Epiphytic and parasitic plants

Epiphytic and parasitic plants grow on other living plants. Typically, epiphytic plants are not rooted in the soil; instead, they live above ground level on the stems and branches of other plants. Epiphytes obtain water from trapped rainwater and from moisture in the air, and minerals from organic matter that has accumulated on the surface of the plant on which they are growing. Like other green plants, epiphytes produce their food by photosynthesis. Epiphytes include tropical orchids and bromeliads (air plants), and some mosses that live in temperate regions. Parasitic plants obtain all their nutrient requirements from the host plants on which they grow. The parasites produce haustoria, rootlike organs that penetrate the stem or roots of the host and grow inward to merge with the host’s vascular tissue, from which the parasite extracts water, minerals, and manufactured nutrients. As they have no need to produce their own food, parasitic plants lack chlorophyll, the green photosynthetic pigment, and they have no foliage leaves. Partial parasitic plants (e.g., mistletoe) obtain water and minerals from the host plant but have green leaves and stems and are therefore able to produce their own food by photosynthesis.

Carnivorous plants

 Carnivorous plants feed on insects and other small animals, in addition to producing food in their leaves by photosynthesis. The nutrients absorbed from trapped insects enable carnivorous plants to
thrive in acid, boggy soils that lack essential minerals, especially nitrates, where most other plants could not survive. All carnivorous plants have some leaves modified as traps; many use bright colors
and scented nectar to attract prey; and most use enzymes to digest the prey. There are three types of traps. Pitcher plants, such as the monkey cup and cobra lily, have leaves modified as pitcher-shaped pitfall traps, half-filled with water; once lured inside the mouth of the trap, insects lose their footing on the slippery surface, fall into the liquid, and either decompose or are digested. Venus fly traps use a spring-trap mechanism; when an insect touches trigger hairs on the inner surfaces of the leaves, the two lobes of the trap snap shut. Butterworts and sundews entangle prey by sticky droplets on the leaf surface, while the edges of the leaves slowly curl over to envelop and digest the prey.

Wetland plants

Wetland plants grow submerged in water, either partially (e.g., water hyacinth) or completely (e.g.,
pond weeds), and show various adaptations to this habitat. Typically, there are numerous air spaces inside the stems, leaves, and roots; these aid gas exchange and buoyancy. Submerged parts generally have no cuticle (waterproof covering), enabling the plants to absorb minerals and gases directly from the water; in addition, being supported by the water, they need little of the supportive tissue found in land plants. Stomata, the gas exchange pores, are absent from plants that are completely submerged; in partially submerged plants with floating leaves (e.g., water lilies), stomata are found on the upper
leaf surfaces, where they cannot be flooded.

Dryland plants

Dryland plants (XEROPHYTES) are able to survive in unfavorable habitats. All are found in places where little water is available; some live in high temperatures that cause excessive loss of water from the leaves. Xerophytes show a number of adaptations to dry conditions; these include reduced leaf area, rolled leaves, sunken stomata, hairs, spines, and thick cuticles. One group, succulent plants, stores water in specially enlarged spongy tissues found in leaves, roots, or stems. Leaf succulents have enlarged, fleshy, water-storing leaves. Root succulents have a large, underground water-storage organ with short-lived stems and leaves above ground. Stem succulents are represented by the cacti (family Cactaceae). Cacti stems are fleshy, green, and photosynthetic; they are typically ribbed or
covered by tubercles in rows, with leaves being reduced to spines or entirely absent.

Vegetative reproduction

Vegetative reproduction: Many plants can propagate themsel ves by vegetative reproduction. In this process, part of a plant separates off, takes root, and grows into a new plant. Vegetative reproduction is a type of asexual reproduction; that is, it involves only one parent, and there is no fusion of gametes (sex cells). Plants use various structures to reproduce vegetatively. Some plants use underground storage organs. Such organs include rhizomes (horizontal, underground stems), the
branches of which produce new plants; bulbs (swollen leaf bases) and corms (swollen stems), which produce daughter bulbs or corms that separate off from the parent; and stem tubers (thickened underground stems) and root tubers (swollen adventitious roots), which also separate off from the parent. Other propagative structures include runners and stolons, creeping horizontal stems that take root and produce new plants; bulbils, small bulbs that develop on the stem or in the place of flowers, and then drop off and grow into new plants; and adventitious buds, miniature plants that form on leaf margins before dropping to the ground and growing into mature plants.

