more than 380,000 species, of which more than two-thirds

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1 The plant world contains more than 380,000 species, of which more than two-thirds are green plants. From the most complex flowering plants to single-cell sea algae, plants present a surprising diversity of organization, way of life, and mode of reproduction the result of more than of evolution. 3 billion years

2 8 Plant classification Growing complexity 10 Plant cells The building blocks of plants 12 Algae The first plants to grow on Earth 14 Fungi Organisms lacking chlorophyll 16 Lichens Symbiosis of an alga and a fungus 18 Mosses Plants of the humid zones 20 Ferns Undergrowth plants 22 Conifers The first trees

3 Plant classification Growing complexity Within the plant world, green plants form the largest group, with about 278,000 different species. The four main subdivisions of the green plants group are mosses, ferns, conifers, and flowering plants. The last are by far the most numerous, with almost 234,000 species. Fungi are traditionally presented alongside plants, although they are not part of the group. They form a separate, homogeneous group, composed of species that draw their nutrition from other organisms. On the other hand, algae do not all come from a common ancestor and form a heterogeneous group. Brown algae comprise a separate group, while red and green algae are part of the plants group. THE FAMILY TREE OF PLANTS Humans have always wanted to classify the living species around them. Early classifications, based on the physical appearance of organisms, were refined through study of the internal organization of individuals, then through genetic analysis. Genes are characteristics specific to an individual and its species, and are coded within its cells. Comparison of the genes belonging to each species reveals the relationships between these species. The resulting classification allows us to trace the evolution of life, from the most primitive to the most complex organisms. Living beings are classified in lineages. Plants are divided among three distinct lineages : the lineage of fungi, the lineage to which brown algae belong, and the lineage of plants. Each lineage is subdivided into phyla, and each phylum contains organisms with a common ancestor. The main plant phyla are the bryophytes, filicophytes, coniferophytes, and angiosperms. PLANTS about 284,000 species BROWN ALGAE about 2,000 species FUNGI 100,800 species Today, there are almost 278,000 species of green plants. There are about 6,000 species of red algae. The group of terrestrial plants comprises almost 270,000 species. Vascular plants have sap-conducting vessels. Green algae form a heterogeneous group of about 8,000 species. The bryophytes (15,000 species, mainly mosses) are primitive plants that do not have sap-conducting vessels. Spermatophytes (234,700 species) reproduce through seeds. The filicophytes (9,500 species, mainly ferns) have sap-conducting vessels and roots, but they do not have seeds. 8 The coniferophytes (600 species, all conifers except the gingko) are resinous trees whose seeds are not protected in a fruit. Angiosperms, or flowering plants (234,000 species), are the most highly evolved and most numerous. They have true flowers, and their seeds are protected in fruits.

4 NAMING A PLANT A phylum may contain a number of classes, which are, in turn, divided into orders, genera, and species. To identify a plant, there is no need to list all of the groups to which it belongs in the classification system. In general, the genus and species, given in Latin, are enough. For instance, a daisy is known by the Latin name Leucanthemum vulgare. In cases where there are several varieties of a single species, it is necessary to add the variety name to avoid any ambiguity. ANGIOSPERM PHYLUM dicotyledon class Dicotyledons are flowering plants, either woody (walnut) or herbaceous (begonia). This class comprises about 50 orders, divided into 230 families containing some 200,000 species. monocotyledon class The monocotyledons are flowering plants most of which are herbaceous (wheat); a few are arborescent (palm tree). This class is divided into four orders containing more than 80 families. The Asterales order contains herbaceous flowering plants of the temperate regions. The largest family in the Asterales order is the Asteraceae. Leucanthemum is one of the 1,528 genera in the Asteraceae family. The Leucanthemum genus contains eight species, including the species vulgare. Carboniferous ( MY) blue-green algae terrestrial plants Precambrian (4, MY) ferns conifers THREE BILLION YEARS OF EVOLUTION The first algae appeared in the oceans during the Precambrian, more than 3 billion years ago; the first green algae appeared 1.5 billion years ago. About 420 million years ago, certain green algae adapted to terrestrial life. They evolved to form mosses, then vascular plants without leaves or roots. The first ferns grew at the beginning of the Carboniferous. By the end of this period, the first conifers were growing, and they had their golden age in the Jurassic, between 280 and 145 million years ago. Flowering plants first appeared 30 million years later, in the Cretaceous. Their colors and shapes soon transformed the landscape of Earth. flowering plants Jurassic MY : millions of years ( MY) Cretaceous ( MY) 9

