Biology by Karl Irvin Baguio (the ebook reader TXT) 📕

roots, stems, and leaves. The internal vascular system, composed of xylem and phloem, is housed within these organs and connects the organs to one another.

 

Different combinations of tissues make up the organs of a vascular plant. The four types of tissues are as follows: vascular tissue, ground tissue, dermal tissue, and meristematic tissue.

 

Ground Tissue

 

The ground tissue of the vascular plant is responsible for storing the carbohydrates produced by the plant. Ground tissue comprises the majority of a young plant and lies between the vascular and dermal tissues.

 

The major cells of the ground tissue are parenchyma cells, which function in photosynthesis and nutrient storage. They have thin walls, many chloroplasts, and large central vacuoles, and they form the mass of most leaves, stems, and roots.

 

Another cell of the ground substance is the sclerenchyma cell. Sclerenchyma cells are hollow with strong walls, and they help strengthen the ground tissue.

 

Dermal Tissue

 

Dermal tissue functions to protect the plant from injury and water loss. Dermal tissue covers the outside of the plant, except in woody shrubs and trees, which have bark. The most common cell type in dermal tissue is the epidermal cell. Generally, a thin, waxy layer called a cuticle covers the epidermal cells and protects them. Other cells in the dermal tissue are guard cells that surround the stomata, which are openings in the leaves. Gases and water enter and leave the dermal tissue through the stomata.

 

Meristematic Tissue

 

Meristematic tissue is growth tissue and the location of most cell division. It is known as undifferentiated tissue because cells in the meristematic tissue will eventually become vascular, ground, or dermal tissue.

 

Plants generally grow where meristematic tissue is present. At the tips of roots and stems, the meristematic tissue is called the apical meristem. The primary growth of the plant occurs in the apical meristem.

 

Lateral growth in a plant is called secondary growth; it occurs in lateral meristem tissue. Woody trees and shrubs display secondary growth when the plants become enlarged and thickened.

 

Roots

 

The primary functions of roots in plants are to anchor the plants to the ground and to take in water and minerals from the soil. Substances usually enter the roots by diffusion, but facilitated diffusion may also occur. In addition, roots may be specialized for storage.

 

Various plants have different types of roots. A taproot is a root that grows straight downward and has strong lateral roots growing out of it. Dicots often have taproots. A fibrous root system, consisting of slender, branching roots, occurs in most monocots. Adventitious roots occur on the stems or leaves of some plants, such as corn.

 

An apical meristem forms the root’s tissues. Cells at the tip of the root form the root cap, which functions mainly for protection. Behind the root cap is the apical meristem, and behind the apical meristem is the region of elongation, followed by the region of maturation. Many of the epidermal cells of the root develop extensions called root hairs that increase the surface area of the root.

 

Stems

 

The stems of vascular plants have several functions, including support of the plant, transport of water and minerals by the vascular system, and generation of energy through photosynthetic cells (in some plants). Some stems also function in food and water storage.

 

Stems arise in the apical meristem. The outer stem layer is the epidermis, the next layer is composed of vascular tissues, the next is the cortex of parenchyma cells, and at the center of the stem is the pith.

 

In herbaceous plants (for example, clover, potatoes, and wheat), the stem is soft and composed primarily of meristematic tissue. In woody plants, in contrast, the stems are hard, with secondary tissues formed after the primary tissues have been laid down. Secondary tissues arise from the vascular cambium, a thin layer of dividing cells between the xylem and phloem. The vascular cambium is responsible for the lateral growth in the diameter of the woody stem as the cells lay down secondary xylem toward the inside of the stem and secondary phloem toward the outside of the stem. Xylem becomes the wood of the stem, while phloem, together with a tough tissue called cork, becomes the bark of the stem. Between the phloem and cork is a thin layer of cells—the cork cambium—that produces the cork. Annual rings form from xylem tissue.

 

Monocots and dicots contrast in the construction of stem tissue. In monocot stems, the vascular bundles are scattered throughout the parenchyma. In dicot stems, the vascular bundles are arranged in a ring around the margin of the stem. Most of the interior dicot stem is occupied by pith. In both monocots and dicots, the xylem provides some structural support for the cell.

 

In woody stems, the apical meristem is embedded in the tip of the stem within a structure called the terminal bud. Along the side of the stem are smaller lateral buds from which new branches and twigs emerge. Leaves generally unfold at intervals along the stem as the terminal bud moves upward. The leaves are attached at points on the stem called nodes. Spaces between the nodes are called internodes. Openings called lenticels are found along woody stems. Lenticels function as pores to permit the exchange of gases between the stem tissue and surrounding air.

 

Leaves

 

The leaves are the principal organs of photosynthesis in the vascular plants. The cuticle surrounds the epidermis of the leaf to reduce water loss, while gases pass through pores called stomata. Beneath the upper epidermis of the leaf is a layer of elongated palisade cells. The palisade cells contain numerous chloroplasts, where photosynthesis takes place. Below the palisade cells is the spongy mesophyll, an arrangement of loosely packed cells that also contain chloroplasts for photosynthesis. The air spaces around the cells permit efficient gas exchange to take place during photosynthesis.