Germination

Germination is the growth of seeds into seedlings. It starts when seeds become active below ground, and ends when the first foliage leaves appear above ground. A seed consists of an embryo and
its food supply, surrounded by a testa (seed coat). The embryo is made up of one or two cotyledons (seed leaves) attached to a central axis. The upper part of the axis consists of an epicotyl, which has a
plumule (embryonic shoot) at its tip. The lower part of the axis consists of a hypocotyl and a radicle (embryonic root). After dispersal from the parent plant, the seeds dehydrate and enter a period of dormancy. Following this dormant period, germination begins, provided that the seeds have enough water, oxygen, warmth, and, in some cases, light. In the first stages of germination, the seed takes in water; the embryo starts to use its food supply; and the radicle swells, breaks through the testa, and grows downward. Germination then proceeds in one of two ways, depending on the type of seed. In epigeal germination, the hypocotyl lengthens, pulling the plumule and its protective cotyledons out of the soil. In hypogeal germination, the cotyledons remain below ground and the epicotyl lengthens, pushing the plumule upward.

Dry fruits

Dry fruits have a hard,dry pericarp (fruit wall) around their seeds unlike succulent fruits, which have fleshy pericarps (see pp. 148-149). Dry fruits are divided into three types: dehiscent, in which the pericarp splits open to release the seeds; indehiscent, which do not split open; and schizocarpic, in which the fruit splits but the seeds are not exposed. Dehiscent dry fruits include capsules (e.g., love-in-a-mist), follicles (e.g., delphinium), legumes (e.g., pea), and siliquas (e.g., honesty). Typically, the
seeds of dehiscent fruits are dispersed by the wind. Indehiscent dry fruits include nuts (e.g., sweet chestnut), nutlets (e.g., goosegrass), achenes (e.g., strawberry), caryopses (e.g., wheat), samaras (e.g., elm), and cypselas (e.g., dandelion). Some indehiscent dry fruits are dispersed by the wind, assisted by “wings” (e.g., elm) or “parachutes” (e.g., dandelion); others (e.g., goosegrass) have hooked pericarps to aid dispersal on animals’ fur. Schizocarpic dry fruits include cremocarps (e.g., hogweed), and double samaras (e.g., sycamoremaple); these are dispersed by the wind.

Succulent fruits

Succulent fruits:  a fruit is a fully developed and ripened ovary (seed-producing part of a plant’s female reproductive organs). Fruits may be succulent or dry. Succulent fruits are fleshy and brightly colored, making them attractive to animals, which eat them and so disperse the seeds away from the parent plant. The wall (pericarp) of a succulent fruit has three layers: an outer exocarp, a middle mesocarp, and an inner endocarp. These three layers vary in thickness and texture in different types of fruits and may blend into each other. Succulent fruits can be classed as simple (derived from one ovary) or compound (derived from several ovaries). Simple succulent fruits include berries, which typically have many seeds, and drupes, which typically have a single stone or pit (e.g., cherry and peach). Compound succulent fruits include aggregate fruits, which are formed from many ovaries in one flower, and multiple fruits, which develop from the ovaries of many flowers. Some fruits, known as false fruits or pseudocarps, develop from parts of the flower in addition to the ovaries. For example, the flesh of the apple is formed from the receptacle (the upper end of the flower stalk).

Fertilization

Fertilization is the fusion of male and female gametes (sex cells) to produce a zygote (embryo). Following pollination, the pollen grains that contain the male gametes are on the stigma, some distance from the female gamete (ovum) inside the ovule. To enable the gametes to meet, the pollen grain germinates and produces a pollen tube, which grows down and enters the embryo sac (the inner
part of the ovule that contains the ovum). Two male gametes, traveling at the tip of the pollen tube, enter the embryo sac. One gamete fuses with the ovum to produce a zygote that will develop into an embryo plant. The other male gamete fuses with two polar nuclei to produce the endosperm, which acts as a food supply for the developing embryo. Fertilization also initiates other changes: the integument (outer part of ovule) forms a testa (seed coat) around the embryo and endosperm; the petals fall off; the stigma and style wither; and the ovary wall forms a layer (called the pericarp) around the seed. Together, the pericarp and seed form the fruit, which may be succulent or dry.
In some species (e.g., blackberry), apomixis can occur: the seed develops without fertilization of
the ovum by a male gamete but endosperm formation and fruit development take place as in other species.

