1. Plasmodesmata run through cell walls to link adjacent cytoplasms into a single, much larger cytoplasm.

2. The middle lamella is composed largely of pectins, and it is the first part of the cell wall to develop.

3. Plant cells lay down cellulose inside the middle lamella to form a primary cell wall.

4. Each plasma membrane contains rosettes made of six molecules of cellulose synthase (Figure 37.2).

5. Cells that become highly elongated, like some xylem elements, orient these microfibrils circularly, perpendicular to the long axis, while in cells that grow more uniformly, such as parenchyma, the fibrils run in all directions (Figure 37.3).

6. Some cells have an additional secondary cell wall, located inside the primary wall, which is usually thicker and is strengthened with lignin (Figure 37.4).

1. A vacuole is set off from the rest of the cell by a membrane called the tonoplast.

2. The tonoplast helps control ion concentrations in the cytosol.

3. Plants grow considerably due to elongation of cells after they have divided, rather than by cell division alone. By accumulating water in the vacuole, a cell builds up internal pressure, which can force it to elongate (Figure 37.6).

1. Chloroplasts are the sites of photosynthesis and are very common in plant cells.

2. Chromoplasts are plastids that are filled with carotenoid pigments and give color to fruits, such as tomatoes and peppers.

3. Leucoplasts are colorless bodies where starch or oils are synthesized.

4. Amyloplasts are plastids that accumulate large amounts of starch. Some amyloplasts are located in the root cap and play a role in detecting the force of gravity.

1. Spherosomes are rich in lipids and are involved in lipid storage and transport.

2. Peroxisomes are the sites where photorespiration occurs.

3. Glyoxysomes are bodies where lipid reserves are converted into carbohydrates.

1. Parenchyma cells have thin primary cell walls.

2. Chlorenchyma is parenchyma with abundant chloroplasts, specialized for photosynthesis and production of the carbohydrates needed by the rest of the plant.

3. Aerenchyma is parenchyma characterized by extensive intercellular spaces, allowing the construction of large but lightweight tissues in which gases can exchange easily.

4. Some parenchyma is specialized for secretion of nectar or gelatinous material around roots.

5. Some parenchyma tissues are meristems, which are stem cells that retain the ability to grow and divide.

1. Along with sclerenchyma, collenchyma is one of the two mechanical tissues of plants.

2. Collenchyma cell walls may be quite uniformly thickened by the deposition of cellulose layers, pectins, and water or thickened mostly at the angles where cells meet.

1. Sclerenchyma is either conducting or nonconducting.

2. Conducting sclerenchyma forms the xylem elements that carry water throughout the plant.

3. Nonconducting sclerenchyma includes elongated fibers and more rounded sclereids.

1. Tracheids are elongated cells with no cytoplasm, whose tapered ends fit together closely; where tracheid cells meet, water passes between them through bordered pits in the cell walls (Figure 37.8).

2. Vessels are tubes formed by a series of cells called vessel elements, which have perforated end-walls and are laid end-to-end.

1. Sieve-tube members join end-to-end to form sieve tubes; two members meet and communicate at sieve plates.

2. The conducting cells of phloem retain their cytoplasm and as they mature, they lose their nuclei but stay attached to nucleated companion cells.

1. Root epidermis is generally the plant's source of water and is highly permeable to water and even has ionic pumps for taking water up actively.

2. Shoot epidermis is covered with the lipid polymer cutin because it must be waterproof to prevent evaporation.

1. The only light that plants can use for photosynthesis fall in the blue and red areas of the visible spectrum.

2. Ultraviolet radiation can bleach chloroplasts, destroying their ability to photosynthesize.

3. Infrared radiation can overheat a plant, inactivating its enzymes.

4. Plant epidermis, plus very reflective cutin, can reflect much of the damaging radiation and protect tissues underneath.

1. No organism is known to have enzymes that can digest cutin, which can be found over the epidermis of many plants.

2. Epidermis may protect against fungi and bacteria simply by virtue of its smoothness, which allows the spores to slide off.

3. Some plants combine stinging hairs with toxins to repel insects and other animals.

1. A mature, viable seed that has not been stimulated to develop is in a state of dormancy.

2. Dormancy ends with germination, when the embryo plant begins to grow.

3. Factors such as temperature, light, and amount of moisture control the beginning of germination.

1. Cotyledons can be photosynthetic, but their primary function is to store food that nourishes the young plant.

1. Grasses such as wheat, barley, corn, and rye are all monocots.

2. Rushes, sedges, cattails, and duckweeds are all monocots.

3. Palms and flowers like orchids, lilies, and irises are all monocots.

1. At least three-quarters of all flowering plants are dicots, including most deciduous trees and an enormous list of shrubs and other plants.

2. Figure 37.13 summarizes the principal differences in organization between monocots and dicots.

1. A plant develops secondary meristems, where particular organs develop.

2. Secondary meristems are responsible for meristematic layers called cambia, where stems, roots, or branches grow in diameter.

