Tisues - Parenchyma - Collenchyma & Sclerenchyma
This lab is designed to give you information on the primary nonvascular tissues. These are relatively simple compared to xylem and phloem. However, we will see that there is a considerable amount of variation within these tissues. In addition, you will observe the major components of the protoplast that are visible with the light microscope.
Study cell shape, contents, and wall structure, the relation of cells to one another for intact tissues, the presence of intercellular connections via pits, and the presence or absence of intercellular spaces. The cell walls and air spaces constitute the Apoplast. The Plasmalemma and all within it constitute the Symplast. These are Extremely Important concepts, which must be appreciated to understand Plant Physiology!
Within the Symplast, look for the cytoplasm, nuclei, chloroplasts, other plastids, crystals, and vacuoles colored with anthocyanins. Use polarizing filters to locate starch grains and crystals. Also use polarizers and stains to study cell wall organization and composition.
Cortex of Pereskia stem:
Parenchyma consists of relatively large, thin-walled cells. The cells are arranged loosely, that is, there are intercellular spaces among them. The protoplasts of these cells contain chloroplasts. Some of these cells may have amyloplasts and crystals. Pereskia is a member of the cactus family. It has spines but it also has normal leaves. Its flowers are extremely beautiful like those of most cactaceae.
Aerenchyma & Stellate Parenchyma
A strikingly different shape of parenchyma cells is illustrated by stellate parenchyma. These are branched and adjacent cells are connected with each other by means of the branches. Parenchyma composed of branched cells is highly lacunose; that is, it has a large volume of intercellular space. The spongy layer in leaves has branched cells with large intercellular spaces. The term Aerenchyma is often used to describe parenchyma, which has large air spaces.
Locate Stellate Parenchyma cells in petioles and midribs of Canna (aliiope) leaves. Cut hand sections and examine with a dissecting scope before observing with a compound microscope. Do these have 3D branching?
Examine the Parenchyma in Papyrus (Cyperus papyrus) stems by making transverse sections. Find the Aerenchyma with a dissecting scope and examine with a compound microscope. What is the shape of the individual cells which comprise the Aerenchyma? Are they branched in 2D or 3D?
Stain these with IKI and look for starch containing Amyloplasts
Aerenchyma in Cyperus javanicus
the demonstration slide of persimmon or palm (niu)
endosperm. This material will also show fine lines traversing the thick walls from cell lumen
to cell lumen. These lines are pits, which connect the symplast of adjacent
fresh sections of unroasted coffee "beans" or palm
fruits to observe the thick walls and their pits.
What is the basic component of cellulose?
Other than the typical function of cell walls what might the function of these thick walls be?
You have already observed cytoplasm and chloroplasts. Other protoplast components include several more types of plastids, vacuoles, and various kinds of crystals.
Chromoplasts and Pigment Bodies. They may be yellow, red, and orange colored plastids and similarly colored crystal-like bodies. The latter are called pigment bodies because there is some question whether they may be classified as plastids.
Observe chromoplasts and pigment bodies in free-hand sections of bell pepper fruits, various flower petals, and the root of carrot. Chloroplasts are Chromoplasts, as well!
Red Bell Pepper Fruit
The color of Alamanda Flowers is due to Chromoplasts
Amyloplasts are filled with starch, which sometimes occupies the entire organelle. They are also regarded as Leucoplasts because they lack color.
Observe thin free-hand sections of Papyrus and stain with IKI. We will have a demo of potato amyloplasts.
Observe an unstained specimen and use the polarizers.
View a stained slide and then use the polarizers.
Leucoplasts clustered around the Nucleus of a Parenchyma Cell stained with Toluidine Blue
Amyloplasts from Canna seen with normal illumination
Amyloplasts from Canna seen with crossed Polarizers
"Statoliths" are amyloplasts, which contain many large multifaceted starch grains, similar to those above. Their function may be related to gravity perception.
Make thin longitudinal sections of Hibiscus petioles or stems and stain with IKI. The statolith-containing cells occur just outside the vascular bundles. Their starch grains are much larger than other starch grains in the stem. Statoliths may also be seen in corn root tips.
Make slides of Rhoeo or Zebrina epidermis (see above for Leucoplasts). These demonstrate vacuoles, which contain anthocyanin.
The pigmentation in many flower petals, like Erithrina (wiliwili), is also contained in vacuoles. This is best observed by looking at fresh cross sections of the petals. How can you tell if the color is due to chromoplasts or vacuolar pigments?