Pollination

Pollination is the transfer of pollen(which contains the male sex cells) from an anther (part of the male reproductive organ) to a stigma (part of the female reproductive organ). This process precedes fertilization. Pollination may occur within the same flower (self-pollination), or between flowers on separate plants of the same species (cross-pollination). In most plants, pollination is carried out either
by insects (entomophilous pollination) or by the wind (anemophilous pollination). Less commonly,
birds, bats, or water are the agents of pollination. Insect-pollinated flowers are typically brightly
colored, scented, and produce nectar, on which insects feed. Such flowers also tend to have patterns that are visible only in ultraviolet light, which many insects can see but which humans cannot. These features attract insects, which become covered with the sticky or hooked pollen grains when they visit one flower, and then transfer the pollen to the next flower they visit. Wind-pollinated flowers are
generally small, relatively inconspicuous, and unscented. They produce large quantities of light pollen grains that are easily blown by the wind to other flowers.

Flowers

Flowers are the sites of sexual reproduction in flowering plants. Their component parts are arranged
in whorls around the receptacle (tip of the flower stalk). The sepals (collectively called the calyx) are outermost; typically small and green, they protect the developing flower. The petals (collectively called the corolla) are typically large and brightly colored; they are found inside the sepals. In monocotyledonous flowers (see pp. 126-127), sepals and petals are indistinguishable; individually they are called tepals (collectively called the perianth). The petals surround the male and female reproductive structures (androecium and gynoecium). The androecium consists of stamens (male organs); each stamen is made up of a filament (stalk) and anther. The gynoecium has one or more carpels (female organs); each carpel consists of an ovary, style, and stigma. Some flowers (e.g., lily) occur singly on a pedicel (flower stalk); others (e.g., elder, sunflower) are arranged in a group (inflorescence)on a peduncle (inflorescence stalk).

Photosynthesis

Photosynthesis is the process by which plants make their food using sunlight, water, and carbon dioxide. It takes place inside special structures in leaf cells called chloroplasts. The chloroplasts contain chlorophyll, a green pigment that absorbs energy from sunlight. During photosynthesis, the absorbed energy is used to join together carbon dioxide and water to form the sugar glucose, which is the energy source for the whole plant; oxygen, a waste product, is released into the air. Leaves are the main sites of photosynthesis, and have various adaptations for that purpose: flat laminae (blades)
provide a large surface for absorbing sunlight; stomata (pores) in the lower surface of the laminae allow gases (carbon dioxide and oxygen) to pass into and out of the leaves; and an extensive network of veins brings water into the leaves and transports the glucose produced by photosynthesis to the rest of the plant.

Leaves

Leaves are the main sites of photosynthesis and transpiration (water loss by evaporation) in plants. A typical leaf consists of a thin, flat lamina (blade) supported by a network of veins; a petiole (leaf stalk); and a leaf base, where the petiole joins the stem. Leaves can be classified as simple, in which
the lamina is a single unit, or compound, in which the lamina is divided into separate leaflets. Compound leaves may be pinnate, with pinnae (leaflets) on both sides of a rachis (main axis), or palmate, with leaflets arising from a single point at the tip of the petiole. Leaves can be classified further by the overall shape of the lamina, and by the shape of the lamina’s apex, margin, and base.

Stems

The stem is the main supportive part of a plant that grows above ground. Stems bear leaves (organs of photosynthesis), which grow at nodes; buds (shoots covered by protective scales), which grow at the stem tip (apical or terminal buds) and in the angle between a leaf and the stem (axillary or lateral buds); and flowers (reproductive structures). The stem forms part of the plant’s transport system: xylem tissue in the stem transports water and minerals from the roots to the aerial parts of the plant, and phloem tissue transports nutrients manufactured in the leaves to other parts of the plant. Stem tissues are also used for storing water and food. Herbaceous (non-woody) stems have an outer protective epidermis covering a cortex that consists mainly of parenchyma (packing tissue) but also has some collenchyma (supporting tissue). The vascular tissue of such stems is arranged in bundles, each of which consists of xylem, phloem, and sclerenchyma (strengthening tissue). Woody stems have an outer protective layer of tough bark, which is perforated with lenticels (pores) to allow gas exchange. Inside the bark is a ring of secondary phloem, which surrounds an inner core of secondary xylem.

Roots

Roots are the underground parts of plants. They have three main functions. First, they anchor the plant in the soil. Second, they absorb water and minerals from the spaces between soil particles; the roots’ absorptive properties are increased by root hairs, which grow behind the root tip, allowing maximum uptake of vital substances. Third, the root is part of the plant’s transport system: xylem carries water and minerals from the roots to the stem and leaves, and phloem carries nutrients from the leaves to all parts of the root system. In addition, some roots (e.g., carrots) are food stores. Roots have an outer epidermis covering a cortex of parenchyma (packing tissue), and a central cylinder of vascular tissue. This arrangement helps the roots resist the forces of compression as they grow through the soil.

