What Do Pine Resin Mint Oil Rubber And Opium Have In Common

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Figure 4.8 How water flow is controlled in adjacent pairs of bordered pits. The pits are separated by a pit membrane consisting of the middle lamella and two thin layers of primary walls. A. Water moves relatively freely through the pit openings and pit membrane when the torus (a thickened region of the pit membrane) is in the center. B. If the flexible pit membrane swings to one side so that the torus blocks an opening, water movement through the pit pair is restricted.

increase absorptive parts of roots. Accordingly, the epidermis and tissues with secretory cells are discussed in this section.

Periderm, which comprises the outer bark of woody plants, consists mostly of cork cells, but it is included in this discussion because it contains pockets of parenchyma-like cells.

Xyh em

Xylem tissue is an important component of the "plumbing" and storage systems of a plant and is the chief conducting tissue throughout all organs for water and minerals absorbed by the roots. Xylem consists of a combination of parenchyma cells, fibers, vessels, tracheids, and ray cells (Fig. 4.6). Vessels are long tubes made up of individual cells called vessel elements that are open at each end, with barlike strips of wall material extending across the open areas in some instances. The tubes are formed as a result of the cells being joined end to end.

Tracheids, which, like vessel elements, are dead at maturity and have relatively thick secondary cell walls, are tapered at each end, the ends overlapping with those of other tra-cheids. Tracheids have no openings similar to those of vessels, but there are usually pairs of pits present wherever two tracheids are in contact with one another (Fig. 4.7). Pits are

Pit Pairs Tracheids
Figure 4. Q Spiral thickenings on the inside wall of a tra-cheid, x400.

areas in which no secondary wall material has been deposited and, as indicated in Chapter 3, they allow water to pass from cell to cell. Figure 4.8 illustrates how pit pairs function in regulating the passage of water between adjacent cells.

In cone-bearing trees and certain other non-flowering plants, the xylem is composed almost entirely of tracheids. The walls of many tracheids have spiral thickenings on them that are easily seen with the light microscope (Fig. 4.9). Most conduction is up and down, but some is lateral (sideways). The lateral conduction takes place in the rays. Ray cells, which also function in food storage, are actually long-lived parenchyma cells that are produced in horizontal rows by special ray initials of the vascular cambium. In woody plants, the rays radiate out from the center of stems and roots like the spokes of a wheel (see Figs. 6.6 and 6.8).


Phloem tissue (Fig. 4.10), which conducts dissolved food materials (primarily sugars) produced by photosynthesis throughout the plant, is composed mostly of two types of

Chapter 4

Phloem Parenchyma

phloem parenchyma sieve plate sieve tube member companion cell phloem parenchyma

Sieve Tube Members

Figure 4.10 Longitudinal view of part of the phloem of a black locust tree (Robinia pseudo-acacia), x1,000.

cells without secondary walls. The relatively large, more or less cylindrical sieve tube members have narrower, more tapered companion cells closely associated with them. Phloem is derived from the parent cells of the cambium, which also produce xylem cells; it often also includes fibers, parenchyma, and ray cells. Sieve tube members, like vessel elements, are laid end to end, forming sieve tubes. Unlike vessel elements, however, the end walls have no large openings; instead, the walls are full of small pores through which the cytoplasm extends from cell to cell. The porous regions of sieve tube members are called sieve plates.

Sieve tube members have no nuclei at maturity, despite the fact that their cytoplasm is very active in the conduction of food materials in solution throughout the plant. Apparently, the adjacent companion cells form a very close relationship with the sieve tubes next to them and function in some manner that brings about the conduction of the food.

Living sieve tube members contain a polymer called callose that stays in solution as long as the cell contents are under pressure. If an insect such as an aphid injures a cell, however, the pressure drops, and the callose precipitates. The callose and a phloem protein are then carried to the nearest sieve plate where they form a callus plug that prevents leaking of the sieve tube contents.

Sieve cells, which are found in ferns and cone-bearing trees, are similar to sieve tube members but tend to overlap at their ends rather than form continuous tubes. Like sieve tube members, they have no nuclei at maturity, and they also have no adjacent companion cells. They do have adjacent albuminous cells, which are equivalent to companion cells and apparently function in the same manner.


The outermost layer of cells of all young plant organs is called the epidermis. Since it is in direct contact with the environment, it is subject to modification by the environment and often includes several different kinds of cells. The epidermis is usually one cell thick, but a few plants produce aerial roots called velamen roots (e.g., orchids) in which the epidermis may be several cells thick, with the outer cells functioning something like a sponge. Such a multiple-layered epidermis also occurs in the leaves of some tropical figs and members of the Pepper Family (Piperaceae), where it protects a plant from desiccation.

