Reproduction in Plants (part 2): Pollination

1. Pollination

2. Pollination Definition: • The transfer of pollen grains from the anther to the stigma

3. Self-pollination vs Cross-pollination • Self pollination – transfer of pollen grains to stigma of same flower or different flower of the same plant • Cross pollination - pollen grains transferred to flower in another plant of the same kind

4. Pollination • Usually effected by insects OR wind • Characteristics of insect-pollinated and wind-pollinated flowers differ Insect pollination

5. In the table below list the possible differences between both wind and insect pollinated flowers Characteristic Insect-pollinated flower 1) Wind-pollinated flower 2) 3) 4) 5) 6)

6. Characteristic 1) Flowers Insect-pollinated flower Wind-pollinated flower Usually large, brightly coloured and scented to attract insects.If flower small → form an influorescence Usually small, dull-coloured and scentless (unattractive to insects) 2) Nectar Often present to attract insects Usually absent 3) Scent Flowers are fragrant or sweet- smelling Flowers do not have scent 4) Pollen Fairly abundant; large, sticky and heavy, rough surfaces to cling onto insects’ bodies Abundant; small, smooth, dry, light → buoyant & easily blown about by wind 5) Stamens May not be pendulous Usually have long, slender filaments that sway in the slightest wind → pollen grains easily shaken out from anthers 6) Stigmas Usually small and compact, not feathery and do not protrude; sticky so that pollen grains setting on them are not easily displaced Protrude + large & feathery → large s.a. to catch pollen floating in air 7) Nectar guides (marking that guide insects to nectar) May be present absent

7. Advantages of self-pollination • Only one parent is required • Offspring inherits its genes from parent plant. Hence beneficial qualities are more likely to be passed down to the offspring • It does not depend on external factors e.g. insects or wind for pollination • Anthers are close to the stigmas of the same flower hence there is a high possibility that self – pollination will occur • Less pollen and energy is wasted is self- pollination compared to cross pollination

8. Disadvantages of self-pollination • Less varieties of offspring are produced as the offspring’s genes are similar to those of the parent plant, therefore the species is less adapted to changes in the environment • Continued self-pollination may lead to offspring becoming weaker, smaller and less resistant to diseases

9. Features favouring self-pollination • Flowers are bisexual with anthers and stigmas maturing at the same time • Stigma is situated directly below the anthers • In certain plants with bisexual flowers, some flowers never open (cleistogamous flowers). Only self-pollination can occur in these flowers

10. ***Advantages of cross-pollination • Offspring may have inherited beneficial qualities from both parents • Abundant and more viable seeds tend to be produced (i.e. seeds are capable of surviving longer before germination) • More varieties of offspring can be produced (greater genetic variation) → increases chance of survival of species to changes in the environment

11. Disadvantages of cross pollination • Two parent plants are required • Depends on external factors e.g. insects or wind for pollination • Lower probability that cross pollination will occur compared to self-pollination (because it involves transfer of pollen grains from the anther of one plant to the stigma of another plant) • More energy and pollen is wasted as compared to self-pollination

12. Adaptations of plants to favour cross- pollination (genetic variability) • Dioecious plants bear either male or female flowers so that self-pollination is impossible e.g. paw paw • Maturation of anthers and stigmas at different times (bisexual flowers) e.g. custard apple • Stigmas of bisexual flowers may be situated a distance away from the anthers (less chances for self-pollination)

13. Structure and Pollination of an Insect-pollinated Flower (pg. 303-304) e.g. Clitoria

14. Characteristics of Clitoria • Butterfly-shaped flowers • Complete • Bisexual • Bilaterally- symmetrical

15. Calyx • Is green • 5 sepals (at free end of calyx) • 2 leaves that enclose the base of the calyx = epicalyx Corolla • Brightly coloured with 5 petals of differing shape and size • Consists of i) large purple petal ii) 2 lateral wing petals iii) 2 small yellowish-green keel petals

16. Androecium Definition: The male (stamen) parts of the flower • 10 stamens with long filaments • 9 stamens fused together with 1 free stamen • Nectar secreted collects at bottom of stamen trough • Can only be reached by an insect with long proboscis e.g. bee or butterfly

17. Gynoecium Definition: The female portion of the flower consisting of the ovary, stigma (a sticky surface to which pollen grains attach and germinate) and style (which connects the stigma to the ovary) • Consists of single carpel • Ovary long and narrow with a single row of ovules • Style is long, curved structure and is hairy (situated below stigma)

18. Pollination mechanism in Clitoria • Can be 1) insect-pollinated OR 2) self-pollinated (because flower is inverted) Insect pollination (cross-pollination): - bees force its way b/w 2 wing petals and move in to collect nectar - back forces keel petals upwards exposing stigma and anthers which brush the hairy back of the insect - pollen grains on the insect’s back (from another flower the insect had visited earlier) adhere to the sticky stigma - when insect leaves, keel springs back to original position and enclose the stamens and stigma

19. Structure and Pollination of an Wind-pollinated Flower (pg. 305-307) e.g. Grass flowers (Ischaemum muticum)

21. Characteristics of Ischaemum muticum • Wind-pollinated flowers • Usually small dull-coloured flowers (scentless and without nectar), massed in influorescences • Influorescence consists of short stalks bearing flowers (in pairs) • Each pair of flower is enclosed and protected by bracts (leaf-like structures)→ forming a spikelet (occurs in pairs) • The lower spikelet is sessile while the upper one is stalked and smaller • Base of spikelet is a pair of glumes (non-flowering bracts that protect the 2 flowers in a spikelet)

22. Characteristics of Ischaemum muticum • Lower flower is unisexual - consists only of 3 stamens with long filaments • Upper flower is bisexual – consists of an ovary + 2 long feathery stigmas, 3 stamens + 2 lodicules at base of ovary • When lodicules swell, they force the 2 flowering bracts slightly apart so that stigmas and anthers can emerge

23. Adaptations of Ischaemum to wind pollination • Mature stamens have long and pendulous filaments • Filaments protrude out of bracts, expose mature anthers to wind • Delicate filaments sway in the slightest breeze • Dust-like pollen sheken free and carried away by wind • Mature stigmas do not hand freely but project out of the bracts (as they are large + feathery → large s.a. to receive pollen floating around)