Woody flowering plants

Woody flowering plants are perennial, that is, they continue to grow and reproduce for many years.
They have one or more permanent stems above ground, and numerous smaller branches. The stems and branches have a strong woody core that supports the plant and contains vascular tissue for transporting water and nutrients. Outside the woody core is a layer of tough, protective bark, which has lenticels (tiny pores) in it to enable gases to pass through. Woody flowering plants may be shrubs, which have several stems arising from the soil; bushes, which are shrubs with dense branching and foliage; or trees, which typically have a single upright stem (the trunk) that bears branches. Deciduous woody plants (e.g., roses) shed all their leaves once a year and remain leafless during winter. Evergreen woody plants (e.g., ivy) shed their leaves gradually, so retaining full leaf cover throughout the year.

Herbaceous flowering plants

Herbaceous flowering plants typically have green,non-woody stems,and tend to be relatively short-lived. Many herbaceous plants live for only one or two years. Annuals (e.g., sweet peas) grow from
seed, produce flowers and then seeds, and die within a single year. Biennials (e.g., carrots) have a two-year life cycle. In the first year, seeds grow into plants, which produce leaves and store food in underground storage organs; the stems and foliage then die back in winter. In the second year, new stems grow from the storage organs, produce leaves, flowers, and seeds, and then die. Some herbaceous plants (e.g., potatoes) are perennial. They grow back year after year, producing shoots and flowers in spring, storing food in underground tubers or rhizomes during summer, dying back in the fall, and surviving underground during winter.

Monocotyledons and dicotyledons

 Monocotyledons and dicotyledons:Flowering plants are divided into two classes: monocotyledons (class Monocotyledoneae) and dicotyledons (class Dicotyledoneae). Typically, monocotyledons have seeds with one cotyledon (seed leaf); their foliage leaves are narrow with parallel veins; the flower components occur in multiples of three; sepals and petals are indistinguishable and are known as tepals; vascular (transport) tissues are scattered in random bundles throughout the stem; and, since they lack stem cambium (actively dividing cells that produce wood), most monocotyledons are herbaceous. Dicotyledons have seeds with two cotyledons; leaves are broad with a central midrib and branched veins; flower parts occur in multiples of four or five; sepals are generally small and green; petals are large and colorful; vascular bundles are arranged in a ring around the edge of the stem; and, because many dicotyledons possess wood-producing stem cambium, there are woody forms as well as herbaceous ones.

Gymnosperms 1

Gymnosperms 1:The gymnosperms are four related phyla of seed-producing plants; their seeds, however, lack the protective, outer covering that surrounds the seeds of flowering plants. Typically,
gymnosperms are woody, perennial shrubs or trees, with stems, leaves, and roots, and a well developed vascular (transportat) system. The reproductive structures in most gymnosperms are cones: male cones produce microspores in which male gametes (sex cells) develop; female cones produce megaspores in which female gametes develop. Microspores are blown by the wind to female cones, male and female gametes fuse during fertilization, and a seed develops. The four gymnosperm phyla are the conifers (phylum Coniferophyta), mostly tall trees; cycads (phylum Cycadophyta), small palmlike trees; the ginkgo or maidenhair tree (phylum Ginkgophyta), a tall tree with bilobed leaves; and gnetophytes (phylum Gnetophyta), a diverse group of plants, mainly shrubs, but also including the horizontally growing welwitschia.

Horsetails, clubmosses,and ferns

Horsetails, clubmosses,and ferns: are primitive land plants, which, like higher plants, have stems, roots, and leaves, and vascular systems that transport water, minerals, and food. However, unlike higher plants, they do not produce seeds when reproducing. Their life-cycles involve two stages. In stage one, the sporophyte (green plant) produces spores in sporangia. In stage two, the spores germinate, developing into small, short-lived gametophyte plants that produce male and female gametes (sex cells); the gametes fuse to form a zygote from which a new sporophyte plant develops. Horsetails (phylum Sphenophyta) have erect, green stems with branches arranged in whorls; some stems are fertile and have a single spore-producing strobilus (group of sporangia) at the tip. Clubmosses (phylum Lycopodophyta) typically have small leaves arranged spirally around the stem, with spore-producing strobili at the tip of some stems. Ferns (phylum Filicinophyta) typically have large, pinnate fronds (leaves); sporangia, grouped together in sori, develop on the underside of fertile fronds.