Angiosperm Cells and Tissues

Angiosperm Cells and Tissues

Some cell types and tissues which are not found in any other groups of plants occur in angiosperms (flowering plants).

Angiosperms are a group of plants with seeds that develop within an ovary and reproductive organs in flowers. They are commonly referred to as flowering plants and represent the most successful group of plants on earth, with approximately 235,000 species.

Various cell types and tissues, many of which are not found in any other groups of plants, occur in angiosperms. These cells and tissues perform varied functions, which are very efficient compared to their counterparts in other plants. These include dermal, vascular (xylem and phloem), and ground tissues (such as parenchyma, collenchyma, and sclerenchyma).

Angiosperm Life Cycle

Angiosperm Life Cycle

The word "angiosperm" comes from the Greek words for "vessel" and "seed" and translates roughly as "enclosed seed". In part, angiosperms (the flowering plants, phylumAnthophyta) are defined by the fact that their seeds are enclosed by an ovule. The life cycle of an angiospermis defined by the formation of the seed and its development to a full-grown plant which, in turn, produces seeds.

Angiosperms are vascular plants with flowers that produce seeds enclosed in an ovule—a fact that is recognized as the angiospermy condition.

Reproductive Flower Parts

In general, angiosperms have a floral axis with four floral parts, two of which are fertile. At the receptacle, or tip, of the axis there is an ovule-bearing leaf structure known as the carpel. The ovule or ovules can be found inside the pistil. Three portions compose the pistil: the ovary, the style, and the stigma, where the pollen usually germinates.

Angiosperm Plant Formation

Angiosperm Plant Formation

Angiosperms are flowering plants. Their formation entails development from embryo to seed, through germination to seedling, and finally to mature plant.

The life cycle of angiosperms (flowering plants) involves an alternation of generations between a dominant sporophytic (spore-producing) phase and a reduced gametophytic (gamete-producing) phase. The first cell of the sporophyte is the fertilized egg, or zygote, which undergoes repeated divisions, growth, and differentiation to form an embryo enclosed in the ovule.

After fertilization, the ovule is transformed into the seed, which germinates into a seedling. The seedling becomes the adult plant; the plant produces flowers inwhich the sperm and egg—representing, respectively, the male and female gametophytic generations—are formed. Fertilization occurs, and seeds are produced to continue the life cycle.

Bulbs and Rhizomes

Bulbs and rhizomes are modified stems, stem bases, or other underground organs used by plants for food (or energy) storage and in asexual reproduction.

Plants reproduce both sexually and asexually. Although sexual reproduction is part of the typical life cycle of plants, for a variety of reasons a plant may reproduce asexually. Exact duplicates of a plant, called clones, are formed by asexual reproduction.

Asexual Reproduction

Asexual reproduction involves the production of offspring through the formation of propagules by mitosis (the process of nuclear cell division). Because genetic recombination does not occur in mitosis, the offspring are genetically identical to the parent plant.

Asexual reproduction does not occur in all plants; some reproduce asexually only when humans intervene. Asexual reproduction occurs when a single plant produces a vegetative propagule that develops into a separate free-living plant. Many of the propagules that support asexual reproduction are actually highly modified branches. Others are modified roots.

Cell Wall

The cell wall is the outer, rigid wall of a cell, dividing the protoplast (the interior, including the cytoplasm and nucleus) from the cell’s external environment. The plant cell wall is both unique to and a major feature of plants, perhaps second only to the plant’s photosynthetic ability.

The primary functions of the cell wall in plant cells include are to provide protection for the enclosed cytoplasm and give mechanical support to the entire plant structure. Plant cell walls are part of the extracellular matrix, a complex mixture of extracellular materials found between cells.

These materials are synthesized by the intracellular contents and transported through the plasma membranes. All plant cell types consist of at least a primary cell wall, and many also produce a secondary cell wall. In addition, certain cells also secrete specialized substances into the extracellular matrix.

Cytoplasm

Plant cell

The cytoplasmis defined as all of the living matter within the plasma membrane of a cell, except for the nucleus, which is isolated from the cytoplasm by the nuclear envelope.

The cytoplasm, bounded by the plasma membrane, is composed of fluid called the cytosol in which floats a large variety of molecules and molecular assemblages, ribosomes (responsible for polypeptide synthesis), and a variety of other structures called organelles (literally meaning “little organs”).

Numerous biochemical processes occur in the cytosol, including protein synthesis (translation) and glycolysis.

Cytoskeleton

Cytoskeleton

The cytoskeleton is a complex network of fibers that supports the interior of a cell. Cross-linked by molecular connectors into systems that support cellular membranes, it holds internal structures, such as the nucleus, in place and controls various kinds of cell movement.

Virtually all eukaryotic cells, including plant cells, have a cytoskeleton. Cytoskeletal systems extend internally from the membrane covering the cell surface to the surface of the membrane system surrounding the cell’s nucleus. There are indications that a cytoskeletal support system reinforces the interior of the nucleus as well.

