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Lesson Overview What is a Plant? Lesson Overview 22.1 What is a Plant? Lesson Overview What is a Plant? The Plant Kingdom Plants are classified as members of the kingdom Plantae. Plants are eukaryotes that have cell walls containing cellulose and carry out photosynthesis using chlorophyll a and b. Lesson Overview What is a Plant? What Plants Need All plants have the same basic needs: sunlight, a way to exchange gases with the surrounding air, water, and minerals. Lesson Overview What is a Plant? Sunlight Plants use the energy from sunlight to carry out photosynthesis. Leaves are typically broad and flat and are arranged on the stem so as to maximize light absorption. Lesson Overview What is a Plant? Gas Exchange Plants require oxygen to support cellular respiration, as well as carbon dioxide to carry out photosynthesis. Plants must exchange these gases with the atmosphere and the soil without losing excessive amounts of water through evaporation. Lesson Overview What is a Plant? Water and Minerals Land plants have evolved structures that limit water loss and speed the uptake of water from the ground. Minerals are nutrients in the soil that are needed for plant growth. Lesson Overview What is a Plant? Many plants have specialized tissues that carry water and nutrients upward from the soil and distribute the products of photosynthesis throughout the plant body. Lesson Overview What is a Plant? Origins in the Water The ancestors of today’s land plants were water-dwelling organisms similar to today’s green algae. Although not as large and complex as many plants, green algae have cell walls and photosynthetic pigments that are identical to those of plants. Green algae also have reproductive cycles that are similar to plants. Studies of the genomes of green algae suggest that they are so closely related to other plants that they should be considered part of the plant kingdom. Lesson Overview What is a Plant? The First Land Plants The greatest challenge that early land plants faced was obtaining water. They met this challenge by growing close to the ground in damp locations. Fossils suggest the first true plants were still dependent on water to complete their life cycles. One of the earliest fossil vascular plants was Cooksonia, shown here. Lesson Overview What is a Plant? The First Land Plants Several groups of plants evolved from the first land plants. One group developed into mosses. Another lineage gave rise to ferns, cone-bearing plants, and flowering plants. Lesson Overview What is a Plant? An Overview of the Plant Kingdom Botanists divide the plant kingdom into five major groups based on four important features: embryo formation, specialized waterconducting tissues, seeds, and flowers. Lesson Overview What is a Plant? An Overview of the Plant Kingdom Lesson Overview What is a Plant? The Plant Life Cycle The life cycle of land plants has two alternating phases, a diploid (2N) phase and a haploid (N) phase. The shift between the haploid phase and the diploid phase is known as the alternation of generations, as shown in the figure. Lesson Overview What is a Plant? The Plant Life Cycle The multicellular diploid phase is known as the sporophyte, or spore-producing plant. Lesson Overview What is a Plant? The Plant Life Cycle The multicellular haploid phase is known as the gametophyte, or gamete-producing plant. Lesson Overview What is a Plant? The Plant Life Cycle A sporophyte produces haploid spores through meiosis. These spores grow into multicellular structures called gametophytes. Lesson Overview What is a Plant? The Plant Life Cycle Each gametophyte produces reproductive cells called gametes— sperm and egg cells. During fertilization, a sperm and egg fuse with each other, producing a diploid zygote that develops into a new sporophyte. Lesson Overview What is a Plant? Trends in Plant Evolution An important trend in plant evolution is the reduction in size of the gametophyte and the increasing size of the sporophyte.