Lecture 7 Importance Of Seeds

Question 1

Importance of seeds

Answer 1

A spore must germinate immediately once shed or die whereas seeds wait for germination stimulation e.g. water swelling increasing chance of survival

2000 yr old dessicated Israeli seeds have been successfully germinated (Salton et al 2008)

Question 2

31800yr old plant tissue

Answer 2

From an arctic squirrel burrow has been cloned by in vitro tissue culture and micro-propogated ( Yashina et al 2012)

Question 3

Plants general evolutionary trend

Answer 3

Reduction in size/duration of haploid gametophyte phase
evolution of heterospory - 2 types of spore that generate 2 types of gametophyte
In seed plants haploid gametophytes develop partly/entirely dependently on/attached to sporophyte

Question 4

Wind pollinated plant

Answer 4

Example Rancaria 385MYA

Sporangial tip - stigma equivalent, wall must be dissolved to allow fertilisation

Megasporangium inside which megaspore and megagametophyte develop

Question 5

Reproduction in gymnosperms - heterospory

Answer 5

Sporophyte (2n)
Develops mega & microsporangium
Which each develop a spore mother cell (2n) which split by meiosis into 4 forming 4 mega spores and 4 microspores
One or more megaspores develop into a megagametophyte : megasporangium+ sporophytic integument =ovule (egg in embryo sac) retained in megasporangium (ovary)
and microspores develop into microgametophytes (pollen) which are released from the anther aka microsporangium

Question 6

Seed plants

Answer 6

Angiosperms

Gymnosperms: cycads, ginkos, gnetophytes, conifers

Question 7

Pollination

Answer 7

Gymnosperm pine pollen is winged for wind pollination

Angiosperm hibiscus pollen is spread by pollinators which is more effective than wind. It germinates by forming pollen tubules that grow down into stigma

Question 8

Seed development in gymnosperms

Answer 8

1) unfertlised female pinecone. Megaspore in megasporangium contains egg nucleus gamete (n.) There is an opening called a micropyle at bottom

2) In fertilisation pollen grain enters micropyle and develops a pollen tubule (germinates) penetrating the integument and releasing sperm nucleus (n) into egg

3) mature seed: diploid embryo (new sporophyte) surrounded by haploid female gametophyte nutritious tissue surrounded by seed coat (diploid parental sporophyte tissue)

Question 9

Seed plant characteristics

Answer 9

Reduced gametophytes - microscopic male & female gametophytes (n) nourished and protected by sporophyte

Heterospory - microspore - male gametophyte / megaspore - female gametophyte

Ovules- ovule (gymnosperm) integument female nutritional tissue (2n) contains megaspore (n) inside megasporangium (2n)

Pollen - makes water unnecessary for fertilisation

Seeds - survive better than unprotected spores can travel long distances

Question 10

Gymnosperms

Answer 10

Cycad e.g. Sago palm

Ginko - ginko biloba from meteozoic era - radiating veins on leaf

Gnetophytes - double fertilised 1 for embryo and one for tissue - ‘aryl’ “berry” modified seed

Conifers - cone bearers transient male pollen cones and long lasting female cones that harden and may take up to 2 years to develop. Female cones develop higher up the tree to reduce chances of self pollination increasing intercrossing.

When female cone is fully matured it opens releasing pine nuts or winged seeds in case of Scots pine.

Question 11

Pine tree life cycle

Answer 11

1) pine tree sporophyte

Female cone) Egg producing megastrobilus cone with megasporocyte integument and ovule produces functional megaspore with open pollen chamber aka micropyle

Male cone) Microstrobilus cone has microsporangium (2n) producing pollen microspores by meiosis

2) pollen(n) enters micropyle develops pollen tubule (germinates) and sperm nucleus releases into egg (n) nucleus - fertilisation (2n)

3) zygote develops a seed coat and becomes a scale of megastrobilus cone

4) megastrobilus cone matures producing winged seeds or pine nuts

5) seeds are released from cones and develop into pine trees (sporophytes)
Back to 1)

Question 12

cone types

Answer 12

Microstrobilus - male, pollen cone, herbaceous scales

Megastrobilus - female, seed cone, woody scales

Question 13

Soft cone conifers

Answer 13

Yew - Taxus baccata - seeds surrounded by flesh ‘aril’ which is fruit like and prompts bird dispersal
The red edible fruit-like aril is the only non toxic part of tree

Juniper- Junipers horizontalis - soft comes w/fleshy fused scales each w/single seed

Question 14

Plant table

Answer 14

Moss and non-vasc - Dom gametophyte, reduced sporophyte dependent on gametophyte for nutrition

Seedless vasc. - reduced gametophyte - independent, photosynthetic and free living , sporophyte dominant

Gymno/angiosperm seeded vasc - microscopic gametophyte Dom sporophyte provides nutrition. Gymno e.g. ovulate/pollen cone angio e.g. ovule and anther

Question 15

Summary

Answer 15

Heterospory (distinct male microspores and female megaspores evolve from homosporous ancestors)

Pollen increasingly independent of water

Seeds offer long-term protection for embryonic sporophyte

Gymnosperms once dominant plant life had naked seeds produced by scales of cones, pollen produced in strobilus - wind dispersed

Question 16

Trophic cascade

Answer 16

Apex predator impact on food chain biodiversity

Too many dragonfly larvae > dragonflies eat all pollinators reducing plant life

Fish population can control dragonfly larvae improving plant life