Lore of Plants

Although it often seems that bacteria are the aggressors, nodule formation is an example of plants taking the first step in controlling the metabolism of another organism. Roots of alfalfa and other legumes secrete chemicals called flavonoids that bind to and activate a bacterial nodulation gene called nodD. The nodD gene product activates other nod genes, whose products then turn on plant genes that control the growth and function of nodules. Nitrogen-fixing bacteria live inside these nodules...

Plants Obtain Essential Elements from the Soil

Over 300 years ago, Jan-Baptista van Helmont, a Dutch physician, studied how a plant gains weight. Up to the time of his experiments, people believed that plants obtained their weight from the soil itself. Van Helmont measured the growth of a willow tree for several years. That tree gained more than 70 kilograms, yet reduced the weight of the soil by less than 60 grams. Van Helmont thus disproved the idea that plants get their weight from soil, but he attributed the growth of the willow tree to...

Deficiencies of essential elements disrupt plant growth and development

Mineral Deficient Leaves Tree

Obtaining some of the essential elements is not a problem for plants. For example, carbon dioxide is always present in the air, and water is usually abundant enough to support at least minimal growth. As a result, deficiencies of carbon, hydrogen, and oxygen rarely occur in plants. Plants also require large amounts of nitrogen, phosphorus, and potassium low levels of these elements in soils almost always limit plant growth, and, as a result, these are the elements most likely to be deficient in...

The root consists of layers of different tissues

Buttercup Mature Root Ranunculus

A cross section through a mature dicot root gives us a look at its primary structure fig. 7.8a . A typical dicot root is surrounded by a layer of epidermis cells. The root epidermis usually either lacks a cuticle a thin coating of wax or has a thin cuticle that does not significantly affect water absorption. On leaves and stems, the waxy cuticle helps keep water from evaporating from the plant. Xylem and phloem cells are packed together in the center of the root. Cortex cells constitute the...

Inquiry Summary

Most dicots have a taproot system consisting of a large taproot and smaller branch roots that maximize storage. Monocots have fibrous root systems consisting of similarly sized roots that maximize absorption. Adventitious roots form on organs other than roots. Roots, as well as other plant structures, conform to the principle of form and function. Ends of roots are covered by a root cap, which produces mucigel, a substance that protects a root, lubricates it, and helps it absorb materials from...

Soils in rain forests are deficient in most nutrients

Selenium Plant

In Amazon rain forests, soils are notoriously deficient in most nutrients. The frequent deluges of rain in these forests leach large amounts of calcium, phosphorus, and magne- The prince's plume Stanleya elata , shown here growing in Death Valley, California, requires selenium. Its selenium level is so high that this plant is toxic to browsing mammals. The prince's plume Stanleya elata , shown here growing in Death Valley, California, requires selenium. Its selenium level is so high that this...

Essential elements are required by plants for normal growth and reproduction

Mineral Nutrition Plants Experiment

More than 60 elements have been found in plant tissues. These elements range from those as common as carbon and hydrogen to those as exotic as platinum, uranium, and gold. Are all of these elements essential for growth If so, what functions do they perform Answering these questions has proven to be a formidable task. We must first define what we mean by essential. An element is essential if 1 it is required for normal growth and reproduction, 2 no other element can replace it and correct the...

Roots Play Key Roles in the Ecology and Evolution of Plants

Indicator Plants For Minerals

Soil teems with life on average, about 0.1 of the weight of soil is living organisms. Although this percentage may not impress you, consider that nearly 170,000 species of soil organisms have been identified. One kilogram of fertile soil contains about 2 trillion bacteria, 400 million fungi, 50 mil- Modified roots. Some tropical trees produce planklike buttress roots at the base of their trunk. Buttress roots help stabilize and support the tree. Modified roots. Some tropical trees produce...

Because of their root systems some plants affect the ecology of an entire community

As we saw in chapter 2, ecological relationships between organisms in a community are often complex as in the food web of a community. An example of a complex ecological relationship involves the tamarisk, or saltcedar Tamarix species , which thrives along the stream banks of many western states. Originally introduced to America from its native habitats in Asia and the Mediterranean, the tamarisk was planted as an ornamental tree and as a way to stabilize t r- - mta t r- - mta Anabaena and rice...

Roots of legumes often establish mutualistic relationships with nitrogenfixing bacteria

Nitrogen Fixing Plants Southeast Asia

Nitrogen is often deficient in soils this deficiency limits the growth of plants and, as a consequence, animals. The deficiency of nitrogen in soil is somewhat paradoxical, because the atmosphere the air you breathe is almost 80 nitrogen gas N2 . The chemical bond holding the two atoms of nitrogen gas together makes the N2 molecule extremely stable and therefore unusable by most organisms, including humans. However, a few species of soil bacteria contain a complex enzyme called nitrogenase that...

