Chapter 9 Flashcards
Plant biology
1
Q
Gas exchange in leaves
A
Plants photosynthesise- require CO2 and water to produce carbohydrates and oxygen
- also require oxygen for cell respiration
- plants obtain gases needed through diffusion, mainly through leaves
- most leaves have tiny pores, usually located on their underside (stomata)
2
Q
Stomata
A
Tiny pores where gases can diffuse in and out
- two guard cells that surround stoma control whether it’s open or closed, and so can control gas exchange
when stomata are open, gases can be exchanged, but water vapour can also escape from the leaves. This water must be replaced by water taken into the roots and carried through the plants to the leaves. This process is called transpiration and is the result of gas exchange in the leaves.
3
Q
Stomata are open
A
- gases can be exchanged, but water vapour can also escape from leaves
- water must be replaced by water taken into roots and carried through plants to leaves- transpiration
- transpiration is the result of gas exchange in the leaves
4
Q
Why is transpiration essential to plants?
A
It pulls water up from the root to:
- provide one of the raw materials for photosynthesis
- transport minerals to leaves for use in synthesis of important molecules
- cool the leaves
5
Q
Transpirational pull
A
- it’s important for a plant to replace water that’s lost during transpiration - otherwise it’ll wilt and won’t photosynthesise efficiently
- loss of water vapour through stomata creates a transpirational pull
- helps to pull water from roots to the leaves
- structure of xylem and water’s adhesive and cohesive properties make this movement possible
6
Q
Porous pot
A
- shows transpirational pull caused by evaporation of water from a porous pot
- water evaporates from surface of pot and leaves of twig pulling more water up from beaker
- a net upwards movement of coloured water in tube is observed
7
Q
Potometer
A
- models transpiration
- inside surface of pot absorbs water, which then evaporates
- cohesion between water molecules causes water to be drawn into pot from potometer- similar manner to transpiration water movements
- similar changes in transpiration rates for both pot and plant can be observed for changing environmental conditions
- a useful model that demonstrates passive nature of transpiration
8
Q
Xylem vessels
A
- in vascular plants
- transports water from roots to all other parts of plant
- long continuous tubes that run from roots through stems of plants
- walls of xylem vessels are thickened w/ lignin (a woody tissue), can be in form of a thickened wall, rings or spirals
- pressure inside is lower than atmospheric pressure- but they don’t break because support from lignin makes tubes v. strong and rigid
- when formed, they’re elongated living cells, but when mature, they’re no longer living- lose their cytoplasmic content and are converted to hollow tubes that transport water within plant
- they’re no longer living, so flow of water is driven by passive forces
9
Q
Lignin
A
- woody tissue
- walls of xylem vessels are thickened w/ lignin
- prevents walls of xylem vessels from collapsing under pressure
- adds strength to woody material of older plants
10
Q
Primary xylem vessels
A
- first xylem that forms from root or shoot tip
- walls contain v. little lignin in form of annular and spiral thickening
11
Q
Process of transpiration
A
- Leaves lose water vapour through their stomata
- More water evaporates from mesophyll cell walls into intercellular spaces, replacing water vapour
- Water adheres strongly to cellulose in cell walls
- Loss of water vapour from mesophyll cells causes water to be drawn from neighbouring xylem vessels- movement of water occurs via small pores present in cellulose cell wall
- Hence, adhesive property of water, and evaporation, generates tension in leaf cell walls that generates transpiration pull
- This causes upward movement of water in xylem vessels to replenish water lost through transpiration
12
Q
Cohesion
A
Water molecules are polar and stick to each other
- allows water to be moved up over long distances
13
Q
Adhesion
A
- water molecules are polar
- polarity interacts w/ hydrophilic parts of xylem vessels
- interaction between water and wall of xylem vessel
14
Q
Cohesion and adhesion in transpiration pull
A
Result of these forces is a continuous stream of water through plant
- supported by structure of xylem vessels
- due to their lignin deposition, xylem vessels can withstand great pressure
- also, transport water under tension without breaking
15
Q
Root hair cells
A
- water and minerals, that the plant needs, are taken up by the roots of a plant from the soil
- root hair cells are tiny extensions on specialised root cells that take up water through osmosis
16
Q
Water uptake through osmosis in root hairs
A
- Water enters root hairs by osmosis
- Water passes across root, from cell to cell by osmosis
- it also seeps between the cells - Water is drawn up by xylem vessels
- transpiration is constantly removing water from the top of them
