Plant Tissue Slides

UNIT 4: PLANT TISSUES

Plant tissues can be divided into two major categories: meristematic (perpetually young tissues important in growth) and permanent tissues. The permanent tissues fall into three subcategories: dermal, ground, and vascular tissues. Tissues composed of one type of cell are called simple tissues; those composed of two or more cell types are called complex tissues. Parenchyma, collenchyma, and sclerenchyma are simple tissues; xylem, phloem, and epidermis are complex tissues.

A set of slides has been set up for you here on the course website to help visualize many of the concepts on plant growth. Use the following notes and pages 744-754 in your text as guidelines to analyzing the slides.

I. Meristematic Tissue

Meristematic tissues are composed of embryonic, undifferentiated (i.e., unspecialized) cells that play a crucial role in plant growth. In plants, most of the cell division occurs in meristematic cells that are restricted to particular regions of the plant body. Apical meristems, found at the growing tips of stems, roots, and in lateral buds, are responsible for the sustained increase in length of the plant body. The tissues produced by the apical meristems are said to be primary tissues. In many plants there are also meristematic areas towards the periphery of the roots and stems; these lateral meristems (the vascular cambium and cork cambium) are responsible for the sustained increase in diameter. In both the apical and lateral meristems, certain cells are able to divide repeatedly; after each division one of the daughter cells remains in the meristem while the other becomes part of the plant body. The tissues produced by the lateral meristems are said to be secondary tissues.

Slide 1: a longitudinal section through a growing shoot tip showing apical meristematic tissue. Note that the cells are small, have dense cytoplasm, and are very tightly packed.
Slide 2: a longitudinal section through a root tip. The meristematic tissue is located just above the root cap. This too is apical meristem; division of these cells followed by cell elongation results in the root growing in length.
Slide 3: is primarily for orientation; it is a cross section of a eudicot stem. Focus on the two large vascular bundles in the center of the slide. The xylem tissue is stained red. Just above the xylem is a layer of meristematic tissue, the vascular cambium. The phloem tissue is found outside of the vascular cambium.
Slide 4: is a high-power view of a cross-section showing a lateral meristem, the vascular cambium, in the same plant shown in Slide 3. Again, the xylem tissue is stained red, and the large cells on the top of the slide are phloem. The green brick-like cells between the xylem and phloem is the area in which the vascular cambium is located. The new cells produced by the cambium are initially like those of the cambium itself, but, as they grow and mature, their characteristics slowly change as they differentiate into other tissues. The vascular cambium is a single layer of cells within this brick like region; it is responsible for the growth in diameter of a stem. The tissues produced by the vascular cambium are secondary tissues.

II. Permanent Tissue

A. Dermal (Surface) Tissues
Slide 5: shows a face view of the undersurface of the epidermis of a leaf. Most epidermal cells are relatively flat and usually the epidermis is only one layer thick. Here the cells are irregularly shaped and lock together like pieces of a puzzle, leaving no intercellular spaces. Such tight connections aid in protection against loss of water, mechanical injury, and invasion by parasitic fungi. Notice the three stomatal openings and their associated guard cells.
Slide 6: Many epidermal cells secrete a waxy, water-resistant cuticle on their aerial parts, which minimizes water loss. Here the cuticle is stained a pale pink. It is unusually thick in this specimen as this is a desert plant.
Slide 7: Often epidermal cells are modified to form hairlike structures and glandular hairs. These are epidermal hairs on the surface of a plant stem. Such hairs may provide protection against insects. In some plants the hairs are hooked and actually impale insects and larvae.
Slide 8: is a high-power view of one glandular hair. Secretory hairs may provide a chemical defense against insects.
Slide 9: is of another type of dermal tissue, the outer bark or periderm (stained red in this slide). Periderm is found on the surface of woody plants; it includes the cork cells on the surface of older woody stems. The periderm replaces the epidermis in plants that have secondary growth. The cork cells are dead; it is their waterproofed cell walls that function as the protective outer covering of plants. Meristematic cells within the periderm (cork cambium, the other lateral meristem) produce the cork cells.
B. Ground Tissues
1. Parenchyma tissue. Parenchyma tissue occurs in roots, stems, and leaves. The cells usually have thin primary walls and no secondary walls. Ordinarily the cells are loosely packed with numerous intercellular spaces (spaces between the cells). These cells are living at maturity, and are capable of cell division. Parenchyma cells are the all-purpose plant cells; they are involved in photosynthesis, storage, secretion, movement of water, and transport of food depending on their location in the plant body. Parenchyma is by far the most common of the ground tissues.
Slide 10: shows a cross section of a monocot (left) and eudicot (right) stem. In a eudicot stem there are two areas of parenchyma tissue: the cortex and pith. The cortex is the area on the periphery of the stem, between the epidermis and the vascular bundles. The pith is the area in the center of the stem, inside the vascular bundles. In a monocot the vascular bundles are scattered throughout the stem so there is no clear distinction between pith and cortex. All of the tissue surrounding the vascular bundles is parenchyma. Parenchyma of stems and roots functions in the storage of nutrients and water. When turgid, parenchyma is important in giving shape and support to the plant.
Slide 11: Focus on the parenchyma cells in the pith (on the lower portion of the screen) of the eudicot stem. Note that the cells are loosely packed and have the thin (primary) cell walls. The parenchyma cells under the epidermis have chloroplasts and are photosynthetic.
Slide 12: is a cross section of a monocot root. The bright red cells in a circle make up the endodermis, a tissue which in roots separates the cortex from the vascular tissues. Inside the endodermis, the xylem and phloem alternate with one another. The very large cells are the xylem and the smaller green cells are phloem. Monocot roots have a pith composed of parenchyma tissue in the center of the root, inside the vascular ring, in addition to the parenchyma tissue in the cortex. Pith and cortex are very similar, both structurally and functionally, being distinguished from each other primarily by their location relative to the vascular tissue. What is the primary function of the parenchyma cells of the root?
2. Collenchyma tissue
Slide 13: is collenchyma tissue from a cross section of a stalk of celery. Collenchyma cells are typically elongated. The primary cell walls (stained light pink) are irregularly thickened, with the walls being their thickest at the "corners" of the cell. Collenchyma is an important supporting tissue in young plants when the cells are turgid. (They provide good support only when hydrated.) The cells are living at maturity.
3. Sclerenchyma Tissue
Slide 14: is a cross section of sclerenchyma tissue (fibers). Note the uniformly very thickened secondary cell walls that give support to the plant body. Here the walls are so thick that the lumen (internal space) of the cell is nearly obliterated. These cells are dead at functional maturity.
C. Vascular Tissue
Slide 15: is a longitudinal section of xylem showing the elongated conducting cells (vessels in this case) which transport water and inorganic materials from the roots to the leaves. These cells do not have cytoplasm at maturity; the nucleus and cytoplasm disintegrate during development, leaving the thick cell walls as the functional structures. Notice the pits (holes) in the secondary walls.
Slide 16: shows the second vascular tissue, phloem. The large, elongated sieve elements shown here are the actual conductive cells; they transport organic materials both up and down the plant body. Sieve elements are living cells, and very sensitive to manipulation. When this slide was made the cytoplasm and proteins contracted and accumulated at one end of the cell, forming a "plug" (the triangular reddish areas in the center of the slide). Note the sieve plates. We shall learn more about the sieve elements in Unit 5.

