Plants are divided into four broad categories based on their mode of reproduction. Mosses and ferns, the two lowest categories in terms of evolution, reproduce using spores. As plants evolved, more complex reproductive strategies developed, most notably the development of the seed. Gymnosperms, such as evergreen trees, fertilize seeds on their cones. Angiosperms, or flowering plants, have the most complex and effective reproductive strategies, allowing them to dominate the plant kingdom.
Spores and Sporangium
Ferns differ from flowering plants and trees because they do not produce seeds. Instead, ferns reproduce using spores. The sporangium is the structure responsible for producing and releasing spores. Sporangium form in many different ways, sometimes appearing as brown "fruitdots" on the undersides of leaves. Others grow as slender stalks up from the plant. When the sporangia burst, the wind carries spores to new ground, where they form reproductive structures that, if fertilized, will grow into a new fern.
Pollen often calls to mind the misery of spring allergies but, to the plant, serves an essential reproductive function. Pollen contains the male sex cells and is necessary to fertilize a seed. Pollen is produced in microsporangia. In gymnosperms, the wind carries pollen to fertilize seeds formed on pine cones. In flowering plants, a variety of pollination methods exist. The wind plays a role, but insects and birds--attracted to the bright blooms and sweet nectar of the flower--also transport pollen to the female structures located inside of flowers.
Stamens arise as delicate threads, usually from the center of a flower, and contain the structures that produce pollen. Stamens consist of a filament topped by a larger node, called the anther, which contains the microsporangia. Pollen forms as yellow-colored dust on the anthers where it can easily be transported by pollinating insects and birds.
The pistil is a structure within a flowering plant that contains female sex cells. The pistil contains one or more tubes called carpels, each leading to an ovary. When pollen lands on a carpal or is delivered there by a pollinating insect, it forms a thin pollen tube that reaches down to the ovary and fertilizes it. Within the ovary, multiple ovules eventually form into seeds. In some plants, the ovary swells into fruit, which when consumed by an animal, effectively transports the fertilized seeds to new locations.
When the female sex cells in a plant are successfully fertilized by the male sex cells found in pollen, the plant produces seeds. Seeds contain an embryonic plant in addition to a material called endosperm that includes the nutrients the young plant will need to germinate. Seeds are easily portable by a variety of means, increasing plant diversity and accounting for the success of seed-bearing plants over mosses and fern.