Transpirational pull caused by transpiration sucks water from epidermal root hair cells deeper into root
- ultimately into xylem vessels
17
Q
Once in root, water can move to xylem in 2 ways
A
- Through apoplast:
- water moves through cell walls of epidermal cells of the root - Through symplast:
- water moves through cytoplasm of epidermal cells of the root
18
Q
Endodermis
A
An inner layer of cells that surrounds the core of the root and the vascular tissue
- from endodermis onwards all water flows through symplast pathway to reach xylem vessels
19
Q
Casparian strip
A
- a band of suberin, waxy substance that is impermeable to water
- found in cell wall of endodermis of plant roots
20
Q
Mineral uptake in the root
A
- conc. of minerals is higher in root hair cells than in soil surrounding root
- protein pumps in plasma membrane of root hair cells actively pump minerals into cytoplasm
- this causes absorption of water by osmosis
- hence, minerals are transported while dissolved in water to xylem vessels
21
Q
Xerophytes
A
Plants that have adapted to thrive under dry conditions
- adapted to survive in environments where water is scarce
22
Q
Halophytes
A
Plants that can survive in environments of high salinity
- plants that have adapted to living in or near the sea
- have evolved mechanisms to cope w/ high levels of salt
23
Q
Adaptations of xerophytes
A
- Leaf size is reduced
- minimise SA through which water can be lost - Stems or leaves are succulent to store water (if these aren’t reduced to spines)
- Stomata are on the plant stem w/ few if any on the leaves
- Stomata open up only during the night when it’s cooler
- so there’s less evaporation - Leaf epidermal cells are coated in a waxy cuticles to reduce water loss
- Leaves rollup to increase level of humidity around stomata
- reduces gradient for loss of water vapour from leaves - Root systems may spread out wide
- forms an extensive network of shallow roots
- or go deep into the ground to absorb water
24
Q
Crassulacean acid metabolism
A
- During the night, when stomata are open, CO2 is absorbed and converted into malic acid
- during the day, CO2 is released from the malic acid
- this can be used for carbon fixation in light-independent reactions
NB/ cacti have evolved this system
- because only having stomata open during the night would affect photosynthesis
- as plant would quickly run out of CO2
25
Adaptations of halophytes
1. Leaves are reduced in size
2. Water storage structures develop in the leaves
3. Some plants have a thick cuticle and a thick epidermis
4. Stomata are sunk into pits
5. Some plants have structures to remove salt build-up
6. In some plants, root cells actively pump excess sodium chloride ions out into surrounding soil/water
26
Rate of transpiration
Determined by measuring amount of water lost from a plant (or part of it) per unit time
27
Internal factors that affect rate of transpiration
Internal factors: factors directly related to the plant
- root to shoot ratio
- SA of leaves
- total no. of stomata per unit leaf area
- structure of leaf, e.g. presence of hair or thick waxy cuticle
28
External factors that affect rate of transpiration
External factors: factors linked to environment
- light
- wind
- temperature
- humidity
- water supply
29
Effect of light on plant transpiration
- as light intensity increases, rate of transpiration increases
- stomata are closed in the dark
- but, as light intensity increases, stomata open and allow water vapour to escape from air spaces of leaves
- hence, bright sunlight increases rate of transpiration
30
Effect of wind on plant transpiration
- increase in wind velocity increases rate of transpiration
Wind speed is low:
- air surrounding a leaf becomes increasingly humid
- reduces water vapour conc. gradient from intercellular spaces to surrounding air- reduces rate of transpiration
Wind speed is high:
- humid air is carried away faster and is replaced by drier air
- this is due to a steeper diffusion gradient of water vapour between air spaces in leaves and surrounding atmosphere
- increases rate of transpiration
- if wind speed gets too high, stomata close reducing rate of transpiration
31
Effect of temperature on plant transpiration
- as temp. increases, rate of transpiration also increases
Higher temp.:
- provides more energy for evaporation of water from cell surfaces
- decreases humidity of external atmosphere.
High temp.:
- if temp. gets too high ( beyond 35°C), rate of transpiration gradually falls
- due to inactivity of protoplasm and closure of stomata
32
Effect of humidity on plant transpiration
- higher the relative humidity of outside atmosphere, lower th rate of transpiration
- lower the relative humidity of outside atmosphere, the higher the rate of transpiration
Low humidity:
- air surrounding a leaf is dry
- gradient for diffusion of water vapour from air spaces within the leaf to the outside, is steep
- Hence, rate of transpiration is high
Opposite occurs when the level of humidity in the air is high.
