Angiosperms

Question 1

Why study angiosperms

Answer 1

Dominated planet since the late Cretaceous and early Tertiary periods
Evolution and rise to dominance closely associated with evolution of many animal groups, particularly insects and mammals
Provide with livestock and food, raw materials
Huge, largely untapped resource of medicines and biotech products
Integral part of the ecology and biodiversity of the earth

Question 2

Features of angiosperms

Answer 2

Flowers
Ovules enclosed in carpels
Carpels develop into fruit
Nutritional endosperm in seed
Xylem with vessels
Phloem with sieve tube elements
Heterosporous with a very reduced male gametophyte (3 cells) and female gametophyte (usually 7 cells)
No antheridia and no archegonia
Diverse life forms – e.g woody and herbaceous forms

Question 3

Life forms of angiosperms

Answer 3

Enormous range in side from many tiny aquatic plants to massive trees
Herbaceous forms – non woody = therophytes (annual), monocarpic (flower, set seed, die), perennials (live more than 2 years)
Woody plants = climbers, shrubs, trees (evergreen, deciduous)
Parasitic forms

Question 4

Angiosperm families

Answer 4

Like other vascular plants, flowering plants are classified into families
Dicots – Fabaceae (legume fam), Asteraceae (dandelion fam), Fagaceae (oak fam)
Monocots – poaceae (grasses), ochidaceae (orchids), Liliaceae (lillies)

Question 5

Differences between monocots and dicots

Answer 5

Mono has one cotyledon in seed, di has two
Mono has root xylem and phloem in ring, di has phloem between arms of xylem
Mono has vascular bundles scattered in stem, di has vascular bundles in a distinct ring in stem
Mono has leaf veins that form a parallel pattern, di has lead veins that form a net pattern
Mono flower parts in threes and multiples of three, di flower parts in fours or fives and their multiples

Question 6

Life cycle of angiosperms

Answer 6

Seed germinates – seedling (young sporophyte) – mature sporophyte – flowers
Flowers produce stamens (m) and/or carpels (f)
Stamens produce pollen, carpels produce ovules
After pollination and fertilisation, seeds are produced in a fruit
Seeds are dispersed but may remain dormant

Question 7

Flower structure of angiosperms

Answer 7

There is a huge diversity of flower structure
Flowers may be borne singly or grouped together in an inflorescence
Each flower is formed on a receptacle
Sepals form the calyx and petals form the corolla
Stamens from the androecium (m) and carpels form the gynoecium (f)

Question 8

Stamens and pollen

Answer 8

Stamens – male
- Represent macrosporophylls in the angiosperm flower
Stamens are made up of a filament and a two-lobed anther
- Each anther lobe has a pair of microsporangia (pollen sacs) – 4 pollen sacs per stamen
- Diploid microspore mother cells in the pollen sacs undergo meiosis to produce tetrads of haploid microspores
Pollen
- Each haploid microspore develops into a pollen grain
- At maturity the pollen grain consists of 3 cells – tube cell and 2 sperm cells
- Pollen grain has inner wall (intine) and a very resistant outer wall (exine) which is often elaborately sculptured
- Exine is made up of sporopollenin
- Mature pollen grains are often packed with nutritious starch or oils
- Pollen grains are released from the pollen sacs when the anthers split open (dehisce)

Question 9

Carpels and embryo sac

Answer 9

Carpels – female
- Represent the megasporophylls
- Made up of ovary, styles and stigmas
- Often ovary is compound – made up of 2 or more fised carpels
- Ovules are attached to the placenta in the ovary
- Fertilised ovules become the seeds
- Seeds are eventually enclosed within the fruit which has developed from the ovary (carpel)
Embryo sac
- Diploid ovule has a stalk called the funiculus and nucellus enclosed by 1 or 2 integuments
- Single diploid megaspore mother cell in the nucellus undergoes meiosis to produce 4 haploid megaspore cells but usually only 1 survives
- Surviving haploid megaspore enlarges and its nucleus divides mitotically to produce 8 haploid nuclei

