Gymnosperms Flashcards
Features of seed plants
Evolution of the ovule and seed were key events for terrestrial plants
Female gametophyte (megagametophyte) is retained within the ovule on the mother sporophytes
Ovule develops into the seed
Seeds consist of a seed coat, an embryo (next sporophyte gen) and stored food
Seed replaced the spore as a unit of dispersal
Independence from water as a medium of transport of the sperm to the egg
All seed plants heterosporous
Megaspore develops in the ovule and the microspores develop into pollen
Pollen represents the male gametophyte (microgametophyte)
Gymnosperms differ from angiosperms
Gymno do not produce flowers or tree fruit, no herbaceous forms, production of cones, xylem does not contain vessels, phloem does not contain sieve tube elements, low biodiversity – fewer species
Conifers
Largest and most widespread group
50 genera and about 550 species (low bd)
Pines, spruce, firs, cedars, yew, redwoods, cypress, juniper, monkey puzzle, larch
Dominate many ecosystems – e.g alpine and boreal forests
Include some of the oldest and biggest organisms on the planet
Three native species in uk – scots pine, yew, juniper
More introduced and grown in forests, parks and gardens
Commercially important (timber industry)
Pine
Small to very large, evergreen trees with needle leaves borne on short shoots
Leaves produced in bundles up to 8, leaves usually live for 2-3 years but exceptionally for 30 or more years (bristlecone pine)
Leaves have thick cuticle and sunken stomata – suitable for growth under conditions where water is scarce or difficult to obtain
Life cycle of pine
- Is the mature sporophyte gen
- Separate male and females cones on the same trees (monoecious)
- Male cones produce pollen grains – found in clusters, usually lower part of tree
- Female cones produce ovules, megametophytes and seeds
- Seed production takes a long time – e.g 3 years from pollination to seed in scots pine
- Microspores develop into pollen, megaspores develop into megagametophyte
Male cones
- Small, clustered, usually lower part of tree
- Simple with central axis and macrosporophylls that bear microsporangia
- Microspore mother cells undergo meiosis to produce 4 haploid microspores
- Each microspore develops into a tiny microgametophyte (pollen grain) of 4 cells
- Pollen grains have 2 air bladders for buoyancy, a tube cell and a generative cell
Female cone
- On upper part of tree
- Larger, more complex with ovuliferous scales
- Megasporangium on ovuliferous scale contains megaspore mother cell which divides by meiosis to give 4 haploid megaspores
- Only one megaspore survives
- Free nuclear division inside megaspore gives rise to coenocytic cell which eventually becomes cellular megagametophyte
- Each megagametophyte contains up to 3 archegonia each with a single haploid egg
- The ovule is composed of the haploid megagametophyte embedded in the diploid megasporangium tissue (nucellus)
- The ovule (n megagametophyte in 2n tissue) is enveloped in an integument which has an opening called the micropyle at its apex
- Integument becomes seed coat after fertilisation
- Pollen enters via micropyle
- Pollination and fertilisation
- In spring, pollen grains are released form the male cones and carried in the wind to female cones
- At this time, female cone is small and in its first year
- A pollination drop exudes from the micropyle of the ovule
- Pollen is drawn in towards the nucellus as this evaporates
- Pollen germinates to produce a pollination tube, from the tube cell
- Pollen tube grows through the nucellus to the gg in the archegonium
- The generative cell in the pollen divides to produce 2 non-motile sperm
- Both sperm are released into the archegonium but only one units with the egg cell nucleus to produce the diploid zygote
- Each egg in each archegonium may be fertilised, but usually only one zygote develops into an embryo in an ovule
The seed
- 2 ovules on the upper surface of each ovuliferous scale of female cone
- Each one develops into a seed
- Seed protected by a seed coat which develops from diploid integument, derives from one original megasporangium
- The diploid embryo (new sporo gen) is inside seed
- Seed embedded in haploid megagametophyte tissue which serves as a store of food
- The embryo consists of a hypocotyl-root axis, an apical meristem, seed leaves (cotyledons) and a root cap
Seed dispersal of gymnosperms
In many species of pine, the seeds have wing-like appendages which aid in their dispersal away from the mother tree
Some are adapted for bird dispersal, particularly in association with jays (pinyon pine) or nutcrackers (white-bark pine)
Others are adapted for dispersal after fire
Closed-cone pines only open the scales on the female cones in response to intense heat of forest fires – e.g lodgepole pine, Monterey pine
Cycads
Small palm-like trees found in tropics and subtropics
None native to Britain but some can be grown here
Dioecious
Female produces very large cone
Male produces pollen and motile sperm
Beetles may be involved in pollination
May contain neurotoxins
Gingko
Only one representative – maidenhair tree
Deciduous tree
Diocious
Motile sperm
Survived in temple garden and holy places in China and Japan
Commonly grown in parks
Gnetophyta
Odd mixture of gymnosperms compromising just 3 living genera
Perhaps closest living ancestors of angiosperms
Gnetum – shrubby vine of tropical forests
Welwitschia ¬ - only found in Namib dessert, SW Africa
Ephedra – a genus of gymnosperm shrubs, typical of dry climates and sandy soils
