1. Herbaceous (nonwoody) plants are about 75—80 percent water, and even fresh wood is 40—65 percent water.

2. Analysis of the dry mass of a plant–what remains after completely drying it–shows it is 90—95 percent organic matter: protein, nucleic acid, lipid, and polysaccharide.

3. The inorganic minerals which come from the soil make up only the remaining 5—10 percent.

4. Water supplies most of the hydrogen and some of the oxygen for organic structure in plants.

5. 90—99 percent of the water a plant takes up merely passes through and is lost by transpiration.

6. The bulk of the organic material of a plant comes from the CO2 that is assimilated from the atmosphere through photosynthesis.

7. Figure 40.1 summarizes the process of nutrition and materials flowing through a plant.

1. An element is considered an essential nutrient if a plant requires it to complete its life cycle.

2. The essential nutrient cannot be replaceable by another element, and it must be required for normal function.

3. The essential nutrients in Table 40.1 are divided into two categories: the macronutrients, which plant tissues require in large amounts (at least 1 milligram per gram of dry mass), and the micronutrients, or trace elements, which are required in concentrations of less than 100 micrograms per gram of dry mass.

4. The six main macronutrients–carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus–are obviously the components of organic molecules.

1. Calcium is a constituent of cell walls, and as one of the principal second messengers in cells, it has an essential role in the common signal-transduction pathway that processes signals from hormones and other external ligands.

2. Calcium also regulates the actions of the cytoskeleton and is essential for spindle formation in mitosis and meiosis.

3. Potassium is the principal monovalent cation in plant tissues.

4. Potassium also has a role in moving water from one compartment to another inside cells.

5. Magnesium has multiple functions–as the principal divalent cation of cytosol, as a cofactor of numerous enzymes, and as a constituent of chlorophyll.

1. Iron is part of the heme group of cytochromes, the proteins that compose the bulk of electron transport systems in chloroplasts and mitochondria.

2. Zinc, molybdenum, and nickel are required by certain enzymes, and with few exceptions each enzyme requires one particular element, which cannot be replaced by another.

3. Plants need only minute quantities of these elements because they have catalytic functions and the enzymes they serve are not abundant.

1. An element is considered essential if a plant grown hydroponically fails to grow, flower, or produce viable seed in the absence of that element.

2. Plants require some elements in such minute amounts that contamination traces in the experimental apparatus have been enough to support plant functions.

3. A seed may contain enough of an essential trace element to supply the entire plant that grows from it and even part of the next generation.

1. Magnesium deficiency shows up as chlorosis, a yellowing of the leaves (Figure 40.3).

2. Iron deficiency can also cause chlorosis, even though chlorophyll contains no iron, because one enzyme in the biosynthetic pathway of chlorophyll requires iron as a cofactor.

1. If a nutrient can move freely from one part of the plant to another, young, growing tissues are generally able to accumulate it; so if the nutrient is in short supply, deficiency symptoms will appear first in older tissues.

2. Even though deficiencies of magnesium and iron produce similar symptoms, the difference in the age of affected organs may distinguish between them.

1. Deficiencies of nitrogen, potassium, and phosphorus–the most common problems in domestic plants–are relatively easy to identify.

2. Shortages of micronutrients tend to only appear in geographically localized areas, due to differences in soil composition.

3. Considering the minute amounts of some micronutrients that are needed, any such deficiency can be easily corrected.

1. Roots with enormous surface areas grow down through the soil, extracting nutrient ions from the surrounding water through osmosis and active transport.

2. Plants are able to take up other compounds, such as sulfur oxide, through their leaves.

3. Plants can absorb more copper, manganese, and iron from foliar spray than from the soil.

1. Growth is a plant's way of moving to a better environment, and a plant can potentially reach sources of essential minerals by extending its root system.

2. Through its roots, a plant may mine the soil so thoroughly that it depletes the local environment of essential materials.

1. Plant roots are selective and often can match their uptake of minerals to the plant's nutritional needs.

2. The mineral composition of plants often reflects the composition of the soil and water in which they grow.

3. Occasionally a plant surprises us by concentrating unusual amounts of an unusual material; locoweeds and poison vetch (Figure 40.4) accumulate selenium, so animals that eat them are killed or driven to wild behavior.

