FRUITS (PS 16) & SEED STRUCTURE AND GERMINATION (PS17) Flashcards
State the meaning of FRUIT and SEED
Fruit: a fruit is formed from the OVARY after fertilisation.
SEED: a seed is formed from the OVULE after fertilisation
A fruit is formed from the ovary after fertilisation, the fruit contains the seeds which develops from the ovule after fertilisation.
State the function of fruits and seeds
FRUIT:
- Provides the developing seeds with protection, food and water
- aids dispersal and distribution of seeds
- presence of chemicals in the fruit imposes dormancy on the seeds prior to dispersal
SEEDS:
- Distribution and protection of the embryo
- may impose dormancy
- gives rise to new plants
Describe the means by which seeds are dispersed
Plus 2 plant examples for each except water - which needs only 1
WIND (wing)
- can often be dispersed hundreds
- often first plants to colonise a new land ie volcanic islands
- seeds can be very small/dust like
- have adapted ways to float on wind such as wings or parachute
Example WING Acer campestre (field maple) Fraxinus excelsior ( ash)
PARACHUTE
Example:
Taraxacum officinale (dandelion)
Clematis vitalba (old mans beard)
CENSER - pepper pot- like fruit , held up high on stiff stalks . contain seeds which are shaken out of perforations in the top when shaken / wind blows them. Example - Papaver rhoeas (poppy) Nigelle damascena (love-in-a-mist)
WATER
- many tropical plants have developed s Ed’s that float and are waterproof
- can travel great distances
-during spring tides they are left stranded at the top of beaches where they may germinate
- fresh water plants such as water lilies also use floating seeds
EXAMPLE:
Cocos nucifera (coconut)
EXPLOSIVE (dehiscent) self- dispersal
- rapid movement of the fruit wall, or of a seed appendage, that propels the seed away from the parent.
- most common is an explosive mechanism
EXAMPLES
Ulex europaeus (gorse)
As fruit dries, tension builds up until suddenly the fruit splits so quickly the seeds are thrown out with great force
Geranium Himalayense ‘Gravetye’
As the fruit dries the carpel suddenly splits from the base and curls upwards, throwing seeds out.
ANIMALS (attachment, scattering) external dispersal
- seed dispersed by chance, in soil on the feet of animals
- sticky fruit of mistletoe can prove irresistible to many birds, seeds are sticky and hairy, stick to birds beak then brushed off onto suitable
Tree where it may germinate and grow into a new plant
- some plants (goosegrass) have modified fruit and seeds with tiny hooks which catch in animal fur , then deposited far away from mother plant
EXAMPLE:
Viscum album (mistletoe)
Galium aparine (goosegrass)
ANIMALS (hoarding) -birds (jays) and squirrels, collect and bury large food stores of seeds, such as acorns, to sustain them through winter. If not retrieved they may germinate and produce new plants EXAMPLE Quercus Robur (oak) Corylus avellana (hazel)
ANIMALS (frugivory)
- plants that produce fruit that is succulent, sweet and brightly coloured
Black, blue, red and orange fruits attract mammals and birds
- seeds in the fruit are protected from damage when passing through the gut- have tough seed coats or stones
- many will not germinate unless they have passed through the animal first , where chemicals in the digestive tract often soften the seed coat
- released in feces
EXAMPLE
Ilex aquifolium (holly)
Rubus fruticosus (blackberry)
Draw a diagram of the non endospermic seed and endospermic seed,
Describe the internal and external structure of the seed and state the function of the various parts
Testa - this is the seed coat, protection for embryo, formed by outer layers of the ovule after fertilisation
Cotyledon - single cotyledon (seed leaf ) found in monocots, two cotyledons -seed leaf -found in dicotyledons. They occupy a large part of the seed. Function is to store starch/food/sugars. Necessary for keeping embryo alive & germination stage.
Embryo - embryo is the immature plant, consists of the radicle and plumule
Radicle - is the embryonic root system, is the first organ emerging from the seed. Anchors the plant to the soil and startscto take up water from the root hairs
Plumule - the apical part of the epicotyl, contains meristematic tissue.
Hypocotyl - part of the embryo below the cotyledons, bearing the radicle at its end.
Epicotyl - part of the embryo and seedling ABOVE The cotyledons that will develop into the shoot.
Endosperm - a food store found in seeds, holds sugars, starch, food which is needed to keep the embryonic plant alive, used during germination process
Hilum - the place on the seed marking the point of attachment to the ovary.
Micropyle - a pore in the seed through which water enters. Is where the pollen tube grew into the ovule during germination .
