seeds and fruit Flashcards
what is a fruit
= seed bearing structure in angiosperms that is formed from the ovary after flowering
- dry fruits, believed to have evolved first (maybe just better preserved in the fossil record), spread by physical forces (ejection, adherence, wind etc)
- fleshy fruits, often consumed and seeds discarded or dispersed after ingestion
epizoochory
= dispersal by adherence to animal surface
endozoochory
= seeds eaten and passed through animals digestive tract for dispersal
- seeds often only germinate once they have passed through, stomach acids needed to break down outer layer
- faeces nutrient rich, so ideal for a seed to be deposited in for germination
ecological and evolutionary importance of fruits
- majority of angiosperms animal dispersed
- evolved competing to attract frugivores
- birds are main frugivores
- major factor in primate and fruit bat evolution
- 4% angiosperms evolved to attract ants, fleshy structures attached to seeds so ants can drag underground, some not consumed so can germinate
- elephants disperse many tropical plants
other dispersal mechanisms
- fall or scatter e.g. Rhabydopsis, annual so less of a need for dispersal (perennial plants tend to need to disperse further to reduce competition)
- ‘sea heart’ bean of Caribbean liana (and coconuts) is buoyant, so can cross sea to colonise other islands, can survive in saline conditions
development of carpel into fruit
- ovule develops into seed
- ovary wall develops into pericarp, outer structure of fruit, can be fleshy or hard
types of hard, dry pericarps
- pods
- grains
- nuts
- samaras
- achenes
- capsules
pods
e.g. peas
- multiple individual ovules develop into seeds in hard pericarp
- pericarp dries and dies, pod splits open, releasing mature pea seeds (hard and dry)
- annual, so seeds sit on soil over winter
grains
- germ/embryo and triploid endosperm surrounded by hard pericarp/bran
- poaceae = grass family
- wheat, maize, rice, oats, barley
- anthropochory
- most consumed and cultivated crops
anthropochory
human mediated dispersal
wheat production
- white flower from endosperm (mainly carbohydrate)
- wholegrain flour includes bran/pericarp and germ/embryo (some protein and fibre)
- historical reason for white flour, fatty acids in germ taste rancid when oxidised in storage, reduces shelf life, makes flour ‘heavy’
wheat dough from grain endosperm
- endosperm = starch in protein matrix
- bonding of glutenin and gliadin proteins (gluten) forms viscoelastic dough
nuts
- hard thick pericarp
- for storage, can remain in ground for years before germination
- synzoochory
synzoochory
seeds deliberately carried and stored by animals (jays, squirrels etc)
samara
- modified pericarp to produce samaras
- flat, elongated, lightweight aerodynamic structures for wind dispersal
- anemochory
- no nutritional content to attract animals
- e.g. ash, maple keys
achene
- single seed per carpal, very thin pericarp
- separation between pericarp and seed coat, single attachment point at base
- e.g. buttercups, dandelions, sunflowers
achenes, buttercups
- multiple carpels with individual ovaries develop into distinct seed structures
achenes, dandelions
- separate florets, each petal attached to individual carpels, achenes ripen from separate florets
- fruit have pappus for wind dispersal
achenes, sunflowers
‘hearts’ easy to extract from black pericarp
hydrochory
= water dispersal
capsules
- hard pericarp makes outer capsule, dry dehiscent fruit
- multiple individual seeds within capsule with hard seed coats
- e.g. yellow flag iris, opium poppies, cotton
dehiscence
= fruit splits open at maturity to release seeds
capsule, yellow flag iris
- 3 carpels, fuse to form 3-chambered ovary that splits along the seams (dehiscence)
- seeds have buoyancy tissues for hydrochory
capsule, opium poppies
- ovary forms hardened structure with multiple carpels inside
- stigmatic disk at top creates ‘salt shaker’ dispersal mechanism
- opioids in latex of capsule
capsule, cotton
- capsule splits to release seed
- cotton fibres = single celled hairs on seed (epizoochory, anemochory)
anemochory
= wind dispersal
fleshy fruits
- ovary wall develops into pericarp with three structures, exocarp, mesocarp (usually flesh), endocarp
- berry = entire fleshy pericarp
tomatoes
- berry
- exocarp and mesocarp fleshy
- locular cavities = chambers containing seeds
- placenta = thick, fleshy tissue in centre of tomato that extends from central axis to locular cavities
- attachment site for seeds and provides nutrients from mother plant to developing seeds
fused carpals
- multiple carpels that have fused together over evolutionary time
- appears as one floral structure but split into different locules inside (chambers in ovary) surrounding placenta
- e.g. citrus, pomegranate, coffee and cocoa
fused carpals, citrus
- leathery rind exocarp
- segments are carpals
- pith is mesocarp
- placenta in centre, seeds
- juicy tissue is vesicles of endocarp cells
fused carpals, pomegranate
- forms from inferior ovary (beneath male parts), residual stamens on top
- leathery exocarp, fleshy mesocarp
fused carpals, cocoa and coffee
- outer exo/meso/endocarp discarded
- seeds (beans) then process (dried, roasted etc)
drupes
- fleshy exocarp and mesocarp with seed encased in hard lignefied endocarp
e.g. peach - woody endocarp makes seed unpalatable and protects from digestive enzymes
compound fruits
- aggregate fruit from flower with many carpals or from many flowers
- evolved to increase number of seeds and fruitlets
- e.g. raspberry, each fruitlet develops from one of many carpals in a flower
- e.g. pineapple, each segment develops from carpel of one flower
accessory or false fruits
- fleshy tissues not developed from ovary
- developed from receptacle, derived from swelling of base of flower where all floral parts attach
- e.g. strawberries, fruit is achenes on surface
- e.g. apples, outer surface is receptacle, ‘core’ surrounding seeds is exo/meso/endocarp
- calyx at bottom of apple and top of strawberry remnants of stamen, styles, sepals