Question 10

Pollination and fertilisation of angiosperms

Answer 10

If compatible pollen reaches the stigma surface, it takes up water and germinates = pollen tube
Pollen tube grows through the stigma and style to the ovule usually entering via the micropyle
The 2 sperms are liberated into the embryo sac, one fuses with the egg cells to form a diploid zygote, the other unites with the 2 polar nuclei to form a triploid cell = double fertilisation
Diploid zygote develops into the embryo
Triploid cell divides to from the endosperm of the seed

Question 11

Development of seed and fruit in angiosperms

Answer 11

The ovule becomes the seed and ovary (carpel) wall and related structures develop into the fruit
Zygote in the ovule becomes the embryo which is nourished by the endosperm
In some seeds the nucellus (2n) also contributes to a food storage tissue called the perisperm
Integument becomes the seed coat

Question 12

Self pollination of angiosperms

Answer 12

Plants that exhibit this have a low pollen:ovule ratio
Stamens and styles either develop together or are mature together for some time
Stamens and styles often near each other in flower
Flowers must be self compatible
Advantages – pollination is assured
Disadvantages – inbreeding leading to lower heterozygosity and inbreeding depression
Occasional outbreeding will increase heterozygosity
Some flowers are cleistogamous (never open) so only self pollination is possible
Many weedy, annuals self pollinate
Assures a rapid and high degree of success in seed production

Question 13

Cross pollination of angiosperms

Answer 13

Essential if outbreeding (cross fertilisation) is to occur
Flower must be self-incompatible (it must outbreed)
Flower design may prevent self pollination
Flower may show specific adaptations that promote transfer of pollen from one flower to another (on a different plant)

Question 14

Physical characteristics of angiosperm flowers that prevent self pollination

Answer 14

Protandry – stamens develop and mature before carpels have receptive style and stigma – e.g foxglove
Protogyny – carpels develop and mature before stamens – e.g lady’s smock
Heterostyly – stamens and styles at different (variable) positions in the flower – e.g pin-eyed and thrum-eyed flowers of primrose
Dioecy – plant is either male or female – e.g red campion, stinging nettle, dog’s mercury

Question 15

Pollen transfer methods

Answer 15

Wind
Water
Insects
Birds
Bats
Other animals (including primates)

Question 16

Insect pollinated flowers

Answer 16

Insects involved = coleoptera (beetles), Diptera (flies), Hymenoptera (bees, wasps), lepidoptera (butterflies and moths)
Insect pollination syndromes:
Colour
- Brightly coloured flowers – usually corolla (petals)
- Bees can see in uv but not in red. Many petals have uv reflection patterns – yellow petals of tormentil are ‘bee purple’
-Petals often have guide marks – ‘landing lights’ not always visible to us
-Many insects have colour preferences, bees and butterflies ignore certain colours
- Beetles have poor colour vision and will visit dull coloured flowers
Scent
- Different scents associated with bee, butterfly, moth and fly pollinated flowers
- Bees like sweet honey smell – clover, gorse
- Butterflies and moths like heavy scents – honeysuckle
- Flies may be attracted by smell of carrion
- Flower may exhibit a daily rhythm of scent production which matches the activity of the pollinators
Reward for pollinator
- Pollen – rich in protein, starch, oils/fats, collected as food by bees and other insects – beetles (only have biting mouth parts), some plants produce copious amounts of pollen but no nectar – red poppy = pollinated by bees, flies and beetles, bees are excellent pollinators – very diligent pollen gatherers
- Nectar – sugar solution made up from combinations of glucose/fructose/sucrose, food for insects with sucking mouth parts – diptera, hymenoptera, lepidoptera, available in nectaries which may be accessed by various insects or may be concealed deep in flower – e.g spur of zygomorphic flower, tongue length of insect will be matched to nectar access – e.g long tongued moths and honeysuckle
Mimicry
- Orchids – bee orchid, fly orchid, spider orchid
- Produce flowers that resemble arthropod
- Also produce pheromone to attract male fly/bee, resulting in pseudo-copulation
- Insects pick up 2 bags of pollen (pollinia)