Examples to be studied include French bean (Phaseolus vulgaris) and broad bean (Vicia faba)
Describe ONE example of epigeal germination and 1 example of hypogeal germination it French bean (phaseolus vulgaris) and broad bean (vicia faba)
HYPOGEAL GERMINATION - balls below ground
example : (broad bean) Vicia faba
Found in many MONOCOTS
& some DIcots
Cotyledon remains BELOW the soil as the plumule is pushed above the ground by elongation of the epicotyl.
The cotyledons continue to supply food to the seedling until they are exhausted, then they shrivel and die below ground level
EPIGEAL GERMINATION - balls above ground
Example: Phaseolus vulgaris (French Bean)
In most DICOTS.
The hypocotyl elongates , pushing the cotyledons , which carry out photosynthesis for a short time before they shrivel and fall off, above the soil together with the plumule.
Describe process of germination
- seed takes up water through micropyle
- cotyledons absorb water and swell
- testa absorbs water, softened &tears as stretched by selling cotyledons
- Enzymes in cotyledons become active in presence of water & break down to release energy needed for cell division of the embryonic seed, starch broken down to glucose, and proteins to amino acids
- some of the glucose produced is used in respiration to provide energy for the growth of the embryo. At this stage the seed looses weight
- in presence of temperature, Enzymes speed up the metabolic reactions & organs such as Radicle and Plumule start to grow
- remainder of glucose and amino acids are used to build the new roots, stems, leaves and buds of embryo plant
- Radicle is first to grow out if the testa,. Anchors to the soil and grows lateral roots that start to absorb water from the soil
- the plumule stars to grow upright and emerges from the soil a few days after germination begins, and starts to photosynthesise. At this stage the plant starts to increase weight rapidly.
- germination stops when plant grows first TRUE leaves.
- new shoots and roots continue to be produced for the rest of the life if the plant using food supplied by photosynthesis
Explain seed viability
SEED VIABILITY
Seed is viable if it is healthy, and capable of germination when given suitable environmental conditions and when dormancy mechanisms have been overcome.
Length seed remains viable depends on storage conditions and varies with species.
(Large seeds such as Quercus robur are short lived- 6 months, majority of small and medium seeds such as lettuce -Lactuca sativum remain viable for a number of years
Describe germination and essential requirements
The dormant seed carries out little biological activity in its dehydrated state , but continues to respire at a v low rate.
In this way it can remain dormant for long periods without using up food supply, as most is required for germination
ESSENTIAL REQUIREMENTS FOR GERMINATION
1- water/moisture - soil condition needs to be at optimum level, moist NOT wet.
2- OXYGEN
3- APPROPRIATE TEMPERATURE- for the species. Most seed require 18-21c, but some germinate best at 10-15c, (celery/lettuce) cannot exceed 24c
4- LIGHT/NO LIGHT
Most species will germinate in light or dark, but some such as Primula Vulgaris (primrose) also require LIGHT to germinate so are left uncovered
Describe seed Dormancy
A seed may be viable, but dormant.
Seed is in dormancy when it is viable, supplied with suitable levels of warmth, oxygen, & moisture and yet still dies not germinate.
It requires tenancy to be broken before it will germinate
Some seeds are dormant when shed from plant , others develop dormancy conditions for successful germination are not present as the seed is shed.
Types of dormancy
List methods of SCARIFICATION & STRATIFICATION
Physical (hard seed coat) dormancy
May present a physical barrier to germination need to be broken artificially by SCARIFICATION methods
1- soaking - to soak testa from inside example : Ipomea
2- hot water - some need to be in very hot water and left to soak for 24hrs at 46-48c before testa will soften eg: Caburnum
3- nicking / chipping - chip or nick tyectrsta on seeds with hard tests to allow water to enter
Eg: Lathynus odoratus (check spelling)
4- Abrasion - rubbed testa with sandpaper until thin, small seeds can be placed in a hat lined with sand poa per and tumbled. Eg: Lupins arboreus
MORPHOLOGICAL (immature embryo ) dormancy
Some seeds are immature and only have few specialised cells when fruit ripens. Seeds will not germinate after fruit has dropped until embryo is fully developed. Needs aid if food stores in endosperm - process is called ‘after ripening’ eg Fraxinus excelsior (Ash)
PHYSIOLOGICAL (chemical) dormancy
Chemicals present in seed coat inhibit growth in the interior of seed or tissues of the fruit - may also induce dormancy . Need to wash out inhibitors so germination can occur.
Can be broken artificially by STRATIFICATION
Stratification methods
1- warm and cold treatments (fridge then warm) to mimic winter /spring
Ex Crataegus monogyna, Rosa canina
2- placing seed in plastic bag containing damp sand or vermiculite , place in fridge , check for germination, take out and plant Sorbus aucuparia (Rowan)