Most essential elements have several functions in plants

The functions of the 16 elements essential for plant growth are summarized in table 7.1. A quick survey of this Some Functions and Deficiency Symptoms of Essential Elements NUMBER OF PERCENTAGE ATOMS OF DRY RELATIVE TO a plastid pigment present in lignin of xylem elements activates enzymes pollen involved in auxin synthesis maintenance of ribosome structure activates enzymes 1,000 photosynthetic O2 evolution enzyme activator electron transfers pollen tubes regulation of enzyme function possible...

Several Factors Control the Growth and Distribution of Roots

Gravitropism Corn

Much of what we know about roots comes from studies of potted plants or seeds germinated in artificial environments such as sterile dishes or moist paper towels. However, there is little evidence that roots grow similarly in the ground. Understanding how roots grow in their natural environment requires some ingenious as well as tedious work and involves underground cameras and painstaking excavations see fig. 2.12 . The most elaborate methods for studying roots involve laboratories called...

Roots Often Possess Special Adaptations to the Environment

Specialized Roots

Although most roots absorb water and minerals and anchor the plant in place, the following are a few of the most common functions of modified roots that is, roots for which other major functions have evolved K Storage. In plants such as beets, turnips, radish, dandelion, and cassava, roots store large amounts of starch. Sugar produced in leaves during photosynthesis moves throughout the plant via the phloem and is transformed into starch in the roots. Roots of other plants store carbohydrates...

Mycorrhizae are mutualistic relationships between a plant and a fungus

Phosphorus Lack

Many relationships have evolved between roots and beneficial fungi. Such associations between a root and a fungus are called mycorrhizae. In mycorrhizal roots of trees such as pine and oak, the fungus produces a mass of filaments on the surface of the root. These filaments invade the root and form an extensive netlike structure between the cells of the cortex. A mature tree may have thousands of mycorrhizae. Mycorrhizae are a type of mutualism, meaning that both the plant and the fungus benefit...

Taproot systems have one large primary root and many smaller branch roots

Recall that the first structure to emerge from a germinating seed is the radicle, or primary root see fig. 1.16 . In most di-cots, the radicle enlarges and forms a prominent taproot that persists throughout the life of the plant. Many progressively smaller branch roots also called secondary or lateral roots grow from the taproot. This type of root system consisting of a large taproot and smaller branch roots is called a taproot system and is common in cone-bearing trees and dicots fig. 7.1 . In...

Some plants accumulate large amounts of various elements

Plants such as alpine pennycress Thlaspi caerulescens are hyperaccumulators, meaning that they concentrate certain elements in their bodies at levels 100 times or more greater than normal. The hyperaccumulation of metals also benefits the plants by helping them to evade weak-stomached predators, including caterpillars, fungi, bacteria, and humans high-metal plants can poison unwelcome guests. The saltcedar gets its common name because it accumulates salt and then gets rid of it by pumping the...

The subapical region of the root includes the zones of cell division cell elongation and cell matura

The part of the root just behind the root cap is called the subapical region. This region is traditionally divided into three regions the zones of cell division, cell elongation, and cell maturation fig. 7.7 . These zones intergrade with one another and are not sharply defined. In general, however, the zone of cell division consists mainly of the apical meristem of the root, where new root cells are produced. These cells divide every 12 to 36 hours in some plants, the meristem produces almost...

The root cap protects the growing parts of a root as the root grows through the soil

Mucigel Root

The tips of most roots are covered and protected by a thimble-shaped root cap fig. 7.5 . Cells of root caps can secrete large amounts of mucigel, a slimy substance fig. 7.6 containing sugars, enzymes, and amino acids. A root weighing only 1 gram can secrete as much as 100 milligrams of mucigel per day. Although this rate of secretion may seem rather insignificant, the total amount of mucigel secreted by an actively growing group of plants can reach impressive proportions. For example, the roots...

The rhizosphere surrounds each root

Function The Casparian Strip Root

The narrow zone of soil surrounding a root is called the rhizosphere fig. 7.13 . The rhizosphere extends up to 5 millimeters from the root's surface and is a complex and ever-changing environment. Growth and metabolism of roots modify the rhizosphere in several ways. First, roots force their way through crevices and between soil particles. Later, when the roots die and decay, they leave open channels that help aerate the soil this aeration may improve the root growth of other plants. Roots also...

Mutualistic relationships may evolve between two organisms that require different resources

Mineral Nutrition Plants

When plants invaded the land between 400 and 500 million years ago, one of the biggest problems they faced was obtaining enough nutrients. Their aquatic habitat contained Nitrogenase is a bacterial enzyme complex found in nitrogen-fixing bacteria that converts atmospheric nitrogen gas N2 to ammonia NH3 . Plants can use ammonia to make animo acids and other nitrogen-containing compounds for growth. Nitrogenase is a bacterial enzyme complex found in nitrogen-fixing bacteria that converts...