Most epidermal cells secrete a fatty substance called cutin within and on the surface of the outer walls. Cutin forms a protective layer called the cuticle (Fig. 4.11). The thickness of the cuticle (or, more importantly, wax secreted on top of the cuticle by the epidermis) to a large extent determines how much water is lost through the cell walls by evaporation. The cuticle is also exceptionally resistant to bacteria and other disease organisms and has been recovered from fossil plants millions of years old. The waxes deposited on the cuticle in a number of plants (see Fig. 7.7) apparently reach the surface through microscopic

Tissues 61

€co LogicaL Review

The first point of contact between plants and the environment is a surface layer of cells called the epidermis. The structure of the epidermis, particularly the thickness of waxes on its surface, determines the potential rate of water exchange between a plant and the environment. The resistance of the epidermis to water loss is generally higher in the plants of arid environments. A second critical ecological function of the epidermis is a barrier to attack by pathogens. Pathogens, which may restrict the distribution of plant species and strongly influence plant population size, are an important part of a plant's biological environment. The extent of development of a plant's xylem and sclerenchyma cells also is related to the plant's environment, with, for example, aquatic plants having weakly developed xylem, large trees having well-developed xylem, and fire-resistant trees, such as redwoods, having thick bark.

channels in the cell walls. The susceptibility of a plant to herbicides may depend on the thickness of these wax layers. Some wax deposits are extensive enough to have commercial value. Carnauba wax, for example, is deposited on the leaves of the wax palm. It and other waxes are harvested for use in polishes and phonograph records. In colonial times, a wax obtained from boiling leaves and fruits of the wax myrtle was used to make bayberry candles.

In leaves, the epidermal cell walls perpendicular to the surface often assume bizarre shapes that, under the microscope, give them the appearance of pieces of a jigsaw puzzle. Epidermal cells of roots produce tubular extensions called root hairs (see Fig. 5.4) a short distance behind the growing tips. The root hairs greatly increase the absorptive area of the surface.

Hairs of a different nature occur on the epidermis of above-ground parts of plants. These hairs form outgrowths consisting of one to several cells (Fig. 4.12). Leaves also have numerous small pores, the stomata, bordered by pairs of specialized epidermal cells called guard cells (see Figs. 7.8 and 9.13). Guard cells differ in shape from other epidermal cells; they also differ in that chloroplasts are present within them. The stomatal apparatus is discussed in Chapters 7 and 9. Some epidermal cells may be modified as glands

Transverse Section Lilium Leaf
Figure 4.11 A portion of a cross section of a kaffir lily (Clivia) leaf, showing the thick cuticle secreted by the epidermis, x1000.

Chapter 4

Chapter 4

Pictures Common Mint Plant Pests
Figure 4.12 Hairs on the surface of an ornamental mint plant, x50.

that secrete protective or other substances, or modified as hairs that either reduce water loss or repel insects and animals that might otherwise consume them (Fig. 4.13).


In woody plants, the epidermis is sloughed off and replaced by a periderm after the cork cambium begins producing new tissues that increase the girth of the stem or root. The periderm constitutes the outer bark and is primarily composed of somewhat rectangular and boxlike cork cells, which are dead at maturity (see Fig. 4.14). While the cytoplasm of cork cells is still functioning, it secretes a fatty substance, suberin, into the walls. This makes cork cells waterproof and helps them protect the phloem and other tissues beneath the bark from drying out, mechanical injury, and freezing temperatures. Some cork tissues, such as those produced by the cork oak, are harvested commercially and are used for bottle corks and in the manufacture of linoleum and gaskets.

Some parts of a cork cambium form pockets of loosely arranged parenchyma cells that are not impregnated with suberin. These pockets of tissue protrude through the surface of the periderm; they are called lenticels (Fig. 4.14) and function in gas exchange between the air and the interior of the stem. The fissures in the bark of trees have lenticels at their bases. The various tissues discussed are shown as they occur in a woody stem in Figure 6.6.

Stem Piperaceae Vascular Cambrium
Figure 4.13A Tack-shaped glands and epidermal hairs of various sizes on the surface of flower bracts of a western tarweed. Scanning electron micrograph ca. x 200.
Stoma Opium

Figure 4.13B Hairs on the surface of a tomato plant stem. Note the raised stoma to the right of center. Scanning electron micrograph ca. x300. (A. Courtesy Robert L. Carr; B. Courtesy Dr. Tahany H. I. Sherif)

Secretory Cells and Tissues

All cells secrete certain substances that can damage the cytoplasm, if allowed to accumulate internally. Such materials either must be isolated from the cytoplasm of the cells in which they originate or moved outside of the plant body. Often, the substances consist of waste products that are of no further use to the plant, but some substances, such as nectar, perfumes, and plant hormones (discussed in Chapter 11), are vital to normal plant functions.

Tissues 63

Periderm Tissue

h cork J cells

Figure 4.14 Periderm and a lenticel. A cross section through a small portion of elderberry (Sambucus) periderm, showing a large lenticel, ca. x 250.

h cork J cells

Figure 4.14 Periderm and a lenticel. A cross section through a small portion of elderberry (Sambucus) periderm, showing a large lenticel, ca. x 250.

Secretory cells may function individually or as part of a secretory tissue. Secretory cells or tissues, which often are derived from parenchyma, can occur in a wide variety of places in a plant. Among the most common secretory tissues are those that secrete nectar in flowers; oils in citrus, mint, and many other leaves; mucilage in the glandular hairs of sundews and other insect-trapping plants; latex in members of several plant families, such as the Spurge Family; and resins in coniferous plants, such as pine trees. Latex and resins are usually secreted by cells lining tubelike ducts that form networks throughout certain plant species (see Fig. 6.11). Some plant secretions, such as pine resin, rubber, mint oil, and opium, have considerable commercial value.

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  • alicia millar
    What do pine resin, mint oil, rubber, and opium have in common?
    8 years ago
  • balbo
    Do mint have cuticle mint?
    8 years ago
  • Nerea
    Do pine have lenticels?
    8 years ago
  • Arianna
    Do mint plants repel bugs?
    7 years ago
  • emppu
    What are resins, oil, rubber are they by product of photosynthesis?
    3 years ago

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