Question 17

Morphology of insect pollinated flowers

Answer 17

Landing platform
Flowers evolved zygomorphy (bilateral symmetry) – insect can only approach from one direction becomes dusted with pollen underneath (fabaceae) or above (lamiaceae), evolution of elegant mechanisms – e.g hinged stamens of sage deposits pollen accurately on bees back
Flowers grouped into inflorescences – maximum advertisement and high foraging potential

Question 18

Bird pollination syndromes

Answer 18

Birds visit flowers to feed on nectar, floral parts and insects that inhabit flowers
Bird pollinated flowers produce copious nectar, but usually have little or no scent (sense of smell poorly developed in many birds)
Most bird pollinated flowers are brightly coloured, particularly red (also yellow)
Flowers are often large and hanging – e.g fuschia, pointsettia, columbine
Hummingbirds, sunbirds, sugar birds, honeyeaters
Nectar from flowers fuels bird flight

Question 19

Wind pollination

Answer 19

First angiosperms were possibly wind pollinated
Not a primitive trait, since it arose many times in evolution of flowering plants
Grasses, one of the most successful groups of flowering plants, are wind pollinated
Massive production of pollen
Dense inflorescences of flowers, plants grow in dense strands
Pendant tassels of flowers – e.g catkins, easily jostled in wind, promoting release of pollen
Stamens exposed – long filaments suspending the anthers
Stigmas exposed and with papillose or feathery appearance – increased surface area for intercepting airborne pollen
Small, smooth, buoyant pollen grains
Explosive anthers – stinging nettles
Pollen released in dry weather in huge amounts (nb hay fever)
Flowers usually green or drab
No scent nor nectar, zygomorphy rare
Lots of familiar trees – hazel, oak, elm, alder, birch (many others)
Many herbaceous species – docks, plantains, stinging nettle, dog’s mercury
Grasses

Question 20

Types of fruit

Answer 20

Nature of the fruit wall (pericarp) is important
2 types – dry fruits (dehiscent = e.g peas, indehiscent = e.g buttercup, grass), fleshy fruits = e.g cherry, plum, peach, grape, tomato, banana, gooseberry, apple
Fleshy fruit – outer layer is the tissue that develops from the ovary wall of the flower

Question 21

Fruit and seed dispersal

Answer 21

Wind
Water
Mechanical
Animals
Frugivory and seed dispersal
Endozoochory – animal ingests fruit and seeds, pericarp may be fleshy or dry, often attractive and inevitably nutritious dispersal occurs because seed is = excreted (common with many birds and vertebrates, but seeds have to withstands passage through gut; seeds may be transported long distance and deposited with source of nutrients), regurgitated (seed has emetic properties), discarded (e.g mistletoe species and birds)

Question 22

Animal dispersal agents

Answer 22

Animal dispersal agents
Mammals
Birds
Reptiles
Fish
Invertebrates (e.g ants)

Question 23

Coevolution and specialisation

Answer 23

Obligate frugivory - Specialist frugivore birds in tropics
- Fruit has large seed which is voided
- Very nutritious (high protein, lipid, carbohydrate)
- High quality food for quality dispersal
- Not available in abundance (requires diligent foraging)
- Resource allocation (plant) matched with diligent foraging strategy (animal)
- e.g palmae, lauraceae, Burseraceae
Facultative frugivory – non specialist, opportunistic frugivores
- Fruits with many small seeds
- Carbohydrate rich only
- Eaten by wide variety of non-specialist animals
- Fruit found easily
- E.g rubus fruticosus

Question 24

Endozoochory in temperate regions

Answer 24

Birds are very important – good sight, colour vision (including uv), poor sense of smell
Most European frugivore birds are relatively small – limited gape and limited ability to carry fruit