REPRODUCTION IN PLANTS
Sexual reproduction in plants occurs when a detached reproductive cell unites with another reproductive cell. Unlike animals, plants do not have to have separate male and female sexes, in many plants both sexes are located on the same species. In asexual reproduction, the cell, tissue, or organ develops directly into a new organism. Sexual reproduction allows combinations of different genetic material to be introduced into a new individual (this is the method by which humans reproduce and provides for genetic rearrangement and assortment.) In asexual reproduction an exact copy is made from the mother cell, therefore, the genetic material remains the same. Vascular plants are more complicated than non-vascular plants. The development of complex tissues and organs in vascular plants to aid in the movement of water and food, created the need for different strategies to reproduce.
ASEXUAL REPRODUCTION OF NONVASCULAR PLANTS.
Unicellular algae reproduce by the simplest type of reproduction which consists of cell division. In simple cell division, two daughter cells are produced from one mother cell. Some algae have asexual spores which produce specialized reproductive cells that are capable by themselves of producing new plants by cell division and growth.
ASEXUAL REPRODUCTION OF VASCULAR PLANTS.
Vegetative reproduction involves no special reproductive cells. One of the most common types of vegetative propagation occurs in plants with horizontal stems growing either above ground (runners or stolons) or underground (rhizomes). Several plants propagate vegetatively by tubers or the thickened, fleshy ends of rhizomes. Some plants have long slender stems that develop roots when they touch the ground. Bulbs such as those of onions form large buds that grow. Corms are the short, fleshy vertical underground stems have more stem tissue and smaller scale leaves than a bulb. Some plants reproduce by having horizontal roots while others have leaves that will start to ground. Some plants can propagate by cutting either stems or tubers into pieces that will grow. There are many different types of asexually reproducing vascular plants.
SEXUAL REPRODUCTION OF NONVASCULAR PLANTS.
Although not common, many small algae will reproduce sexually by forming specialized sex cells. This usually happens when the asexual cycle has terminated its cycle. (This is called alteration of generations.) Some spores are also formed by asexual means.
SEXUAL REPRODUCTION OF VASCULAR PLANTS.
Trees reproduce sexually through seeds (female) and pollen (male). Gymnosperms produce a naked seed whereas angiosperms (flowering plants) produce a true seed.
Gymnosperms are the non-flowering seed plants such as cedar, pine, redwood, hemlock, and firs. Gymnosperms are woody plants that bear "naked seeds." They are called naked because their seeds develop exposed on the upper surfaces of cone scales, such as in pine cones. A pollen grain is carried by wind currents to the appropriate "egg" where the growth of the pollen tubes through this tissue brings the sperm to the egg. Gymnosperms are usually of large size with much secondary growth, the leaves are usually evergreen needles or scales.
Angiosperms have flowers and bear seeds enclosed in a protective covering called a fruit. Angiosperms are the dominant types of plants today. Angiosperms are further divided into monocots and dicots. Monocots have one seed leaf. Dicots have two seed leafs. There are at least 250,000 species of angiosperms ranging from small flowers to enormous wood trees. Pollination is accomplished by wind, insects, and other animals. The male part is the pollen grain, and the female part is the ovary. The ovary goes through meiosis to produce an "egg", which is them fertilized by the "sperm" carried by the pollen. The sperm of the male part travels down the pollen tube in the style. Two sperm enter the micropyle of the ovary. After the process of mitosis, it turns into a seed with an embryo. The seed may be inside a fruit.