The fibers of the cytoskeleton also anchor cells to external structures through linkages that extend through the surface membrane. The cytoskeletal material, rather than being fixed and unchanging, varies in makeup and structure as cells develop, move, grow, and divide.

Cytosol

Cell

Within each eukaryotic cell are a number of distinct, membrane-bounded structures, generically called organelles, including the nucleus, mitochondria, the endoplasmic reticulum, and chloroplasts (only found in plants, algae and some protists).

Each organelle is a specialized structure that performs a specific function for the cell as a whole. The rest of the cell, excluding the organelles, cell wall, and plasma membranes, is called the cytosol: the fluid mass that surrounds and provides a home for the organelles.

The cytosol is organized around a framework of fibrous molecules and protein filaments that constitute the cytoskeleton. Although the cytosol consists mostly of water, it contains many chemicals that control cell metabolism, including signal transmission and reception, cellular respiration, and protein transcription factors.

Endomembrane System and Golgi Complex

The endomembrane system is a collective term applied to all of the membranes in a cell that are either connected with or are derived from the endoplasmic reticulum (ER), including the plasma membrane but not the membranes of chloroplasts or mitochondria.

The membrane-bound organelles considered to be part of the endomembrane system are the vacuole, nuclear envelope, endoplasmic reticulum, Golgi complex, and various types of vacuoles.

Some components of the endomembrane system have direct, permanent connections with the endomembrane system (such as between the endoplasmic reticulum and the nuclear envelope), whereas other components share membrane and contents by trafficking vesicles (membrane-bound packages) from one component to another (for example, the ER sends numerous vesicles to the Golgi complex) across the cytosol.

Endoplasmic Reticulum

Endoplasmic Reticulum

The endoplasmic reticulum is a network of sacs in the cytosol of eukaryotic cells that manufactures, processes, transports, and stores chemical compounds for use inside and outside of the cell.

The endoplasmic reticulum (ER) is an extensive, complex system of a more or less continuous distribution of convoluted membrane-bound cavities that take up a sizable portion of the cytosol.

The internal space of the ER is called the lumen. The ER is attached to the double-layered nuclear envelope and provides a connection, or bridge, between the nucleus and the cytosol.

Flagella and Cilia

Flagella and Cilia

Flagella and cilia are hairlike structures,made primarily of protein, found on the surfaces of cells and used for movement by microorganisms and some specialized cells, such as the gametes of certain plants with motile sperm. Because flagella and cilia are so similar, many scientists use the term “undulipodia” for both in reference to eukaryotic organisms.

Although the term “flagellum” is used in reference to both prokaryotes (archaea and bacteria) and eukaryotes (fungi, protists, plants, and animals), the structure and mechanism of action of this structure in prokaryotes are quite different from the structure and mechanism of action in eukaryotes.

Eukaryotic flagella and cilia, however, are structurally and functionally identical. The differences between them are in their number, length, and position. Flagella are less numerous, longer, and usually polar, while cilia are more numerous and shorter, covering much of the cell’s surface.

Flower Structure

Flowers are the modified shoots bearing modified leaves that serve as the sexual reproductive organs of angiosperms. This strategy for reproduction has been so successful that angiosperms now dominate the plant world, and accordingly there are many variations on the basic structure of a flower.

Flowers are organs of sexual reproduction produced by the angiosperms (phylum Anthophyta), the largest phylum of photosynthetic organisms, with roughly 250,000 species. This large number represents a great diversity of flower types, but all flowers have some common structural elements.

Flower Parts

Flowers are modified shoots bearing modified leaves. In the typical flower, the modified leaves can be grouped into four sets based on appearance and function: sepals, petals, stamens, and pistils. The sepals and petals are lowermost on the shoot toward the sides of the flower. The stamens and pistils are at the tip of the shoot at the inside.

Flower Types

Yellow hibiscus flower

The flower is the most distinctive feature of the phylum Anthophyta, commonly referred to as angiosperms or flowering plants, and is responsible in making them the most dominant, diverse, and widespread of all groups of plants.

There are already about 250,000 species of flowering plants that have been discovered and named. The basis for their diversity comes from their incredible reproductive success in a wide variety of habitats.

The success of this group is also reflected by the diversity of their flowers that show astonishing displays of different forms, sizes, shapes, and colors—all of these to lure pollinators and effect sexual reproduction.

Structure and Types of Fruit

Fruits

Fruits are the seed-containing reproductive organs, including nuts and grains, produced by angiosperms (flowering plants).

When most people think of a fruit, what typically comes to mind is a juicy, edible object, such as an apple, orange, or banana. To botanists, however, fruit includes many plant-derived structures, such as grains, nuts, and many vegetables.

