NOTES: CH 38 - Plant Reproduction *Modifications in reproduction were key adaptations enabling plants to spread into a variety of . * Water has been replaced by and as a means for spreading gametes. * Embryos are protected in SEEDS. * Vegetative (asexual) reproduction for propagation of some plants in a variety of environments. Our focus in this chapter will be…SEXUAL REPRODUCTION ALTERNATION OF GENERATIONS the angiosperm ( ) life cycle includes ALTERNATION OF GENERATIONS – alternate with SPOROPHYTE (2n): -the recognizable plant most familiar to us -produces haploid spores by -spores undergo mitosis and develop into a multicellular male or female GAMETOPHPYTES (n): - -gametes fuse to form a ( ) that develops into a multicellular sporophyte… …ALTERNATION OF GENERATIONS! FLOWERS: -the reproductive structure of angiosperm sporophytes -four sets of modified leaves: 1) 3) 2) 4) STAMENS and CARPELS (PISTILS) contain the sporangia and are the reproductive parts of the flower. Female gametophytes develop in carpel sporangia as , which contain the this occurs inside the (at the base of the carpel, surrounded by ) Male gametophytes develop in the stamen sporangia as this occurs at the stamen tips within chambers of the POLLINATION: -occurs when wind- or animal-born pollen released from the anthers lands on the STIGMA at the tip of the carpel -this 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 resulting in fertilization of the eggs; 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. *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. *the embryo starts to grow and develops into a new . *after flowers are produced by the sporophyte, a new generation of GAMETOPHYTES develop and the life cycle continues! FLOWER VARIATIONS: Complete flower = Incomplete flower = (e.g. most grasses do not have petals on their flowers) Perfect flower = Imperfect flower = Monoecious = plants with both staminate and carpellate flowers on the same plant (ex: ) Dioecious = plants having staminate and carpellate flowers on separate plants (ex: ) 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 Steps in pollen grain formation: 1) diploid microsporocytes undergo meiosis to form 4 haploid MICROSPORES; 2) haploid microspore nucleus undergoes mitosis, giving rise to a GENERATIVE CELL and a TUBE CELL; 3) microspore wall thickens and becomes sculptured into a species-specific pattern. Development of the Female Gametophyte (EMBRYO SAC) OVULE = structure that forms within the plant ovary Steps in embryo sac formation: 1) a diploid megasporocyte in each ovule grows and goes through meiosis to form 4 haploid MEGASPORES (only one survives) 2) the remaining megaspore grows and its nucleus undergoes three mitotic divisions, forming one large cell with eight haploid nuclei. 3) membranes partition this into a multicellular EMBRYO SAC. Embryo Sac: (diagram of embryo sac below): egg cell at one end; flanked by two other cells at opposite end are three antipodal cells the other two nuclei (“polar nuclei”) share the cytoplasm of the large central cell POLLINATION = Methods of Pollination / spread of pollen: • • • (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 ; -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: DOUBLE FERTILIZATION: When a compatible pollen grain lands on a stigma of an angiosperm, double fertilization occurs. 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 ; 5) other sperm combines with the 2 polar nuclei to form a 3n nucleus in the large central cell of the embryo sac. this central cell will give rise to the (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) • in monocots, stores nutrients that can be used b y the seedling after germination • in dicots, food reserves of the endosperm are exported to the cotyledons, thus mature seeds have no endosperm 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 integuments of the ovule. Dicot seed structures: (diagram of mature dicot seed): -cotyledons = -hypocotyl = embryonic axis below cotyledons -radicle = -epicotyl = embryonic axis above cotyledons FRUITS: -ovary develops into fruit (adapted for ) -fruits can be classified as: • SIMPLE FRUITS: derived from a single ovary (ex: ) • AGGREGATE FRUITS: derived from a single flower with several separate carpels (ex: ) • MULTIPLE FRUITS: derived from separate tightly clustered flowers (ex: ) *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 as organic acids or starch are converted to sugar; 3) these changes produce an edible fruit which entices animals to feed, thus !