44. It is a pattern of reproduction occurring in the life cycles of many lower plants and some invertebrates, involving a regular alternation between two distinct forms. The generations are alternately sexual and asexual (as in ferns) or dioecious and parthenogenetic (as in some jellyfish).
45. Ferns and mosses reproduce by releasing millions of spores through the air. The spores are fed eggs and are spread through the environment by wind.
46. Gymnosperm reproduction differs from that of angiosperms in several ways. In angiosperms, the female gametophyte in the ovule exists in an enclosed structure, the ovary; in gymnosperms, the female gametophyte is present on exposed bracts of the female cone and is not enclosed in an ovary.
47. Unlike gymnosperms such as conifers and cycads, angiosperm's seeds are found in a flower. Angiosperm eggs are fertilized and develop into a seed in an ovary that is usually in a flower. The flowers of angiosperms have male or female reproductive organs.
48. All species are classified hierarchically. Related species are grouped into a genus; related genera into a family; related families into an order; related orders into a class; related classes into a phylum; and related phyla into a kingdom. Below, the most significant characteristics of the nine phyla of the kingdom Plantae are briefly considered. Bryophyta is a phylum with three classes, the largest of which is the mosses, with about 15,000 species. The gametophyte phase is dominant, and in mosses this is the familiar, small, green, leafy plant. Bryophytes do not have true leaves, stems, or roots, and they lack a vascular system for transporting food and water. They reproduce by making spores, and are mostly found in bogs or moist woodlands, so their sperm can swim through water to reach the eggs. Mosses are particularly prominent in the northern boreal forest and arctic and alpine tundra. The Lycopodophyta is a phylum with about 1,000 species. The sporophyte phase is dominant, and is the familiar, low-growing, green plant in many species which superficially resembles the branch of a pine. Their leaves are tiny structures, termed microphylls, and are arranged in whorls on the stem. The stems of lycopods and all subsequent phyla have vascular tissues for efficient transport of food and water. Like bryophytes, they reproduce by making spores, and are mostly found in wet areas so their sperm can swim to reach the eggs. Lycopods are most abundant in the tropics, although numerous species of Lycopodium (ground pine) grow in woodlands in the temperate zone.