1. Soil gives terrestrial plants mechanical support and oxygen for their roots.

2. Soils have abiotic and biotic components.

3. The abiotic part of a soil is rock fragments whose composition depends on the parent rock they are derived from.

4. Soils change constantly due to climatic influences such as heat and precipitation.

5. The living components in soil include plant roots and an astonishing number and variety of organisms, some whose activities supply essential nutrients and others that tend to damage or kill plants (Figure 40.5).

1. To a depth of only a few centimeters, each square meter of soil contains 103 to 105 insects, spiders, centipedes, slugs, snails, and other invertebrates, forming the usual complex food web.

2. Typically, 30—300 earthworms per square meter mix and aerate the soil by their burrowing and add mucus that holds fine soil particles together.

3. A soil contains from 50—3,000 kg per hectare of plant roots, which have enormous effects on soil chemistry as they remove water and minerals.

1. Soil formation begins with the mechanical weathering of rocks into small fragments through repeated wetting, drying, and freezing.

2. Mechanical weathering is followed by chemical weathering, the chemical alteration of some rock components, particularly by dilute carbonic acid water which is formed by the reactions of carbon dioxide with water.

1. Carbonic acid rainwater filtering through the soil breaks down some mineral components and removes them.

2. Clay particles swell and shrink as they become hydrated and dehydrated, and they strongly determine the physical and chemical properties of soils.

3. Each soil contains a particular mixture of clay minerals, depending on the parent rock.

1. The texture of topsoil depends in large part on the size of its mineral particles.

2. Although water drains away from the larger spaces of the soil, it is held in smaller spaces by capillarity and by its adhesion to the hydrophilic surfaces of soil particles (Figure 40.6).

3. Some of this water is so tightly bound that it is not available to plants, but the more mobile water constitutes the soil solution from which plants draw their water, with its dissolved nutrients.

4. For agricultural purposes, the most fertile soils are loams, rich soils that contain a balance of sand, silt, and clay.

1. Humus is a black or brown complex of decomposing organic matter formed by the actions of bacteria and fungi on all the detritus of the ecosystem.

2. Humus, lignin and other woody compounds that are quite resistant to decay form humic acids.

3. Humic acids are acids that release hydrogen ions, and combine with clay minerals to form clay—humus complexes.

1. The roots determine the nature of the rhizosphere by secreting various organic acids into the soil solution and withdrawing its water and ions.

2. Roots also secrete a slimy mucilage, a hydrated polysaccharide called mucigel that changes their surface properties and forms the interface between the cell surface and the soil solution.

3. The pH of the soil is critical for plant growth.

1. A soil with little humus and clay loses these essential cationic nutrients through leaching, particularly during heavy rain or irrigation.

2. Clay-humus complexes are only able to hold and exchange cations.

3. Soils have no analogous exchange system for anions, and therefore anionic nutrients that are so important to plant nutrition, such as phosphate, nitrate, and sulfate, are easily leached to lower soil horizons.

1. Soil acidity also affects the chemical form of mineral nutrients.

2. Farmers frequently reverse acidification of the soil by the practice of liming, applying calcium-rich materials that release compounds such as calcium carbonate or calcium hydroxide.

3. Phosphorus is mostly available to plants in a very narrow pH range (Figure 40.8).

4. Most crops do best at slightly acidic conditions, around pH 6.3 to 6.8.

1. Even though four-fifths of the atmosphere is nitrogen gas, most organisms have no way to use this abundant resource.

2. N2 is very unreactive, and it takes a great deal of energy to convert it into one of the forms plants can use: nitrate ion (NO3-) or ammonia (NH3).

3. The process of reducing N2 to NH3 is called nitrogen fixation.

4. The fertilizer industry now fixes substantial amounts by a method called the Haber process.

5. Most reduced nitrogen compounds are still made by nitrogen-fixing procaryotes, which convert about 9 x 1013 grams of atmospheric dinitrogen per year.

1. Some nitrogen-fixers are free-living soil and water bacteria, particularly Azotobacter and cyanobacteria.

2. The most important nitrogen-fixation systems develop in mutualistic associations of procaryotes and certain plants or fungi; these mutualisms provide the bacteria with a place to grow and supply the eucaryote with ammonia.

