14.4 The Life Cycle of Flowering Plants Biology 1001 November 25, 2005

Alternation of Generations Figure 29.5!! Angiosperms, like all plants, have an alternation of generations type of life cycle  The two generations are heteromorphic  The gametophytes highly are reduced and dependent on the sporophytes  The gametes and spores are non-motile

Flower Structure The flower is an angiosperm structure specialized for sexual reproduction It is a specialized shoot with up to four rings of modified leaves called floral organs: sepals, petals, stamens, and carpels  Sepals and petals are sterile organs  Stamens, the male reproductive organs, consist of an anther and a filament  Carpels, the female reproductive organs, consist of the stigma, the style, and the ovary Figure 30.7!!

Flower Variety

Figure 38.2!,!! – An Overview of Angiosperm Reproduction

Figure 30.10!,!! - The life cycle of an angiosperm

Development of the Gametophytes The flowers on the sporophyte produce microspores and megaspores that form male and female gametophytes The male gametophytes are the pollen grains, which develop within microsporangia in the anther The female gametophyte is the embryo sac, and it develops within the megasporangium within the ovule, which develops in the ovary Each pollen grain has two haploid cells, a generative cell that divides to produce two sperm, and a tube cell that produces a pollen tube The embryo sac consists of 8 haploid nuclei partitioned into 7 cells: the egg, two synergids, three antipodal cells, and two polar nuclei

Gametophyte Development Figure 38.4

Pollination and Fertilization Pollination is the transfer of pollen from the anther to the stigma of the carpal After pollination, the pollen tube grows through the style and into the ovary and the generative cell divides into the two sperm cells, which are discharged near the embryo sac Pollination precedes and is necessary for fertilization to occur Fertilization is the union or syngamy of the egg and sperm In angiosperms, a process called double fertilization occurs  One sperm fertilizes the egg forming the diploid zygote and the other combines with the two polar nuclei to form the triploid endosperm, which develops into a nutrient supply for the seed After fertilization, the ovule matures into the seed

The Co-evolution of Flowers & Pollinators Co-evolution refers to reciprocal evolutionary adaptations of two interacting species Animals interact with plants by helping to disperse their pollen and seeds, meanwhile receiving a benefit by eating nectar, seeds, and fruits of the plant Wind and water are also means of pollen and fruit dispersal Figure 30.13! Bee Pollinators Hummingbird Pollinators Bat Pollinators