Plant Reproduction – Sexual Reproduction In sexual reproduction, sperm carried in the pollen form the male part of a flower fuses with the egg in the female part of the flower. SR gives the plant species the means to change with a changing environment. Every time SR occurs there is a recombining of genetic material. Plants receiving genes that enable them to adapt to the environment are more likely to survive to pass onto their offspring. Species evolve in order to survive. Birds, insects, bats and other animals and sometimes wind help in the pollination process, which is the transfer of pollen from the male to the female part of a plant. When the pollen of a plant pollinates a flower the same plant, it is called self-pollination. Cross-pollination is when pollen from one plant pollinates a flower on a different plant. Once the pollen lands on the stigma, it grows a thin pollen tube down the style to the ovary. The cell within the grain of pollen divides to form two sperm nuclei. These travel down the pollen tube to the embryo sac that holds the egg. Flowering plants have double fertilization. One occurs when one sperm fuses with the egg. The sperm carries genetic material from the male part of the flower and the eggs contain genetic material from the female part. When the sperm and the egg combine, a cell called the zygote develops. The other fertilization has the second sperm nuclei fusing with 2 nuclei in the embryo sac, it develops into the endosperm. The ovule of the flower becomes the seed. The offspring of genetically different parent is said to be a hybrid. The advantage to hybridizing is the best traits of each parent may be expressed in the offspring. Genetic information is stored in every cell of a plant or animal in long molecular chains made of DNA (Deoxyribonucleic Acid) Genes, segments of DNA, code for life processes and the appearance of a plant. Genes are arranged in a set of chromosomes. Normal cells contain a double set of chromosomes and are called diploid (2n). Reproductive cells have a single set of chromosomes and are haploid (1n). When fertilization occurs, the sperm and the egg both contribute to a single set of chromosomes. After fertilization, the ovary wall enlarges and forms the fruit. Seeds are formed in the fruit. The major parts of the seed include: cotyledon, seed coat, endosperm and the embryo. Seeds of a flowering plant have a seed coat that surrounds the embryo and endosperm and it prevents them from drying and physical injury. It also plays a role in determining when conditions for germination are right. Food in the form of starch and protein is stored in the seed for the embryo. Seeds are designed to wait for favorable conditions to begin growth, this may take years. One mechanism seeds have to help ensure survival is called stratification; this is where a seed must go through a period of cold temperatures before it will germinate. (Example: tulips) Another mechanism is scarification; this is breaking down of the seed coat. For this to happen, the seed must pass through the acid stomach of an animal or lay in the soil where microorganisms can eat away the seed coat. (Example: geranium) Environmental factors play key roles in seed germination. These include: water, oxygen, temperature and light. Water stimulates chemical reactions within the seed. Oxygen is important because starches stored in the seed are converted to energy through cellular respiration. Most plant seeds germinate at 60 to 80 degrees F. Seed quality refers to two factors, viability and vigor. Viability is the ability of a seed to germinate under optimal conditions. Vigor is the ability of a seed to germinate under different conditions but still produce healthy seedlings.