33
Effect of water on plant transpiration
- deficiency of water in soil decreases rate of transpiration
- absorption of water by roots can't keep up w/ rate of transpiration
- leaf cells lose their turgidity
- stomata close- the plant wilts
- immediately reduces rate of transpiration.
34
Humidity
Refers to % of water vapour present in the atmosphere
35
Potometer
A device used for measuring the rate of transpiration of a leafy plant shoot
36
How does a potometer work?
As water is lost by leaves, water is absorbed from cute end of the shoot due to transpiration stem
37
Two ways of collecting data from a potometer
- measure water absorbed over a fixed amount of time
| - measure drop in mass over a fixed amount of time
38
Sieve tube elements
elongated living cells that form the phloem tissue
- no nucleus
- no tonoplast
- no ribosomes
39
Sieve tube
several sieve elements are connected end to end to form a sieve tube
40
Sieve plates
cross walls within the sieve tubes become perforated during development to give rise to sieve plates
41
Sources
Photosynthesising tissues and some plant organs that export sugars to other parts of the plant
42
Sinks
Plant organs that can't produce sugars, but need them for respiration or storage
43
Translocaiton
The steady supply of the products of photosynthesis eg. carbohydrates and other solutes eg. minerals and AA
- based on principle that some areas of the plant have too much (source) while others have too little (sink)
- direction of flow of sap is from source to sink
44
Sap
Water containing carbohydrates, amino acids and plant hormones
45
Sources
- photosynthetic tissues eg. leaves and stems
- storage organs which are unloading their stores at the beginning of the growing season
eg. germinating seeds, potato tubers and bulbs
46
Sinks
- roots
| - developing food stores eg. fruits, seeds or new leaves
47
Steps of translocation
1. Sugars produced by sources are actively (uses ATP) loaded into sieve tubes by companion cells
- causes conc. of solute to build up in sieve tubes
2. Water enters sieve tubes by osmosis from neighbouring xylem vessels
3. Water is incompressible and sieve elements have a rigid cell wall
- this inflow of water creates a great deal of internal pressure
- pressure causes movement of water and carbohydrates through pores of sieve plates, down tube to the sink
- pressure that drives this mass flow is called hydrostatic pressure.
4. At the sink, companion cells actively unload sieve tube
- some carbohydrates are converted into starch and stored- some are used by respiring cells
- as sugars leave sieve tube, conc. of solute decreases
- leads to water moving to neighbouring vessel by osmosis
5. loss of water from sieve tube leads to drop in hydrostatic pressure
- this is important, it allows transport along hydrostatic pressure gradients in sieve tubes
- as phloem sap flows from source to sink , it's transported from region of high hydrostatic pressure to one of lower hydrostatic pressure
- this is referred to as pressure-flow mechanism
48
Phloem loading
Process by which soluble carbohydrates (sugars) enter the phloem
- requires active transport
- resulting high conc. of carbohydrates need to be contained in the sieve elements
- so they don't affect osmotic balance of neighbouring cells
49
Phloem
Column of living cells w/ perforated walls between them
- transports sap from sources to sinks
- ensures that all parts of the plant can perform the functions of life
50
Flow of sap
Sap flows from an area w/ high hydrostatic pressure to an area w/ low hydrostatic pressure
51
Structure of phloem
- consists of living cells w/ reduced cytoplasm and no nucleus
- but, cells do have membranes to maintain high conc. of solutes
- companion cells perform many of the genetic and metabolic functions of the sieve elements or sieve tube cells
- hence, sieve tube cell can maintain membrane structure necessary for high solute conc.