 

Bundles of vascular tissues extend through the leaf and form its veins. The vascular tissue supplies water and nutrients to the photosynthetic cells, and the products of photosynthesis are conducted away from the cells through the phloem. Vascular tissue also runs through the petiole, the stalk that connects the leaf to the node of the stem. The broad, flat portion of the leaf is the blade.

 

One of the most important activities in the leaf is the opening and closing of the stomata. These pores regulate the rate of gas exchange, which regulates the rate of photosynthesis. The opening and closing of a stoma is regulated by osmotic pressure within a pair of guard cells. Guard cells are thicker on their inner sides than on the outside, so when the cells are swollen with water, they bow outward, opening the stoma. The pressure exerted on the guard cells to open is called turgor pressure. Scientists believe that a low concentration of carbon dioxide and an accumulation of potassium ions in the guard cells, along with environmental conditions such as temperature, instigate their opening. ATP provides the energy for opening and closing the guard cells.

 

When the stomata are open, the carbon dioxide needed for photosynthesis enters the leaf, while the oxygen gas produced in photosynthesis leaves the leaf. The water produced during photosynthesis also leaves through the stomata. This water loss is called transpiration.

 

Vascular Tissue

 

The vascular tissues include xylem, which conducts water and minerals from the roots upward and throughout the plant, and phloem, which transports dissolved nutrients in all directions within the plant.

 

The main conducting vessels of xylem are the tracheids and the vessels. Tracheids are long, thin tubes found in most vascular plants, while vessels are large tubes found predominantly in angiosperms. The tracheids and vessels form pipelines that have pores and perforated ends that allow water and minerals to be conducted from one tube to the next and out to the surrounding tissues. Tracheids and vessels also help support the plant body.

 

The main conducting cells of phloem are sieve cells and sieve tube members. Both cell types have numerous pores through which substances are exchanged with adjacent cells. Sieve tube members occur in angiosperms, while sieve cells are found in other vascular plants. In angiosperms, small cells called companion cells assist the sieve tube members in their functions.

 

Plant Hormones

 

The growth and development of many plants are regulated by the activity of plant hormones. Hormones are biochemical substances produced in one part of a plant and transported to a different part where they exert a particular effect.

 

An example of a plant hormone is a series of substances called auxins. Auxins increase the length of most plant cells and thereby contribute to the growth and elongation of the plant.

 

Another plant hormone is abscisic acid, which is produced in mature leaves and inhibits growth in developing leaves and germinating seeds. The inhibition occurs during the winter, contributing to the plant’s dormancy. Another hormone, ethylene, encourages ripening and the dropping of leaves and fruits from the trees. Slight pressure permits the fruit to break loose from the stem.

 

Two important growth-regulating hormones are the gibberellins and the cytokinins. Gibberellins affect plants by stimulating their growth via rapid stem elongation. Cytokinins induce plant cells to undergo mitosis; therefore, they encourage increased growth in the roots and stems of plants. They also enhance flowering and stimulate some types of seeds to germinate.

 

The interactions of hormones and stimuli in the environment often result in a bending or turning response in the plant called a tropism. When the plant turns toward a stimulus, the tropism is said to be positive. If the plant turns away from a stimulus, the tropism is negative.

 

One of the most familiar plant responses is the bending of the stem toward a light source. Light is the stimulus, and the response of the plant is called a phototropism. A geotropism is a turning of the plant away from or toward the earth. A negative geotropism is a turning away from the earth, such as by a plant stem that grows upward. A positive geotropism is a turning toward the earth, such as in a root that grows downward.

Chapter 20: Animals: Invertebrates

 Cnidaria

 

Members of the phylum Cnidaria include hydras, jellyfish, sea corals, and sea anemones. Cnidarians live primarily in marine environments. They have tissue organization and a body plan displaying radial symmetry; that is, the organisms are circular with structures that radiate outward. The ends of the structures have tentacles with stinging devices called cnidocyte that help in defense and in trapping food. Digestion of food occurs within the central body cavity called the gastrovascular cavity. Cells lining this cavity produce enzymes to break down the food, and the products of digestion are taken into the cells. In addition, the animals have a loose network of nerve cells within their tissue. These nerve cells coordinate the animal’s activities.

 

There are two basic body plans in the cnidarians: a hollow, vaselike body called the polyp, and an inverted umbrella shape called the medusa (the plural is medusae). Hydras occur as polyps, while jellyfish appear as medusae. A three-layered body wall makes up the outer surface of the polyp or medusa.

 

Platyhelminthes

 

Members of the phylum Platyhelminthes are flatworms, such as the planarian. Grubs and tapeworms are other examples of flatworms. Flatworms display bilateral symmetry; that is, the left and right halves of the body are mirror images of one another. Another characteristic of the platyhelminthes is cephalization, which means that one end of the