3. More widespread mutualisms with nitrogen-fixers occur between certain angiosperms and two types of bacteria: the actinomycete Frankia or the small bacterium Rhizobium.

4. Both bacteria (Frankia and Rhizobium) grow within root cells, where they induce the growth of round root nodules in which nitrogen is fixed (Figure 40.10).

5. The filamentous Frankia forms actinorrhizal associations with plants such as alders, silverberries, and bayberries, while rhizobia associate with elms and primarily with legumes–clover, peas, alfalfa, soybeans, and many tropical shrubs and trees.

1. A field is planted in alternate years with a nitrogen-demanding crop such as wheat and a legume such as clover or alfalfa.

2. Instead of being harvested, the legume is generally plowed under and allowed to decompose.

3. Root nodules fix so much nitrogen that they secrete excess ammonia, which further increases the fertility of the soil.

1. Although ammonia is toxic to plants, they can take up and tolerate ammonium ions at low concentrations, especially at basic soil pH.

2. Most plants grow better with nitrate than with ammonium ions as a source of nitrogen, and they take up nitrate ions preferentially under more acidic conditions.

3. A plant transports nitrate to its chloroplasts, where some of the energy generated in the light-dependent reactions of photosynthesis is used to reduce it to ammonia for the synthesis of amino acids.

1. In stage one, bacteria oxidize ammonia to nitrite ions.

2. In stage two, bacteria oxidize nitrite to nitrate.

3. The bacteria involved in nitrification are all chemoautotrophs that carry out oxidations as part of their energy metabolism.

1. Some common soil bacteria that normally grow aerobically can grow anaerobically by using nitrate as a terminal electron acceptor in place of oxygen.

2. Animals can only acquire their nitrogen through the food web, so they are entirely dependent on the reactions of nitrogen fixation and nitrification that support plant growth.

1. Sulfur is also a component of coenzymes such as thiamine, lipoic acid, and coenzyme A.

2. Plants can take sulfate ions from the soil or water, reduce them to the sulfhydryl level, and incorporate them into cysteine.

3. Animals depend upon the cysteine, methionine, and vitamins from plants, as they do for nitrogen.

4. Plants can reduce sulfate themselves and do not depend on bacteria.

1. As the civilization of Mesopotamia was replaced by a series of others, wheat production decreased steadily because of salinization, the process in which the soil becomes increasingly salty as irrigation water evaporates year after year, leaving its dissolved salts behind.

2. At first, farmers of the region were forced to grow more and more barley, a plant that is more tolerant of salty soil, but now the ecosystem is a desert.

3. Desertification is the process by which poor farming practices make deserts out of formerly rich ecosystems.

1. The natural ecosystem is in a steady-state condition, with its mineral losses slowed by the structure of the ecosystem itself.

2. Agriculture can seriously deplete the mineral content of the soil.

3. In addition to salinization, agriculture commonly erodes the soil.

4. Soil is a renewable resource whose fertility can be preserved, so it should be able to sustain agricultural production for many human generations.

5. Soil conservation measures are well known: terracing hillsides, using stands of woods as windbreaks between fields, planting and recycling certain crops as manures.

1. Intensive agriculture requires huge amounts of water, and the availability of water is often the main limitation on plant growth (Figure 40.12).

2. Intensive agriculture uses inorganic fertilizers instead of organic fertilizers such as compost, rotted plants, or manure.

3. Intensive agriculture uses chemical pesticides, which can be dangerous if consumed indirectly, and to which many insects become resistant.

4. Intensive agriculture uses genetic selection to modify plants so that they can continue to thrive in damaged ecosystems.

1. Energy-intensive modes of conventional agriculture cannot be sustained, so agriculture must focus on recycling a finite supply of nutrients.

2. Soil quality and nutrient balance are essential if agriculture is to have a future; human and animal health are directly related to the health of the soil.

3. Healthy plants, animals, and humans result from balanced biologically active soil.

4. Monoculture is overspecialized and environmentally unstable.

5. Ecological agriculture promotes person and community independence from energy-intensive production and distribution systems.

1. This supports maintaining a rich, fertile soil with an active biological community, rather than using inorganic fertilizers and pesticides, and it places great emphasis on maintaining the humus.

