing embryo sporophyte.

Question 83

Parental conflict in angiosperms

Answer 83

Unusually in the flowering plants, we see triploidy in the endosperm;
◦ Result of a double fertilisation event: the egg/sperm, and the two haploid maternal nuclei in the endosperm get fertilised by the single haploid sperm nucleus

→Why do we get this 2:1 maternal:paternal ratio (2m:1p)?
Remember there are two central haploid nuclei in the middle of the ovule, we have the pairing with the second sperm cell. So we end up with this triploid endosperm. Theres a really good reason why we have this triploidy and we have the ratio in favour of the maternal parent. This is because of parental conflict. We see parental conflict a lot in animal species but we also see this in plants.

Parental conflict
◦ Genomic imprinting of the DNA means there is a fight over how the quantity & quality of resources that are laid down for the developing embryo in the endosperm. The final ratio gives the maternal parent more control.

Question 84

Angiosperm Key Innovations: The Endosperm

Answer 84

Following fertilisation the triploid endosperm nucleus divides repeatedly to form a mass of protoplasm and nuclei without cell division and acquiring nutrients (lipid, starch, protein) from the sporophyte. As the seed matures the endosperm undergoes cellularization (cell walls form). The resources are not wasted on undeveloped seeds.

Food resources are thus only laid down in an ovule/seed after fertilisation. The reduction in the size of the female gametophyte in the angiosperms is again another benefit because it’s not wasting resources. Reduced it down to only what is necessary.

Question 85

Angiosperm seeds utilise

Answer 85

maternal resources more efficiently than gymnosperm seeds

Question 86

So what is a fruit?

Answer 86

A structure formed from parts of the flower or inflorescence that contains the seeds, e.g. a pod.

In angiosperms after the seed has been fertilised, we get the formation of a fruit. Modifications of the flower post fertilisation occurs, petals fall off and the structure fills with sugar and water. This fleshy substance forms around the seed. It becomes fatter and colourful in order to attract frugivores which eat the fruit and drop the seeds deposits around with. There’s also laxative chemicals that are found in a fruit that motivate the gut, so essentially this is a very clever strategy because as the animal deposits the seed along with them there’s a really nice fertilised patch of ground that the new sporophyte can then develop into.

The fleshy fruit is an adaptation to seed dispersal by animals.

Question 87

Angiosperms (flowering plants)- How are they classified?

Answer 87

Dicotyledons (dicots)

Monocotyledons (Monocots)

Question 88

What are features of dicots?

Answer 88

• Embryonic plant with 2 seed leaves (cotyledons)
• Leaves net-veined, opposite or alternate
• Vascular bundles of stem (usually) arranged in a circle
• Stems often woody - secondary thickening
• Flower parts usually in 4s or 5s or their multiples.
-Dicots are a paraphyletic group so the lineage that exclusively contains dicots in a monophyletic format is actually called the Eudicots

Question 89

What are features of monocots?

Answer 89

• Embryonic plant with 1 seed leaf (cotyledon)
• Leaves typically parallel-veined
• Vascular bundles of stem closed and scattered
• No secondary thickening
• Flower parts usually in 3s or their multiples

Question 90

Modern classifications recognize

Answer 90

Eudicots, Monocots and a mixture of ‘primitive’ dicots

Question 91

What is The most ‘primitive’ flowering plant?

Answer 91

Has very simple flowers- ancient plants. Only bears tracheids rather than vessels in its vascular tissue. Carpels lack styles so it’s got a very primitive version of a flower.

Question 92

What are seeds?

Answer 92

Seeds are highly resistant structures well suited to protecting the plant embryo from environmental stresses and to some extent from predators. In addition, almost all seeds contain a supply of food for the young plant. Lycophytes and pterophytes do not have seeds. They are dispersed via haploid spores.

Question 93

What do fruits add in flowering plants?

Answer 93

Fruits in the flowering plants (angiosperms) add a layer of protection to seeds and have adaptations that assist in seed dispersal, expanding the potential range of the species. Flowers allow plants to secure the benefits of wide outcrossing in promoting genetic diversity.

Question 94

How is the ‘inevitable’ seed produced?

Answer 94

Came about from endosporic development and what this means is the retention of the megaspore within the megasporophyll. The picture demonstrates a heterosporous traceahpyhte because it’s got a megasporangium which houses the megaspores and microsporangia which houses the microspores. So this differentiation means that the megaspores will germinate and become the female gametophyte and the microspores will become the male gametophyte. What happens is there are four megaspores in this particular sporangium. If three of them abort that leaves just a single megaspore. Now what this can do is instead of being released, it can instead germinate within the megasporangium, divide mitotically, and actually become the multicellular female gametophyte. The mega gametophyte then instead of being free-living and germinating on a damp soil somewhere, it can actually retain within the parental sporophyte. Now this is really clever because just like in the mosses, we saw the retention of the sporophyte within the gametophyte. Now we are starting to see a reversal of that. Once the sporophyte, it is now more independent and free living, we see the retention of the megaspore within the megaspores sporangium. And it can germinate within this space and create a reduced but still multicellular being that is still the gametophyte. But it can now then retain all of the benefits of being nurtured and looked after by its parental sporophyte. And this is essentially what an ovule or a seed is, the retention of the female gametophyte.