NOTES: CH 38 – Plant Reproduction

*Modifications in reproduction were key adaptations enabling plants to spread into a variety of terrestrial habitats.

**Our focus in this chapter: * Water has been replaced by wind and animals as a means for spreading gametes. * Embryos are protected in SEEDS. Vegetative (asexual) reproduction is used for propagation of some plants in a variety of environments. **Our focus in this chapter: SEXUAL REPRODUCTION!

 GAMETOPHYTE: reproductive structure of plants -where haploid GAMETES (n) are produced (sperm and egg cells) -gametes fuse to form a ZYGOTE (2n) that develops into a multicellular plant

…ALTERNATION OF GENERATIONS!

FLOWERS: -the reproductive structure of angiosperms -four sets of modified leaves: 1) Sepals 2) Petals 3) Stamens: -anther, filament 4) Carpel (a.k.a. pistil): -stigma, style, ovary

FLOWERS:  STAMENS and CARPELS (PISTILS) are the reproductive parts of the flower.

FLOWERS:  Female gametes develop in EMBRYO SACS, which contain the EGG CELLS  this occurs at the base of the carpel /pistil, inside the OVARY

 Male gametes develop in the stamen inside POLLEN GRAINS  this occurs at the stamen tips within chambers of the ANTHERS

FLOWER VARIATIONS:  Complete flower = flower with sepals, petals, stamens, carpels  Incomplete flower = flower missing one or more of these parts (e.g. most grasses do not have petals on their flowers)

FLOWER VARIATIONS:  Perfect flower = flower having both stamens and carpels (a.k.a. bisexual)  Imperfect flower = flower that is either STAMINATE (male) or CARPELLATE (female) – a unisex flower. (a.k.a. unisexual)

 Monoecious = “one house”; plants with either perfect flowers or with both staminate (male) and carpellate (female) flowers on the same plant (ex: star lily, rose, dandelion) Dioecious = “two houses”; plants having staminate and carpellate flowers on separate plants (ex: willow, juniper)

Development of the Male Gametophyte (POLLEN): POLLEN GRAIN = the immature male gametophyte; develops within the anthers of stamens in an angiosperm  extremely durable; tough coat to prevent biodegradation

 At the time of pollination: A pollen gain consists of 2 cells (both haploid): -GENERATIVE CELL: divides to form 2 sperm cells -TUBE CELL: produces the pollen tube; contains the generative cell

Development of the Female Gametophyte (EMBRYO SAC) ● Female gametes are contained in the EMBRYO SAC

Embryo Sac: egg cell (OVUM) at one end ovum flanked by 2 syngerids (help to attract sperm cells)  two nuclei in center (“polar nuclei”)

POLLINATION: -occurs when wind- or animal-born pollen released from the anthers lands on the STIGMA at the tip of the carpel

POLLINATION POLLINATION = the placement of pollen onto the stigma of a carpel Methods of Pollination / spread of pollen: • wind • animals / insects • self-pollinate (most plants cross-pollinate)

To prevent self-pollination (and thus ensuring sperm and egg from different plants meet), some plants: -have stamens and carpels that mature at different times; -have structural arrangement of flower parts that reduces the chance of pollen getting transferred from stamen to carpel; -are SELF-INCOMPATIBLE (a biochemical block that prevents the pollen grain from developing and fertilizing the egg)

 SELF-INCOMPATIBILITY: the rejection of pollen from the same, or closely related, plant by the stigma

after POLLINATION… -pollination is followed by this sequence: 1) a pollen tube grows from the pollen grain, down the carpel, into the embryo sac; 2) sperm are discharged from the pollen grain resulting in fertilization of the egg; 3) the zygote develops into an embryo; 4) as the embryo grows, the ovule surrounding it develops into a SEED; 5) while seed formation is taking place, the entire ovary is developing into a FRUIT, which will contain one or more seeds.

DOUBLE FERTILIZATION:  When a compatible pollen grain lands on a stigma of an angiosperm, double fertilization occurs. DOUBLE FERTILIZATION = the union of two sperm cells with two cells of the embryo sac

DOUBLE FERTILIZATION: Steps: 1) pollen grain adheres to stigma, germinates, and extends a pollen tube toward the ovary; 2) generative cell divides (mitosis) to form 2 sperm; 3) directed by a chemical attractant, pollen tube enters and discharges its 2 sperm nuclei into the embryo sac; 4) 1 sperm unites with the egg  ZYGOTE; 5) other sperm combines with the 2 polar nuclei to form a 3n (triploid) nucleus in the large central cell of the embryo sac.

this central cell will give rise to the ENDOSPERM (a food storing tissue for the seed/embryo) **after double fertilization, each ovule develops into a seed and the ovary into a fruit surrounding the seed(s).

ENDOSPERM: • rich in nutrients (which it provides to the developing embryo)

Structure of the mature seed: in mature seeds, the embryo is quiescent (dormant) until germination • the seed dehydrates until its water content is only 5-15% by weight. • the embryo is surrounded by endosperm, enlarged cotyledons, or both. • the seed coat is formed from the outer layers / integuments of the ovule.

Dicot seed structures: -cotyledons = embryonic leaves -hypocotyl = embryonic axis below cotyledons -radicle = embryonic root -epicotyl = embryonic axis above cotyledons

*Seeds are dispersed from the source plant when fruits are moved about by the wind or animals. *Seeds deposited in soil of the proper conditions (moisture, nutrients), will GERMINATE.

FRUITS: -ovary develops into fruit (adapted for seed dispersal)

*Fruits ripen about the time seeds are becoming fully developed Ripening fruits are characterized by: 1) fruit becomes softer as a result of enzymes digesting the cell wall components; 2) colors usually change and fruit becomes sweeter as organic acids or starch are converted to sugar; 3) these changes produce an edible fruit which entices animals to feed, thus dispersing the seeds!