C. PRIMARY GROWTH OF THE PLANT BODY (Microscope Slides)

Once you have mastered the online slides of the various tissues, go to the microscopes in the study center and examine the four microscope slides, using the information provided below.

Slide 24: cross section of a dicot stem showing the vascular tissue arranged in a continuous cylinder. The red sclerenchyma cells at the top of the slide are sclerenchyma fibers of the phloem. Under the fibers lie the phloem parenchyma and the conductive phloem, the sieve tube members (stained green). Next are several layers of cells that mark the area of the vascular cambium. The xylem cells (stained red) lie below the cambium. Note the large lumens identifying some of these cells as vessel elements. The parenchyma cells in the center of the stem constitute the pith, where excretory products can be deposited.

Slide 25: cross section of a woody dicot stem in its first season of growth. Note that the xylem tissue is arranged in a continuous cylinder rather than in discrete bundles. The next slide shows the vascular tissues at a higher magnification.

Slide 26: a cross section of phloem tissue, including fibers, is at the top of the slide. The phloem fibers and the phloem cells nearest the fibers are primary phloem, those closest to the vascular cambium were produced by the cambium and are therefore secondary phloem. The vascular cambium lies at the junction where the staining changes from green and red to red. The xylem cells just below the cambium are produced by the cambium and are secondary xylem. The primary xylem can be easily located on this slide because the cells have very large lumens and are closer to the pith. Notice that the tissue on either side of the vascular cambium is secondary tissue produced by the cambium. Where would the ray initials of the cambium be located? ...the fusiform initials?

Slide 27: cross section of another dicot stem in its first year of growth. A ray system can be noted, extending throughout the secondary xylem and phloem. From outside in, locate the epidermis, cortex (with tightly packed collenchyma cells close to the epidermis and parenchyma cells inside), primary phloem, secondary phloem, vascular cambium, secondary xylem, and primary xylem. Note the vascular rays in the secondary xylem and secondary phloem. Vascular rays are radial strands of parenchyma cells that function as pathways for lateral transport of materials and as storage areas. Ray cells are produced by the ray initials in the vascular cambium.

Slide 28: high power view of a vascular ray. Locate the vascular cambium (green brick wall-like cells running from top to bottom). To the right of the cambium is the secondary xylem. To the left side is the secondary phloem. The vascular ray is funnel-shaped; it is several cell layers thick in the xylem, but expands greatly in the phloem. Some of the rays in secondary phloem will become very wide as the stem increases in girth; this is one way in which the tissues outside the cambium keep up with the increase in girth as new xylem cells are produced. Rays function in lateral transport; nutrients move through plasmodesmata from the secondary phloem through the vascular cambium to the living (parenchyma) cells of the secondary xylem. Although some water flows from the xylem to the phloem through the symplast of vascular rays, much of the water movement is through the apoplast system.

Slide 29: cross section of a woody stem in its second year of growth. The cork cambium has become active and produced the non-living cork cells on the outside of the stem. Underneath the periderm lies the phloem, the fibers of which are stained red. Inside the phloem lies the vascular cambium and internal to the cambium is the xylem. There is a small amount of pith in the center of the stem. Where would the primary phloem be located? ...the secondary phloem? ...the primary xylem? ...secondary xylem? Note the two annual rings showing that the stem is in its second year of growth. (See also Fig. 35.22, p. 754.)