1. If photosynthesis is to be productive, a plant must maximize its intake of CO2 and water.

2. Sugar produced by the photosynthetic portions must be transported throughout the plant and perhaps stored.

3. A plant needs a continuous flow of oxygen to support respiration.

1. "Stoma" is sometimes used to mean only the space between guard cells but sometimes includes the guard cells themselves.

2. Stomata occur between cells and are usually concentrated on the lower surfaces of land plant leaves, in the upper epidermis of grasses, and in the leaves of floating plants such as water lilies (Figure 38.2).

1. When the guard cells swell with water, the stoma opens; when they lose water, the stoma closes (Figure 38.3).

2. This control process is not well understood yet, but K+ flow is clearly related to the concentration of CO2, which has a central role in leaf physiology.

1. It is estimated that through transpiration some plants lose well over 90 percent of the water that enters their roots.

2. A plant hormone called abscissic acid, also regulates the stomatal opening to conserve water during times of stress.

3. Some plants have an endogenous stomatal rhythm: The stomata open and close on a regular daily cycle, even when all conditions, including water content, remain the same.

1. To supply all of a plant's tissues, the xylem must overcome the force of gravity and transport water to the topmost leaves, which may mean to the remarkable heights of some trees.

2. The epidermal and cortical cells of the root are joined into one unit, the symplast, through their plasmodesmata.

3. The cell walls and intercellular spaces of the root cells constitute its apoplast.

4. Water moves from the soil through the root cortex via both the apoplast and symplast (Figure 38.4).

1. Because water molecules adhere to the wall of a tube by forming weak bonds with it, they tend to climb the wall slightly.

2. Since water molecules also cohere to one another through hydrogen bonds, those molecules that move up the wall pull others behind them, making a curved surface (meniscus).

3. The thinner the tube, the higher the water can rise against gravity.

4. The Casparian strip, a layer of waterproof suberin, completely blocks the intercellular spaces of the endodermis (Figures 38.6 and 38.7).

1. Root pressures appear in most plants if there is adequate moisture in the soil and the humidity is high so that transpiration is low.

2. Root pressures do not occur in conifers.

1. As long as the stomata are open, leaves continually transpire water, which moves from the mesophyll cells in the middle layer of a leaf into the air spaces and out through the stomata (Figure 38.8).

2. A xylem column contains an unbroken column of water, held together by cohesion between its molecules.

3. The movement of each water molecule into the mesophyll stretches the column slightly, creating a tension throughout the column that ultimately pulls more water into the roots.

1. Since water adheres to the walls of a tube, a layer of water adjacent to the walls is somewhat bound to the sides and is less mobile than water in the middle of the tube.

2. The area of the tube increases with the square of its radius, so a slight increase in radius means a relatively large increase in the amount of unbound water that is free to move.

1. Vessels are common only in angiosperms.

2. Among ferns, gymnosperms, and the more primitive vascular plants, tracheids are the rule, with only a few genera having vessels.

1. Since considerable material may be stored in the roots and in parenchyma throughout the xylem and phloem, phloem may also conduct sugar from these reserves throughout the shoot.

2. Although glucose is a primary product of photosynthesis, about 90 percent of the material dissolved in the sap is sucrose, a disaccharide of glucose and fructose; the remaining 10 percent consists of amino acids and other organic compounds.

3. The sap carries along all kinds of material that cannot be transported by specific pumps, including virus particles.

4. The nutrients of the sap enter the phloem at sources (or exporters), primarily the leaves where most photosynthesis occurs, and are removed at sinks (or importers), where they are consumed or stored.

1. A mature leaf is always a source. A growing leaf is a sink at first, switches over to being a source when it is about half grown, and never becomes a sink again.

2. An active sink is fed by its nearest source. Thus, a growing fruit is fed by the nearest leaves.

3. Upper leaves feed the growing meristems of the top branches; leaves near the bottom of the plant feed the roots and lower parts of the stem; and leaves in between feed either or both, depending on the demands of the sinks.

4. If a source or sink is removed, the plant quickly compensates for the loss by changing its flow pattern.

1. The most generally satisfactory explanation is the pressure-flow model (Figure 38.9).

2. A simple demonstration of pressure-flow movement can be performed with glass tubing, as shown in Figure 38.9 and 38.10.

1. Morning glory flowers open each morning to show their colors and then fold up again every night.

2. The sensitive plant, Mimosa pudica, is famous for reacting to touch (Figure 38.11).

3. Common three-leaved clovers open their leaflets in the day and close them at night, on a regular diurnal cycle.

4. The open leaves of a Venus Flytrap close quickly when an insect wanders onto them.

5. The opening and closing of stomata is a kind of nastic movement effected by guard cells.

6. In other cases, the effective organs are generally called pulvini (Figure 38.12).

7. A typical pulvinus contains large parenchymal cells, which swell or shrink as they gain or lose water ( Figure 38.13).

