Plants reproduce in a number of ways. The most primitive members of the plant kingdom, green algae, reproduce asexually (offspring only have one parent) by fission (splitting). Other plants can reproduce by the asexual process called fragmentation (breaking apart). The remainder of plants reproduce sexually (offspring with parents from each sex) by releasing gametes (reproductive cells). Male reproductive cells are called sperm and female reproductive cells are called eggs. Next we look at some of the different types of reproductive organs used by plants.
Spores are reproductive cells that are able to develop into a new individual without fusing with another cell (in contrast, seeds are formed when male and female gametes join together). The microscopic spore cell has everything it needs to grow into a multicellular plant, and under favorable conditions the cell will divide and grow. In plants, spores can be found on non-seed bearing plants including green algae, mosses and ferns. Often, the spores are located on the underside of the leaves and are carried to a new area by wind or rain. Spores, unlike seeds, are less likely to be eaten by animals, but they are at risk of being consumed by bacteria and fungi.
Flowers are what make flowering plants flowering plants! Flowers provide the mechanisms for sperm to find eggs, thus leading to fertilization and development of seeds.
The outermost parts of the flower which typically surround the rest of the flower are green structures called sepals. Inside of the sepals are the petals, which are typically colourful. Petals on flowers are also modified leaves which serve a similar function as bracts. Next are the stamens, each of which contains a filament topped by pollen-producing cells. The innermost part of the flower is the carpel, which contains the ovary (where the eggs are located). The pollen from another flower must enter the ovary and fertilize the ovule in order for a seed to start developing.
Seeds are embryonic (immature) plants that are enclosed in protective seed coats. Seeds often contain stored nutrients in the endosperm, which is rich in oil, starch and protein. Seeds can be dispersed (sent to new places) by wind (can be lighter or structured to be air-borne), by water (can float so they can drift down rivers) or by animals (can have barbs to catch on animal’s fur or can be eaten and dispersed through an animal's droppings). Seeds are a more evolutionarily advanced form of plant reproduction than spores and are present in both gymnosperms and angiosperms.
In gymnosperms the seeds are covered by the scales of cones, while in angiosperms the seed becomes covered with a fleshy or hard fruit.
Cones are the parts of conifers which contain reproductive structures. Female cones (A) produce ovules and male cones (B), which are much smaller and not as visible, produce pollen (which is visible as a yellowish powder).
The ovule, once fertilized by pollen, becomes a seed. In most conifer species, the male and female cones appear on the same plant, with the female cones on higher branches and the male cones on lower branches. This is to improve cross-fertilization (being fertilized with the pollen of another plant) and avoid self-fertilization (being fertilized with its own pollen). The tiny, lightweight pollen of one conifer is more likely to be carried by the wind to female cones of a different conifer than upward into its own female cones.
Fruit is the result of the maturation of one or many flowers, and is therefore only found in angiosperms. In cooking, a fruit is any sweet-tasting plant product. However in plant science (botany), a fruit is considered the ripened ovary of a seed-bearing plant which contains its seeds. In flowering plants, as a seed develops, its ovary begins to ripen and the ovary wall, called the pericarp, becomes fleshy (as in berries and apples) or forms a hard outer covering (as in nuts). Botanically, fruit also include beans, corn kernels, tomatoes, wheat grains, pumpkins, peanuts, cucumbers and rice!
Earth Rangers provides a list of ten different types of pollinators, their favourite types of flowers, and how the pick up and distribute pollen.
CrashCourse video (10:23 min.) covering the different types of reproduction in plants. Angiosperms (flowering plants) are discussed starting at 5:33 min.
Video (3:55 min.) from Peekaboo Kidz on the parts of a flower and the basic concepts of pollination and fertilization are introduced. Suitable for a younger audience, but applicable for older grades as well.