1. Protoderm produces epidermis.

2. Procambium produces primary vascular tissues (xylem and phloem).

1. Many perennials store food in their roots in the form of carbohydrates.

2. Roots are also the sole or primary sources of certain plant hormones.

1. Most monocots have a highly branched fibrous root system made of many fibers of about the same diameter.

2. Gymnosperms and dicots form one or more heavy taproots that sprout much smaller, secondary roots.

3. Plants sometimes produce adventitious roots, which arise from any organ that is not a root.

4. Prop roots or crown roots are the most obvious example of adventitious roots.

1. The stele is the center of a dicot root. It is made of vascular tissue.

2. Monocots may have parenchyma forming a pith within the stele.

3. The stele is surrounded by a layer of parenchyma cells called the pericycle. The pericycle is a meristematic layer that produces secondary roots.

4. Endodermis, the innermost layer of cortex just outside the pericycle, is made of specialized cells that regulate the transport of water and dissolved materials into the vascular cylinder.

1. The expendable cells of the root cap slough off as the root grows.

2. The root cap, by a poorly understood mechanism, detects gravity and directs the growth of the root in response.

1. A cell originates in the root apical meristem immediately behind the root cap, a zone of rapidly dividing, undifferentiated cells that produce new root tissue as they proliferate.

2. In a zone behind the meristem, slightly older cells are elongating, growing somewhat wider and enormously longer.

3. As the root cells achieve their full length, they differentiate into mature types such as epidermis, cortex or vascular cylinder.

4. Protoderm cells on the outside will become epidermal tissue; the ground meristem inside this will become root cortex; and the central procambium will develop into xylem and phloem.

1. At its tip is a shoot apical meristem with its young, rapidly dividing cells pushing upward and leaving mature cells behind.

2. At intervals, the upwardly growing shoot forms groups of meristematic cells called nodes.

3. The lengths of stem between the nodes are internodes.

1. The cortex, made primarily of parenchyma, may be quite a narrow region in some dicots, but in many herbaceous monocots, it comprises the bulk of the stem.

2. Xylem and phloem are combined in vascular bundles, with the xylem of each bundle facing the center of the stem and the phloem facing outward.

3. Monocots have vascular bundles dispersed through the stem (Figure 37.19).

4. Gymnosperms and most dicots have vascular bundles set in a circle around the perimeter of the stem (Figure 37.19).

5. In a dicot stem, the parenchyma ground tissue outside the circle is then known as cortex, and that on the inside as pith.

1. As the vascular bundles pass the nodes, subsidiary bundles called leaf traces leave them and feed into the leaves.

2. When a plant grows branches, branch traces grow to serve them.

3. The branching pattern is highly variable and sometimes rather complicated as axial bundles merge with one another or as a branch from one bundle leads into another (Figure 37.20).

1. The blade or lamina is the flattened part of the leaf.

2. Vascular bundles in leaves are veins which form very unique patterns.

3. Elongated conifer leaves (e.g. pine needles) have only one or two unbranched veins which extend their whole length.

4. Most monocots have a series of roughly parallel veins running from the base of the blade to its tip.

5. Dicots have a complex branching or featherlike pattern in which smaller veins arise from larger ones.

6. Veins converge at the base of the leaf, run through the petiole, the stalk that connects the leaf to the stem, and then join the leaf traces connecting to the stem's vascular bundles.

1. Leaves may branch off alternately along the stem, spirally, or in opposite pairs.

2. Trees such as ashes and hickories have compound leaves where the whole leaf is a symmetrical arrangement of leaflets along a central vascular strand.

3. A true leaf can be identified by the axillary bud where its petiole joins the stem.

1. Most of the leaf interior is mesophyll, a parenchyma tissue rich in chloroplasts where most photosynthesis occurs.

2. Most dicot leaves have palisade mesophyll near the upper epidermis and spongy mesophyll below (Figure 37.22).

1. In all gymnosperms and many dicots, a layer of permanently meristematic vascular cambium remains between the primary xylem and phloem.

2. Plants grow into woody trees and shrubs as both their stems and roots enlarge in secondary growth through division of the vascular cambium (Figure 37.23).

1. Fusiform initals elongate axially, and their daughter cells develop into the elongated elements of xylem and phloem: the tracheary and sieve elements, companion cells, and fibers.

2. Ray initials develop into rays that run radially through secondary xylem and secondary phloem for radial transport.

1. Periderm is a mixture of tissues formed when some cells of the epidermis, cortex, and secondary phloem acquire a renewed ability to divide and form a layer of cork cambium, or phellogen.

2. The periderm is a combination of cork cambium and cork.

3. The combination of all the exterior tissues–periderm, secondary phloem, and bits of other primary tissues–forms bark.

1. Bark gives mechanical protection from fire, animals, disease, and many other potential threats.

2. Many trees shed their bark continually, thus also shedding various pathogens or animals that might be growing there (Figures 37.26 and 37.27).