1. Plants generally show phototropism, which is growth in response to light.

2. Plants also show gravitropism, which is growth upward or downward in response to gravity.

3. Many plants grow around or along an object they touch (thigmotropism) so as to hold onto a support such as a wall, a wire, or another plant.

1. Because of the properties of their plasma membranes, all cells maintain an electrical potential across the membrane, and in these plant cells the cytoplasm is about 200 millivolts more negative than the surrounding apoplast.

2. Stimulation somehow induces an action potential in a cell, a transient change in the membrane potential (Figure 38.14).

1. The challenge for a terrestrial plant is to perform these processes all at once and to balance them in the face of various environmental influences.

2. The concentration gradient of water vapor is about a hundred times that of CO2, meaning that water can diffuse out of a leaf a hundred times faster than CO2 can diffuse in when the stomata are open (Figure 38.15).

3. Transpiration can occur about a hundred times faster than photosynthesis.

1. Just as a plant received infrared radiation from everything around it, so it radiates away some of its own energy.

2. Convection transfers heat in bulk through air currents (Figure 38.17).

3. Every object is surrounded by a boundary layer, a layer of gas or liquid whose composition and temperature are influenced by the object (Figure 38.18).

4. The leaf's boundary layer is a thin band of air that stays in contact with the leaf surface long enough to be warmed and then moves away.

5. The water loss is transpiration , of course, and it also carries heat away from the plant since a liquid is always cooled as some of it evaporates.

1. As the temperature rises, the ratio of transpiration to net radiation rises dramatically, so above 35û C, transpiration can actually keep a leaf cooler than the surrounding air.

2. The rate of heat loss through convection falls sharply at higher temperatures.

3. Wind velocity has enormous influences on cooling.

4. The rate of transpiration is inversely proportional to the thickness of the boundary layer, so transpiration increases with increasing wind velocity.

1. The boundary layer is thinnest at the leaf edge, especially at the windward edge.

2. A large leaf, with a low ratio of edge to surface, has a heavier boundary layer and is therefore cooled more slowly and loses water more slowly than a small leaf.

3. Heat and water balance have probably been major determinants in the evolution of leaf shape (Figure 38.19).

1. Each plant species is adapted to a range of light, and we broadly distinguish shade plants from sun plants.

2. A plant adapted to deep shade will outcompete others in its light range, and in brighter areas it will be outcompeted by other plants.

3. A few plants, classified as "intermediate," are adapted to partial shade.

1. The photosynthetic rate increases with light intensity, and the intensity at which CO2 production just balances its consumption is called the light compensation point.

2. As the temperature rises, the rate of respiration increases faster than that of photosynthesis, so the compensation point also increases with increasing temperature. At higher temperatures, a plant needs more light to keep up with respiration.

3. Compensation points of shade plants are lower than those of sun plants.

4. Shade plants require less light than sun plants to produce an excess of organic materials.

1. The majority of plant species live in optimal environments with regard to water availability.

2. These plants have enough water around their roots to ensure adequate turgor pressure in their cells.

3. Mesophytes can afford large losses of water through their stomata, even though some of their adaptations keep it to a minimum.

1. One xeromorphic adaptation is the crassulacean acid metabolism (CAM) mechanism of photosynthesis that is common among succulent desert plants.

2. Xerophytes commonly have thick layers of epidermis and heavy wax on the cuticle.

3. The cell walls may be rigid, thus preventing cell collapse during times of water deficiency, and they may have rubbery, shiny leaves that don't look wilted even when they are getting dry.

4. Cactus spines are the ultimate xeromorphic leaf adaptation.

1. Halophytes include representatives from many plant groups, including marine algae, as well as many protists and chromists.

2. Considerable research is being directed toward understanding these adaptations in the hope of developing salt-resistant agricultural strains.

1. Salt surrounding the roots threatens to desiccate the plant by causing osmosis of water out of the roots rather than into them.

2. High salt concentrations threaten the plant's metabolic machinery by inhibiting most enzymes and disrupting membranes, so either the plant must exclude salt, or its enzymes and structures must be modified to resist it.

1. In nonhalophytes, the principal cation of the cytoplasm is K+; such plants don't require sodium and generally exclude it.

2. Many plants, not just halophytes, accumulate excess sodium in their leaves and then drop the leaves to eliminate the salt.

3. Other plants do still better by having salt glands in their leaves that continually excrete salt by active transport.