In essence, a fruit is an enlarge dovary, often with some accessory tissue, that develops after a flower has been pollinated. After pollination, seed development begins, and soon the peripheral parts of the flower fall away, leaving the immature fruit. The fruit subsequently enlarges and then ripens to maturity. It is then often edible.

Germination and Seedling Development

Germination

With germination, the growth of a seedling, spore, or bud begins. Seedling development begins with the close of germination. To germinate, seeds must be nondormant and in a suitable environment. Seeds germinate within a restricted range of temperatures, moisture, oxygen, light, and freedom from chemical inhibitors.

Wild seeds display many adaptations that predispose germination within specific habitats and seasons. By contrast, seeds of crops and other cultured plants usually lack controls that prevent germination.

The control system was lost because some seeds in the population lacked controls and were chosen when they germinated in the care of a culturist. For that reason, most cultivated plants that start from seeds show little or no germination control. Most of the information on germination control, therefore, covers wild species of plants.

Inflorescence

Inflorescence

The term "inflorescence" refers to the arrangement of flowers on a floral axis. Most schemes that define inflorescence types separate solitary flowers from flower clusters and stipulate that an inflorescence is a cluster of two or more flowers.

It is not always easy to distinguish between solitary flowers and an inflorescence. An examination of the evolutionary development of the flower and the inflorescence provides some insight into the problem. It generally is accepted that the flower arose as a modified stem tip that bore male and female reproductive structures at its apex.

These reproductive structures became the pistils and stamen of the flower. Leaves that immediately subtended the reproductive structures became the sterile parts of the flower (petals and sepals) and are typically more leaflike as distance from the apex increases.

Leaf Anatomy

Leaf Anatomy

The leaf has evolved as the chief part of the plant for gathering light energy from the sun and conducting photosynthesis to transform that light energy into biochemical energy. Hence, its structure is adapted to that function.

Leaves are formed by a plant to manufacture food. Photosynthesis—a complicated chemical reaction in which carbon dioxide from the air and water from the soil, in the presence of light, produce sugar—is carried out in the chloroplasts found packed within the leaf cells.

Because energy is derived from light by chlorophyll, either the leaf must be thin enough for light to penetrate all the cell layers or, in the case of plants with succulent leaves, chloroplasts must be most concentrated near the surface of the leaves.

Leaf Arrangements

Leaf Arrangements

The study of leaf arrangements, or phyllotaxy, considers not only the descriptive classification of leaf arrangements but also theories regarding the cause of such arrangements.

The function of the arrangement of leaves (phyllotaxy) is to increase a plant’s ability to carry on photosynthesis by positioning the leaves in such away as tomaximize the surface area available to intercept sunlight. Leaves may be either caulescent (on obvious stems) or acaulescent (with no obvious stems).

Flowering plants have three basic types of arrangements: alternate spiral; opposite; and whorled or verticillate. The alternate spiral arrangement is generally considered to be the most primitive condition, with the opposite and whorled conditions being derived by suppression of internode development.

Leaf Lobing and Division

types of leaves

The pattern of leaf lobes (projections) or divisions, leaf arrangement, the number, and the shape of leaflets composing compound leaves are often useful characteristics for identification of plants.

Leaves, the main photosynthetic organs of plants, are usually green, flattened structures that are formed as lateral outgrowths at stem nodes. Simple leaves are composed of a single lamina, or blade, which may be attached to the stem via a cylindrical structure called a petiole.

Leaves lacking a petiole are called sessile. Laminae of simple leaves may exhibit various patterns and degrees of lobing, which are often characteristic of individual species of plants and, together with reproductive features, are used in plant identification.

Other species have compound leaves, in which the leaf laminae are subdivided into smaller leaflets. The pattern of arrangement, the number, and the shape of leaflets comprising compound leaves are often useful characteristics for identification of plants.

Some species of plants exhibit either gradual or abrupt changes in leaf lobing and division during development and are called heterophyllous. For example, some species exhibit a mixture of pinnatifid (pinnately lobed) and pinnatisect (pinnately compound) leaves on the same stem.

Heterophylly is often observed in water plants, with one form of leaf being produced where the plant stem is submerged and another being produced where the stem is above water.

Leaf Margins, Tips, and Bases

Leaf margins

The flattened part of the leaf is the leaf blade or lamina,which can be subdivided into three discrete regions: The tip or apex is the part of the lamina farthest removed from the point of attachment of the leaf to the stem. The base of the leaf is the part of the lamina that is closest to the point of attachment of the leaf to the stem. The margin is the perimeter of the leaf between the apex and base.

The form, ormorphology, of leaves is often characteristic of individual species of plants and, like the reproductive features, is an important base of plant identification.

Some plants have a more or less cylindrical petiole that joins the base of the leaf to the stem, while others lack a petiole and are called sessile. The midrib is the prominent vein that subdivides the leaf into two halves from base to apex.