Double Fertilisation + Seed development

How is pollen tube formed?

• pollen grain lands on compatible stigma + germination occurs

• pollen grain generative nucleus divided by mitosis into 2 male gametes (+ pollen tube nucleus)

• pollen tube grows out of grain (through gap in cell wall “pit”) down style and into embryo through-micropyle (gap in integuments)

What is role of pollen tube nucleus?

controls growth of pollen tube

How does first fertilisation take place?

• pollen tube nucleus disintegrates + top of tube opens releasing 2 male gametes in embryo sac

• one male nuclei fuses with female nuclei forming diploid zygote

What happens in second fertilisation?

• second male gamete fuses with 2 polar nuclei forming triploid primary endosperm nucleus

• endosperm nucleus generates endosperm tissue (by mitosis) + provides nutrition to embryo

What happens to the ovule after ferilisation?

• develops into seed as diploid zygote divides by mitosis to become an embryo

• embryo contains pumule (shoot) + radicle (root) + 1/2 cotelydons

What happens to triploid endosperm after fertlisation?

• its nucleus divides by mitosis to form endosperm tissue

• food storage for embryo

What happens to the integuments + ovary wall after fertilisation?

• outer integuments dry out, harden + become waterproof with lignin deposists to become testa

• micropyle remains as pore in testa

• ovary wall develops into fruit and encloses the seeds

Define Fruit

structure developing from ovary wall containing 1 or more seeds

Define Seed

structure developing from fertilised ovule containing embryo + food store in testa

Why is maize grain fruit and not seed?

testa + ovary wall have fused together so outer layer is fruit

What are adaptations of seed for survival

• protection - tough outer wall testa protects physical damage + pathogens + water loss

• food store - endosperm to allow survival before it can photosynthesise

• dormancy - until conditions met

  • temp 5-30C for optimum temp for enzymes to speed activity + faster diffusion rate

  • water to mobilise enzymes & transport substances + swell cotyledon

  • oxygen for aerobic respiration to synthesise ATP for metabolism

• dispersed far from parent plant to avoid competition

Describe seed dispersal methods

• wind

• water

• rolling - away from parent