Introduction
 Depending upon their age, function, and position within the plant, plant cells vary greatly in size, shape, inclusions, and in the thickness and chemical composition of the cell wall.  Although many plant cells are still alive at functional maturity, several very important types of cells are dead when functionally mature.  The following is a concise outline of the major plant cell and tissue types that you will be studying in this lab and many later labs (for more detail see Table on p. 586 of text):

Simple Tissues (composed of one cell type):

1.  Parenchyma -   thin 1  cell wall only; alive and   physiologically active (Figs.    3-8, 24-5)

2.  Collenchyma -   unevenly thickened 1  cell wall only; alive but not active (Figs. 24-7 & 24-8)

3.  Sclerenchyma – thick 2  cell wall; usually dead at functional maturity

a) fiber- narrow & elongate (Figs. 24-9 & 24-13f)

b) sclereid – isodiametric to irregular (Figs. 24-10, 24-11)

Complex Tissues (more than 1 cell type, often including cell types described above  ):

1.  Xylem – the H O transporting xylem cells (the traheary elements, a & b below) have a thick 2  cell wall and are dead at functional maturity

a) tracheid – elongate & tapered with broadly overlapping ends; pits or larger gaps present in the 2  cell wall (Fig. 24-13d)

b) vessel member – shorter, wider, & less or not tapered; pits & perforations both present (Fig. 24-13a-b)

c) other – in addition to the functional (water transporting) xylem cells, any of the cell types listed above under simple tissues may also occur in the xylem tissue, parenchyma & fibers are the most common

2.  Phloem – the photosynthate translocating phloem cells (a & b below) have only a 1  cell wall [although sometimes somewhat thickened] & are alive at the functional maturity, but have a greatly reduced protoplast; sieve areas (areas of large plasmodesmata) present in the cell wall.

a) sieve cell – elongate & tapered with broadly overlapping ends; sieve areas mostly scattered (Fig. 24-12)
b) sieve tube member – shorter & less or not tapered; sieve areas localized on end walls as sieve plates (Figs. 24-18, 24-19, & 24-20)

c) other – in addition to the functional (‘food’ translocating) phloem cells, any of the cell types listed above under simple tissues may also occur in the phloem tissue; parenchyma & fibers are especially common

Plant Cells and Their Inclusions:

 In this lab we will study plant cell types found in simple tissues and the various inclusions visible with a light microscope.  For the most part, microscopic study of the complex vascular tissues xylem and phloem requires the use of specially stained and sectioned materials on prepared microscope slides.  Xylem and phloem will therefore be studied in later labs, as we begin looking specifically at the anatomy of the major plant organs.

A.  Plastids and Other Inclusions of Living Plant Cells

 Most of the inclusions we want to observe are characteristic of living, physiologically active cells.  Therefore, keep in mind that most of the cells you will study in this section are parenchyma cells.

 1)  Chloroplasts-

 Mount one leaf from near the stem tip of an actively growing Elodea in a drop of water under a coverslip (Fig. 4-10).  Identify, where possible, the following:  cell wall, cytoplasm, vacuole, nucleus, chloroplasts

  Can each of the above be observed in all the cells?

  Do adjacent cells share cell walls?

 Find an area of the leaf where the chloroplasts can be seen to be moving.
 
  What causes the chloroplasts to move?

  Do the chloroplasts move from cell to cell?
         2) Chromoplasts-

 Another type of pigmented plastid is the chromoplasts, which contain red, orange or yellow carotenoid pigments.  These, along with the anthocyanins and betacyanins discussed below, are responsible for most of the various colors found in flowers, fruits, leaves, and other plant parts (e.g., the carrot root).  Make wet mounts of the following materials and for each determine the shape and color of the chromoplasts as well as their distribution within the cells.

  Red pepper flesh:      Carrot root:

3) Leucoplasts-

Section a small piece of potato tuber and make a wet mount, adding also a small drop of iodine.  Identify the cell walls, and numerous starch grains.  Observe both unstained and stained (blue-black by the iodine) starch grains.  Change to high power and find a leucoplast.

What is the relationship between leucoplasts and starch grains?

Why do starch grains appear with concentric rings?

Following the instructions provided, peel a portion of the lower epidermis from a leaf of the purple Wandering Jew (Zebrina pendula) and make a wet mount with coverslip.

Observe a portion where folding or underlying mesophyll does not obscure detail.  Note the small spheroid leucoplasts clustered around the nuclei of some of the cells (compare Fig. 3-13).  Save this slide preparation for the next exercise.

What is the function of leucoplasts?

4) Vacuoles and Water Soluble Pigments-

 Anthocyanins and betacyanins are pigments (usually purplish but occasionally reddish or yellowish) which, unlike the chromoplast-bound carotenoids, are water soluble and are found in the cell sap of the vacuole.  In most mature parenchyma cells, the vacuole occupies most of the volume of the cell.  Using the slide of Zebrina epidermis prepared above, observe the anthocyanins concentrated in the cell vacuole.

  Is it easy to distinguish the vacuole form the cytoplasm?

  Are other organelles visible (nuclei, etc.)?

B.  Plant Cell Types (see summary page above  )

 1) Parenchyma Cells-

 This cell type is alive at functional maturity, usually thin-walled (with primary cell walls only), and often physiologically very active.  Although often more or less isodiametric, they vary to irregular or elongate in shape.  When mature the vacuole often nearly fills the cell.

 As indicated previously, virtually all the cells observed in the exercises of part 1 above are parenchyma cells.  Notice that the types and numbers of inclusions can vary dramatically depending on the particular function of the parenchyma cell (e.g., photosynthesis vs. food storage).

 Look for these cells in the Zebrina leaf.  Examine the prepared slide of a root cross section for parenchyma cells.

 2) Collenchyma Cells-

 These are elongate cells (this is not obvious of course in a cross section) which commonly serve a supportive (structural) role in herbaceous stems and petioles.  They have an unevenly thickened primary cell wall and are still alive at functional maturity although they are generally considered to be physiologically less active than most parenchyma cells.
 
 Make a thin cross section of a small piece of celery stalk (=petiole), and look for collenchyma cells near the periphery.  These bundles of collenchyma supporting tissue form the ‘strings’ that catch between your teeth when you eat celery.  Inward in the petiole, you can also see bundles of vascular tissue (xylem and phloem) which will be studied more closely in later labs.

 3) Sclerenchyma Cells-

 Sclerenchyma cells have thickened, lignified primary and secondary cell walls.  At functional maturity, the protoplast is dead and gone and in some cases the thick secondary cell wall may almost fill the cell lumen (the central cavity of the cell).  Two major types of sclerenchyma cell are usually recognized, the fibers which are elongate and tapered and sclereids which are isodiametric to irregular or somewhat elongate.

  a) Sclereids-

 Mount some of the fleshy tissue from a ripe pear fruit in a drop of phloroglucinol solution under a coverslip.  This solution will stain the lignin in the cell walls of the “stone cells” (sclereids) red.  Search for the clusters of these small, more or less isodiametric cells (Figs. 3-31 & 24-10).

 Make a wet mount of a pea seed coat and compare its cells to those of the pear “stone cells”.  What is the function of these sclereids?

  b) Fibers-

 Examine the prepared slide of macerated pine wood (red stained).  The elongated, tapered cells are of two types.  Those with few or no circular bordered pits are sclerenchyma fibers (structural) and those with circular bordered pits along their length are xylem tracheids (to be studied in greater detail in later labs).  (Fig. 24-13 shows both types)