Crystals are vacuolar in nature.
Observe star-like (Druses) in Begonia or Pereskia stems. Druse crystals are very common. The other commonly observed crystals are spear-like Raphides. Both are birefringent (bright) in polarized light. They probably deter herbivory and are more abundant in plants that grow in dry environments.
Try your polarizers on these!!!!!
The function of these crystals is relatively uncertain.
Collenchyma is closely related to parenchyma. However, the plastids are not well differentiated in collenchyma while they are well differentiated and obvious in parenchyma. Collenchyma always occurs just beneath the epidermis, while parenchyma occurs throughout the plant. Collenchyma cell walls are unevenly thickened. When the thickening occurs at the corners where cells are joined it is called angular. Lamellar collenchyma has thickenings on their tangential walls, which are parallel with the surface. Lignin is usually not present in collenchyma.
Collenchyma in hand sections of Widelia stem, Celery
or Water Lily (Nymphia) petioles. Determine cell shape by
observing cross sections and a demo of a longitudinal section.
Mount fresh sections in water. After examining them, stain with Toluidine Blue and then examine again. What does the pink color of the cell walls indicate?
Observe (prepared slides) of Sambucus stems.
The distinction between parenchyma, collenchyma and sclerenchyma is largely based on the wall structure. Parenchyma cell walls are usually thin and primary while in sclerenchyma a secondary wall is formed on the inner side of the primary wall. Secondary walls are those, which develop after a cell, has ceased to enlarge. Collenchyma cells have secondary wall thickenings but these are uneven in their distribution. Furthermore, the cellulose fibrils in Collenchyma are not as highly organized or tightly bound as in Sclerenchyma. Finally, Sclerenchyma cells can be found in many locations throughout the plant body but Collenchyma are always just beneath the Epidermis.
Sclerenchyma cells are usually classified into sclereids or fibers on the basis of form as well as the abundance and type of pitting.
Sclereids are generally shorter than fibers and their walls show more abundant pitting. The pits are often branched (ramiform). Walls of sclerenchyma cells are usually lignified and, therefore, stain red with safranin or phloroglucinol-hydrochloric acid. They often show concentric laminations, which indicate different periods of wall synthesis. Sclereids vary in shape and occur in all parts of the plant.
Fibers tend to be highly elongated cells with tapering ends, and they often occur in bundles. There are few pits in the walls of fibers. The pits, when present, are usually simple and unbranched.
Sclerenchyma observe their (1) overall shape;
(2) wall structure; (3) pits; (4) staining reactions to Phloroglucinol & Toluidine Blue; (5) appearance with crossed polarizers.
Study cross sections
|Cross Section of Hoya Stem stained with Toluidine Blue: The sclereids occur in a unicellular band in the outer part of the stem.|
Various Wall Layers in Sclerenchyma
Examine the partly macerated seed coats of peas and beans. Brachysclerids, Macrosclereids and Osteosclereids are present.
Look for Astrosclereids in fresh sections of Nymphaea (water lily) leaves.
Like sclereids, fibers may be found in various parts of the plant. Fibers are particularly common near the phloem (phloem or bast fibers) and the xylem (xylem fibers). In monocots fibers often enclose vascular bundles (fibrovascular bundles) or appear as strands that are independent of vascular tissues.
The best commercial fibers are usually associated with the phloem. This includes hau (Hibiscus tiliaceus) and wauke (Broussonetia papyrifera) plus Cannabis. Coarse fibers can be obtained from monocot leaves like uki uki grass (Dianella sandwichensis) and Agave. Agave was grown in Hawaii but was uneconomical. Some of these plants have escaped cultivation and can be found in nature. They are slow growing but once established, they may be difficult to eradicate. This could present a problem for native species if they can't compete with Agave.
Observe free-hand cross and longitudinal sections of stems of hau (Hibiscus tiliaceus).
Deploy your polarizers!
The phloem fibers of this plant were used by ancient Hawaiians for making rope.
Using prepared slides, compare fibers Linen (Linum) and Hemp (Cannabis). Note the fact that the linen fibers do not stain for lignin. Lignin makes the fibers brittle and it discolors them as well.
Cross sections of flax stem that show the phloem or "bast" fibers which are green. In this case the green color indicates the absence of Lignin.
Examine a DEMO of a fiber from wood. This will provide some information on the overall shape and length of wood fibers. We will encounter these again so don't worry about the details just yet.
Fiber from Oak wood