- reduced cytoplasm increases volume of sap that can be transported by sieve cells
- sieve elements lose most of their cellular components, eg. nucleus, cytoskeleton, ribosomes and tonoplast as they mature
- produces a tube-like structure that allows sap to flow through easily
52
Plasmodesmata
Contact between companion cells and sieve tube cells is mainly through plasmodesmata
53
Xylem vs. phloem
Xylem:
- columns of dead cells
- transports water and minerals
- thickened cell walls consisting of lignin
- continuous hollow tube, allows for an unbroken column of water
Phloem:
- columns of living cells
- transports sugars, AA and plant hormones
- has companion cells for cell functions w/ many mitochondria that provide ATP for active transport
- sieve tubes have sieve plates at an interval that controls flow of sap
54
Examples of two research methods measuring phloem sap
- using aphid stylets
| - radioactively labelled carbon dioxide
55
Aphid stylets
- small sap-sucking insects
- can tap into phloem sap w/ their long stylets
- if stylet is separated from aphid, phloem sap will continue to flow out of the stylet
- it has now been found that sap wasn't sucked up by the aphid
- instead, it is 'pushed' out of the plant, suggests phloem is under pressure
56
Radioactively labelled carbon dioxide
Plant sap = rich source of AA, proteins and carbohydrates
- if plant is exposed to radioactive CO2, resulting organic compounds will be labelled with C-14
- easily visualised in radiograms
57
Cross section of a monocot stem
- ground tissue is NOT differentiated into different layers of tissue
- vascular bundles are scattered randomly in the ground tissue
58
Cross section of a dicot stem
- ground tissue is differentiated into different layers eg. cortex and the pith
- vascular bundles are arranged in a ring to near the edge of the stem
59
Meristems and creation of new cells
Made of undifferentiated cells that divide and grow rapidly
- growth in plants is concentrated here
- new cells created are constantly pushed away from meristematic zone due to continuous cell production
- these cells later differentiate and become specialised for the function they'll perform
60
Apical meristems
The two main areas where growth occurs
1. Root tip
2. Shoot tip
61
Root apical meristem
- elongates the root
| - pushes the root deeper into the ground
62
Mitosis in the shoot apex
Mitosis and cell division at meristem in shoot apex provide cells needed for extension of stem and development of leaves.
63
Dicotyledons
a group of flowering plants whose seed has two embryonic leaves or cotyledons
64
Why do plants continuously grow throughout their lifetime?
Continuous growth of plants throughout their whole lifetime is due to undifferentiated cells in meristems that can continuously produce new cells
65
Hormones
Chemical messengers
Animals:
- can be proteins or other organic compounds
Plants:
- usually small organic molecules that are simpler than animal hormones
What do plant and animal hormones have in common?
- both are effective, even in low conc.
- but their action can be restricted by transport system at their site of production and site of action differ
66
Plant hormones
can be carried over long distances in plant xylem or phloem dissolved in the sap
67
Auxin
- a plant hormone
- a derivative of acetic acid (IAA) (indole-3-acetic acid)
- IAA is synthesised in apical meristem and travels down stem
- further it travels away from apical meristem, lower its concentration
- IAA inhibits growth of axillary (side) buds
- these buds form in the nodes of a plant
- IAA inhibits growth of axillary buds, causing plant to grow vertically up to trap more light for photosynthesis- apical dominance
68
Elongation of cells
- mitosis and cell division in shoot apex meristem provide cells needed for extension of stem and development of leaves
- but, this growth is under direct control of plant hormones eg. auxin, which control elongation of cells
69
Cytokinin
- a plant hormone produced in the root of a plant that promotes growth of axillary buds.
- further down the stem, conc. of IAA is lower and conc. of cytokinin is higher
- ratio between conc. of auxin and cytokinin hormones determines whether an axillary bud will develop
70
Function of IAA
- promotes elongation of cells in the stem
- regulates fruit and leaf development
- inhibits growth of buds in the nodes
71
Functionof cytokinin
- produced in the root
| - promotes axillary bud growth
72
Tropism
a directional response to an external stimulus
73
Phototropism
directional response in plants in response to light
74
Gravitropism
directional response in plants in response to gravity
75
Positive and negative tropisms
Positive: growth to the stimulus
Negative: growth away from the stimulus
76
Shoots and tropisms
- show +ve phototropism
- but -ve gravitropism
Grow directly to the source of light
- and away from gravity
- brings them towards the light
- hence, shoot receives maximum light for photosynthesis
77
Roots and tropisms
- show -ve phototropism
- and +ve gravitropism
Move away from light and towards gravity
- roots grow towards soil
- allows them to absorb water and necessary minerals
- provides a firm support for the plant
78
Shoots and auxins
Under normal light conditions (when light is coming from above):
- auxin produced at the shoot tip diffuses to zone of elongation and is evenly redistributed