2. A soil rich in humus is porous and crumbly, with large air spaces that are essential for respiration by plant roots.

3. Humus makes a soil more drought-resistant by retaining water, up to 80-90 percent of its mass.

4. Because humus is dark in color, a humus-rich soil absorbs heat from sunlight more quickly than a poorer soil and warms up faster.

1. Organic fertilizers become components of humus and as they decompose, they gradually release inorganic nutrients that roots can absorb.

2. Gradual release is important because plants can only assimilate mineral nutrients in proportion to their general growth rate.

3. Minerals in commercial fertilizers are available in large quantities immediately, and farmers may perceive them as superior because they provide an instantaneous supply of nutrients and can be formulated to meet the needs of a particular crop.

1. Ecological agriculture strives for crop diversity, as opposed to monoculture, and other ecologically sound practices such as crop rotation.

2. Ecological agriculture means maintaining farm animals as well as plants, so their manure can be used as fertilizer.

1. Several vegetables have exhibited better storage life when grown organically, apparently due to lower respiration rates and lower levels of the enzymes that produce spoilage.

2. Compared to vegetables grown conventionally, those grown organically have higher levels of several desirable nutrients (protein, vitamin C, iron, potassium, and other mineral nutrients); they also have much lower levels of nitrate.

3. Several studies on the nutrition and growth of animals have shown that they grow considerably better on organically produced food.

1. Malnutrition in humans is most often protein deficiency.

2. The vast majority of the world's people today have a predominantly vegetarian diet, and therefore rely mostly on plants for protein.

3. Many plants contain relatively little protein, and what proteins they do have are often deficient in some of the essential amino acids that animals cannot synthesize for themselves.

4. Some plant breeders have therefore sought to improve world health by developing plants with more and better proteins, and a great deal of agricultural research has been directed toward this goal.

1. Food plants might be modified so that they could respond to rhizobia and form their own root nodules.

2. It might be possible to implant nitrogen-fixation (nitrogenase) genes directly into the plant tissue.

3. Research still has to progress from blackboard plans and laboratory experiments to field applications, but the prospect of food crops fixing their own nitrogen provides a strong incentive for the research.

1. Epiphytes are not parasites, but the relationship with the host plant is one of phoresis or commensalism.

2. Some lichens, mosses, and ferns grow epiphytically, as do a number of flowering plants, especially orchids and some bromeliads or the pineapple family, Bromeliaceae.

3. The vegetative plants of tillandias (Spanish moss) and other epiphytic bromeliads do not have functional roots, and they obtain water and mineral nutrients from rain and fog through their leaves, sometimes using special structures that are very efficient at absorbing water.

1. Nearly half of the organic material of the epiphytic zone is dead, composed of epiphytes that are decaying in place to form a crown humus.

2. Epiphytes are rich in the nutrients they absorb from precipitation, and the mass of epiphytes in the crowns of trees may be several times the mass of the tree's foliage.

3. Many epiphytes change the nutrient balance around themselves and may have a significant influence on the nitrogen balance of the ecosystem.

4. Rather than robbing phorophytes of nutrients, some epiphytes are an additional source.

1. The crown humus is a real soil that supports its own community of microorganisms, earthworms, and insects.

2. This community in turn supports a whole, above-ground animal community.

1. Parasitic plants obtain their food directly from other living plants.

2. Indian pipe is a parasitic plant that obtains its nutrients from nearby photosynthetic plants, with the aid of mycorrhizae (root fungi) (Figure 40.15).

3. Dodders do not carry out photosynthesis at all and draw all their nutrients from the plants they grow on (Figure 40.16).

4. Although mistletoes are green and carry on some photosynthesis, they obtain water and mineral nutrients from the xylem sap of other plants by growing extensions called haustoria. Some mistletoes obtain photosynthetic products from their hosts.

1. The 450 known species of plant carnivores are normally found in boggy regions with acidic soil.

2. Carnivorous plants include pitcher plants, Venus's-flytrap, and sundews (Figure 40.17).

3. Carnivory is an adaptation for supplying a plant with nitrogen from animal proteins in this restricted environment.

4. None of the carnivorous plants require insects for survival.