GENERAL CHARACTERISTICS • Enclose their seed in masses of tissue = fruit • Fruit protects and aids in the distribution of seeds • Their xylem is more specialized and transports water and minerals more efficiently • Found in nearly every habitat
Why Sucessful • Have evolved mechanisms for dispersal of both pollen and seeds • Pollen Dispersal – co-evolution of flowers that provide nectar which increases the probability of successful pollination • Seed Dispersal- co-evolution for fruits covering seeds which increases distance of seed dispersal from the parent plant
THE TWO SUBCLASSES MONOCOTS AND DICOTS • MONOCOTS include corn, wheat, lilies, daffodils, orchids, and palms • DICOTS include roses, clover, tomatoes, oaks, and daisies • There are several differences between these two groups:
Most are terrestrial and all lack locomotion. This poses several problems. • Gametes are delicate single cells. For two plants to cross fertilize, there must be a mechanism for the two gametes to reach each other safely. • There must also be a mechanism to disperse their offspring far enough away from the parent so that they do not have to compete with the parent for light, water, and soil minerals. The functions of the flower solve both of these problems.
The Flower and Its Pollination • In angiosperms, meiosis in the sporophyte generation produces two kinds of spores. • microspores • which develop in the microsporangium and • which will germinate and develop into the male gametophyte generation and • megaspores • which develop in the megasporangium and • which will develop into the female gametophyte generation.
TYPES Both types of sporangia are formed in flowers. • In most angiosperms, the flowers are perfect: each has both microsporangia and megasporangia. • Some angiosperms are imperfect, having either microsporangia or megasporangia but not both. • Monoecious plants have both types of imperfect flower on the same plant. • Dioecious plants have imperfect flowers on separate plants; that is, some plants are male, some female. Examples include willows, poplars, and the date palm.
Parts of the Flower • stamens in which the microsporangia form • carpels in which the megasporangia form.
Stamens • Each stamen consists of a • lobed anther, containing the microsporangia and supported by a thin filament • Meiosis of the diploid microspore mother cells in the anther produces four haploid microspores. Each of these develops into a two-celled pollen grain.
Carpels • Carpels consist of a • stigma, usually mounted at the tip of a • style with an • ovary at the base. • Often the entire whorl of carpels is fused into a single pistil. • The megasporangia, called ovules, develop within the ovary.
Pollination • When a pollen grain reaches the stigma, it germinates into a pollen tube. The generative nucleus divides by mitosis forming 2 sperm nuclei. These, along with the tube nucleus, migrate down the pollen tube as it grows through the style and into the ovule chamber. • The pollen tube with its contents makes up the mature male gametophyte generation.
Double fertilization • The pollen tube enters the ovule through the micropyle and ruptures. • One sperm nucleus fuses with the egg forming the diploid zygote. • The other sperm nucleus fuses with the polar nuclei forming the endosperm nucleus. Most angiosperms have two polar nuclei so the endosperm is triploid (3n). • The tube nucleus disintegrates.
Self-incompatibility • Most angiosperms have mechanisms by which they avoid self-fertilization.
Fruits • Fruits are a development of the ovary wall and sometimes other flower parts as well. • Fruits promote the dispersal of their content of seeds in a variety of ways. • Wind. The maple "key" and dandelion parachute are examples. • Water. Many aquatic angiosperms and shore dwellers (e.g., the coconut palm) have floating fruits that are carried by water currents to new locations. • Hitchhikers. The cocklebur and sticktights achieve dispersal of their seeds by sticking to the coat (or clothing) of a passing animal. • Edible fruits. Nuts and berries entice animals to eat them. Buried and forgotten (nuts) or passing through their g.i. tract unharmed (berries), the seeds may end up some distance away from the parent plant. • Mechanical. Some fruits, as they dry, open explosively expelling their seeds. The pods of many legumes (e.g., wisteria) do this.