- all cells in this area grow at the same rate
- causes the shoot to extend vertically upwards
Presence of light from the side:
- causes auxin to redistribute so it accumulates on the shaded side
- accumulation of auxin on shaded side causes increased cell elongation, results in bending of the stem towards light source
Shoot is horizontal:
- gravity will cause accumulation of auxin on lower side of shoot
- shoot reacts by growing faster than the upper side
- ultimately, shoot will grow away from gravity and towards light- shows -ve geotropism
79
Roots and tropisms
- root cells are a lot more sensitive to auxin
Root is horizontal:
- light from above and gravity from below will cause accumulation of auxin on lower shaded side of the root
- this causes inhibition of cell elongation in this part
- causes elongation of cells on the upper side
- overall effect is the bending of the root towards gravity (away from light)
- shows +ve geotropism and -ve phototropism
80
Regulation of phototropism
- phototropism is regulated by proteins called phototropins
- phototropins regulate transcription of genes that may play a role in transport of auxin
- hence, phototropins are indirectly involved in phototropism
81
Production and movement of auxin
- auxin is produced in the apical shoot meristem
- it moves vertically down the stem by diffusion (to a limited extent) and by passing in and out of the successive layers of cells
- to support this process, plant cells have auxin influx proteins (take in auxin) and auxin efflux proteins (remove auxin)
82
Auxin influx proteins
- take in auxin
| - found at the top part of the apical shoot meristem
83
Auxin efflux proteins
- remove auxin
| - found on the basal membrane of the apical shoot meristem
84
Normal light conditions and auxins
- auxin is pumped into a cell from the meristem
- auxin exists the cell towards the base to be re-pumped into the cell below
- in this way, auxin moves down the stem w/ help of auxin efflux pumps that set up conc. gradients of auxin in plant tissue
- highest conc. is at meristem
- lowest conc. is towards the plant base
- auxin is more or less uniformly distributed in each horizontal layer of cells along the stem- accounts for vertical extension of the stem
85
PIN proteins
auxin efflux pumps or proteins
86
Auxin influx and efflux transporters
Both auxin influx and efflux proteins are collectively called auxin influx and efflux transporters
87
Abnormal light conditions and auxins
- light is shone at the side of a plant
- auxin efflux proteins redistribute laterally towards the shaded part of the cell (shit from basal to lateral membrane of the cell)
- now any auxin entering the cell from the apex is pumped out laterally to the neighbouring cell
- neighbouring cell responds by elongating
- causes the shoot to bend and grow towards the light source
88
Important roles that auxin has at a cellular level
1. Stimulaion of proton pumps in the cell membrane
- to pump H+ into the cellulose cell wall
- causes a drop in pH, essential for enzyme activation to loosen the wall
2. Influencing growth rat by changing pattern of gene expression
- it enhances expression of genes that code for wall loosening enzymes and expansin proteins
- once produced, these are secreted into the cell
- they are activated in presence of H+ ions to break certain bonds to loosen the wall
- allows cell to expand, making cell elongation possible
89
Totipotency of plant cells
Plant cells are totipotent
| - any plant cell can still differentiate into any plant tissue
90
What is a feature that distinguishes plants from animals?
Plant cells are totipotent
| - we can take any plant cell and w/ appropriate conditions and growth media, grow a completely new plant (a clone)
91
Micropropagation
- method used to mass produce specific plants
- involves use of tissue culture techniques
- plants are grown under controlled sterile conditions from meristematic tissue or somatic cells on nutrient media in vitro
92
Process of micropropagation
- it uses totipotent ability of plants
- some cells from apical meristem of shoot apex are taken to produce new plants identical to original (i.e. clones)
1. Tissue sample scraped from the plant
2. Tissue samples placed in agar growth medium containing nutrients and auxins
3. Samples develop into tiny plantlets
4. Plantlets planted into compost
93
Uses of micropropagation
- rapid increase in no. of plants of a new variety
- production of virus-free strains of existing varieties
- propagation of orchids and other rare species
94
Asexual reproduction in plants
- most plants can reproduce asexually or sexually
- roots, stems and leaves of a plant and vegetative structures are all used for asexual reproduction - because all plant cells are totipotent
95
Sexual reproduction of plants
- flowers are needed
- flowers are produced by shoot meristem
- flowers come in many forms and sizes
- flowers and their pollinators have co-evolved, so particular flowers are suited to a particular form of pollination
96
What is required for a plant to flower?
- its apical meristem (produces stem and leaves) has to be converted into a floral meristem
- it's from the floral meristem that all parts of the flower will be produced
97
What are the signals that trigger the change from apical meristem to floral meristem?