5 Plant cells The building blocks of plants Although they have many things in common, plant cells are very different from animal cells. They have a rigid wall, unlike animal cells, and they contain extra organelles : large vacuoles and chloroplasts. This cellular apparatus enables plants to make their own food from elements drawn from the environment. On average, a plant cell measures 0.2 mm in diameter, four times the size of an animal cell. THE STRUCTURE OF THE PLANT CELL The plant cell is the basic building block of the plant. It is surrounded by a cell membrane covered with a rigid wall, characteristic of plants, which gives the cell its shape. The inside of the plant cell is filled with a viscous liquid, the cytoplasm, in which float the small components essential to the cell s life, the organelles. Among the organelles are the nucleus, the ribosomes, the vacuole, the mitochondria, and the chloroplasts. The organelles fulfill various vital functions, such as nutrition, respiration, and making proteins that will be used to help the plant grow. The wall is rigid and relatively impermeable. It limits deformation of the cell and protects it from dehydration. The semipermeable cell membrane controls the entry and exit of small molecules such as minerals. walls of neighboring cells Fats are stored in the form of lipid droplets. The vacuole is a large vesicle that contains reserves of water, minerals, and sugars. Cell organelles float in a clear, gelatinous substance, the cytoplasm. Ribosomes are small globular organelles attached to the elongated vesicles of the endoplasmic reticulum or floating free in the cytoplasm. They produce proteins. The pockets of the Golgi apparatus transport the proteins made by the ribosomes within the cell or to the outside. Mitochondria are responsible for cellular respiration, which generates the energy the cell needs to be active. 10

6 A CLOSE LOOK AT THE ORGANIZATION OF A PLANT Plants are composed of different organs, such as leaves. Each organ is made up of tissues formed of groups of cells of different types. The inside of the leaf is composed of several types of cells. organ tissue plant Chloroplasts contain chlorophyll, a green pigment that absorbs luminous energy and thus enables organic matter (sugars) to be made. This process is called photosynthesis. The sugars produced by photosynthesis are stored in the form of starch grains. Starch is also stored in small organelles called amyloplasts. Channels called plasmodesmata run through the cell membrane and wall. They play a role in exchanges between two neighboring cells. The nucleus contains the plant s characteristics, encoded in chromosomes. It controls all cell activity, including the making of protein. The nucleolus is a small spherical body located inside the nucleus. It is involved in the making of protein. The endoplasmic reticulum is formed of a group of vesicles that synthesize protein. It is also involved in transporting substances within the cell, and between the cell and the surrounding environment. 11

7 Algae The first plants to grow on Earth More than 25,000 species of algae live in aquatic environments, both freshwater and salt water, and in some wetlands. Single-cell algae are microscopic; many live in suspension in freshwater or salt water and are a component of plankton. Multicell algae are made of a number of cells connected in the form of filaments or blades, and may be several meters long. In the sea, some algae float on the surface of the water. Others live attached to rocks on the shore. Algae are rarely found more than 30 m below the surface of the water; below this depth there is not enough light for them to feed themselves. THE STRUCTURE OF AN ALGA Algae are among the simplest species in the plant world. They are thallophytes: an alga is formed of an assemblage of cells that is ramified to various degrees, the thallus, which has no branches, roots, or leaves. The receptacle, a bulbous part at the end of a frond, contains the alga s reproductive organs. The divisions of the thallus, called fronds, are shaped like leaf blades of different widths. thallus The midrib forms a projection that runs along the thallus or the fronds of some algae. Some algae have small pouches filled with gas, aerocysts, which enable them to float. Rockweed is a brown alga. It lives attached to rocks that are uncovered at low tide. The thallus is attached to its substrate by a small holdfast called a hapteron. 12

8 GREEN, BROWN, AND RED ALGAE Algae are colored differently depending on the types of pigments they contain; the main types of algae are green, red, and brown. However, all algae have a green pigment, chlorophyll, which absorbs luminous energy and enables organic matter to be produced by photosynthesis. Algae are thus capable of making their own food: they are autotrophic. Sweet kelp is a brown alga that grows on the shore of the North Atlantic. It clings to rocks immersed in relatively calm waters. hapteron substrate Dilsea is a red alga with a thick, fleshy thallus. It lives attached to rocks in deep water and is rarely above the surface. Green algae often grow in freshwater. With 8,000 species, they are the most numerous. REPRODUCTION OF AN ALGA Algae may reproduce by asexual reproduction simple fragmentation of the thallus or production of cells called spores, which are released into the water and germinate to form new algae identical to the parent alga. Most algae can also reproduce by sexual reproduction. The receptacles of the alga Q have structures that produce gametes W. At maturity, these organs release male and female gametes E into the water. The male gametes have flagella and are mobile in the water R. Fertilization the fusion of a male gamete and a female gamete leads to the formation of a zygote T. This single cell multiplies, gradually forming a new alga Y. The receptacles contain the alga s reproductive organs. SEXUAL REPRODUCTIVE CYCLE OF AN ALGA 0W The structures that produce male and female gametes are tucked into the hollows of receptacles. section of a receptacle 0E male gamete 0Q female gamete Male gametes use their flagella to swim to the female gametes. female gamete new alga 0R 0Y The zygote is a single cell resulting from fertilization. 0T Fertilization occurs when a male gamete fuses with a female gamete. 13