1. Internal factors: plant maturity and gibberellin availability in the plants
2. External factors: temperature and photoperiod
NB/
- conversion of apical meristem into a floral meristem to promote flowering involves change in gene expression in shoot apex
- switch to flowering is a response to length of light and dark periods (photoperiod) in many plants
98
Photoperiod
Relative length of day and night
| - main trigger for change from apical meristem to floral meristem
99
Flowering plants
- split into short-day plants and long-day plants
100
Short-day plants
- start to flower when the no. of daylight hours falls below a critical no.
- Pfr binds to an inhibiting factor to prevent flowering
- but, at the end of a long night, little Pfr remains, so inhibition fails and plant can bloom
101
Long-day plants
- bloom early in summer when the days are long
- lots of Pfr is produced during long days
- leaves enough Pfr at the end of the night to trigger flowering process
102
Phytochromes
- plants use phytochromes to determine relative length of day and night
- pigment is sensitive to light in red (660 nm) and far-red region (730 nm) of visible spectrum
- there are 2 inter-convertible forms of phytochrome: Pfr and Pr
- plant uses time taken for pigment to convert from far-red form (Pfr) to red form (Pr) as measure of length of daylight
103
Conversion of phytochrome from Pfr to Pr
1. Pr is converted to Pfr when it absorbs light with a wavelength of 660 nm (red)
2. Pfr is converted to Pr when it absorbs far red light (730 nm)
- sunlight has more red light (660 nm) than far-red light (730 nm), so during the day, most Pr is converted into Pfr
3. Pr is a more stable molecule than Pfr, so during the dark, Pfr is converted back to Pr
104
What stimulates flowering?
The length of nights (period of darkness)
- long-day plants = high level of Pfr stimulates flowering
- short-day plants = Pfr inhibits flowering
105
Pollination
transfer of pollen from anther of a flower to stigma of another (or same) flower
- allows fertilisation
106
Transporting pollen
- pollen is carried from anther to stigma by wind or animals
- Wind-pollinated plants include many important species of grass and cereals
- vast majority of flowering plants use animals as pollinators
107
Mutualistic relationship
A relationship where both parties benefit
| - most flowering plants use mutualistic relationships w/ pollinators in sexual reproduction
108
Fertilisation in plants
- pollen grain (male gamete) grows a tube that penetrates stigma and grows down into ovary, where egg cell (female gamete) is fertilised
- egg cell is located in the ovule
- once fertilisation takes place, ovule grows into a seed and ovary turns into a fruit
109
Why is seed dispersal important for survival?
- each plant produces many seeds
- seeds stand a better chance to survive if they're dispersed away from parent plant where they won't compete for nutrients, light and space - seed dispersal is important for success of plant reproduction and spread of plant species
110
Ways in which seeds are dispersed
- eaten by birds or other animals and expelled in faeces
- by wind (see Figure 2)
- by water
- attached to fur of passing animals
111
Pistil
- female part of flower
| - made of stigma, style and ovary containing ovule
112
Stigma
where pollen grains are deposited
113
Style
supports stigma and connects it to ovary; pollen tube grows down through this
114
Ovary
at base of style
- contains ovule
- in some species, develops into the fruit
115
Ovule
- contains female gamete - after fertilisation, develops into the seed
116
Stamen
male part of the flower
| - made up of the anther and filament
117
Anther
part of the stamen that produces pollen
118
Filament
supports the anther
119
Petal
modified leaves surrounding reproductive structures of flower
- often brightly coloured to attract pollinators
120
Sepal
forms a covering for bud
| - protects developing flower
121
Peduncle
- stem of the flower
122
Plant seed
A structure that has the potential to grow into a new plant
123
Structures within the seed
1. Embryo root (radicle)
2. Embryo shoot (plumule)
3. One or two cotyledons. - Monocotyledons have one embryo leaf
- Dicotyledons have two of embryo leaves
- cotyledons contain food reserves for developing embryo, usually as starch
4. Seed coat (testa)
In some seeds, there's an endosperm which acts as a food reserve
124
External seed structures
1. Hilum
- a scar on the seed from where it was attached to the ovary wall
2. Micropyle
- opening through which the pollen tube entered the ovule for fertilisation to occur
125
Factors affecting germination
- water
- oxygen
- suitable temperature
126
How does water affect germination?
- germination starts w/ uptake of water by dry seeds through imbibition
- this breaks seed's dormancy, softens testa and allows metabolic activities within seed to increase
127
How does oxygen affect germination?
straight after seed has been activated, seed starts to absorb oxygen for respiration
128
How does suitable temperature affect germination?
ensures that metabolic reactions can occur at an optimum rate, as they're all catalysed by enzymes
