Bot 201 Lab-Vegetative Features   
of Vascular Plants

The purpose of this lab is to acquaint you with the various types of tissues found in Vascular Plants and to show you how they are organized to form a "Typical Vascular Plant". This may seem daunting at first until you realize that basic plant anatomy is very simple.

Cell Types & Tissues

Meristematic Tissue

Meristematic tissue produce all the cells in herbaceous plants. The cells originate in the Root or shoot Apical Meristems. Meristematic Cells are Spherical (Isodiametric) and densely cytoplasmic. They have a relatively large Nucleus, few Vacuoles and thin Cell Walls. Consequently, they readily absorb biological stains and appear as dense areas in the apices of Roots and Shoots. The Apical Meristem in Roots is actually subterminal because it is covered by a Root Cap. The root cap protects the delicate meristem cells and it also secretes carbohydrates which lubricate the root as it grows through the soil.

There are two basic types of Apical Meristems. Nonseed plants have a large Apical Cell which gives rise to the plant body. Seed plants have Multicellular Apical Meristems which function as a unit.

Primary Tissues

There are Three Basic Tissues that comprise all herbaceous (soft bodied) plants. These are Dermal, Vascular & Ground.

Plants are made like reinforced concrete. There is a outer mold, steel rods and concrete which fills in the rest of the volume.

The first tissue is the Epidermis which is typically on the surface (Dermal) and is usually one cell thick. If it is on the surface it is probably Epidermis. This is like the mold for the concrete.

The next  is Vascular Tissue. It is typically found in longitudinal columns. There may be one large central column, a ring of smaller columns or multiple rings of small columns. These columns are the steel rods.

Finally, there is the Ground Tissue which occupies the rest of the plant organ. This is like the concrete above.  I regard the Ground Tissue as BACKGround Tissue.

The Epidermis is readily identified by its location.

Vascular Tissues, especially Xylem,  have strong characteristic traits which make them relatively easy to identify.

If it is neither Epidermis nor Vascular Tissue, it must be Ground Tissue.

There are two Vascular Tissues, Xylem & Phloem. The conducting cells in the Xylem are called Tracheary Elements. These have thick walls that stain red due to the presence of Lignin. Lignin makes their cell walls watertight, inflexible and strong. Consequently, Tracheary Elements have two main functions, water transport and structural support. They usually have a hollow appearance because they are dead at maturity. These traits make Xylem relatively easy to spot in a cross section.

Phloem is associated with Xylem. Phloem cells do not generally have diagnostic traits like xylem but they can usually be discerned by their position. They have relatively thin cell walls and have a narrower diameter than the largest Tracheary Elements. In some cases,  Phloem exhibits a definite pattern of small cells and large cells. The large cells are Sieve Elements (S). These are the cells that are specialized for Sucrose Transport. The smaller cells are Companion Cells (CC). These regulate the physiological activities of the Sieve Elements. Phloem contains Callose. Callose regulates the size of the openings between adjacent Sieve Elements. Callose is a carbohydrate that stains with Aniline Blue and is fluorescent under Violet light. This is the best way to verify the presence of Phloem because Callose is not present in other cell types.

There are two major types of Ground Tissue (Parenchyma & Sclerenchyma).

Parenchyma cells can be Isodiametric (equal size in all directions) to elongate in shape. They have thin cell walls which contain a lot of hydrophilic pectins which have a characteristic staining reaction. They usually have well developed Plastids like Chloroplasts or Amyloplasts. Amyloplasts store starch.

Sclerenchyma cells are usually elongate in shape and have thick walls impregnated with Lignin. They stain red in most prepared slides due to their Lignin content. Lignin makes cell walls extremely strong and inflexible. This makes Sclerenchyma a good support tissue. Sclerenchyma is usually associated with Vascular Tissues and may completely surround them.

There are a wide variety of Epidermal cell types. The typical Epidermal Cell resembles Parenchyma but it has only tiny plastids. All Epidermal Cells have a waxy Cuticle on their outer surface. They may contain Red/Purple Anthocyanins in their vacuoles. This gives the cells a uniform color.  Chloroplasts are present in the Guard Cells which are part of the Stomata. The Guard Cells tend to be smaller than  typical Epidermal Cells, and they occur in discernable patterns. The Epidermis can produce a wide range of hairs (Trichomes). These can be unicellular or multicellular. They may absorb water from the soil, secrete chemicals to the surface, inject chemicals into the skin, trap & digest insects and perform many other functions.

Trichomes are most abundant with flowering plants! Other Divisions generally lack hairs, except for root hairs.

Tissue Organization and Organs

The most simple pattern of tissue organization is seen in Roots. Imagine a stack of ice cream cones. Fill the first cone with raspberry ice cream. This is the Xylem. Coat the inside of a second cone with a layer of blueberry ice cream. The ice cream represents the Phloem. Place this over the first cone. Coat another cone with mint ice cream and place it over the other two cones. The mint ice cream is ground tissue. We should have a stack of three cones. The outermost cone shell is the Epidermis.

Some Stems and Leaves have the same kind of organizational pattern!

Lab Activities

Meristematic Tissue

Examine Longitudinal sections from Root and Shoot tips. Locate the Apical Meristems and note their general features with your 40 X Objective.

Follow cell files as you scan towards the base of each Apical Meristem & look for signs of Cell Enlargement and Differentiation (changes in the Cell Walls).

Epidermis

Observe Commercial Slides of Lily Epidermal. Note the Guard Cells of the Stomata. The Guard Cells can open and close to regulate gas exchange with the atmosphere. Also note the elongate Epidermal Cells

Observe a Demonstration slide which shows an Epidermis with an extremely thick Cuticle.

Take a Pigmented Coleus Leaf.

Cut  a 2 x 2 cm square piece from the middle of the leaf.

Place this upside-down on a Microscope Slide.

Place this on the stage of your microscope and move it into the light path.

Use the 4 X Objective but flip up the High Power Condenser Lens. This should produce a small spot of light

Turn the illumination to MAXIMUM!

Do NOT Look through the Objectives when you do this. The intense beam could damage your eyes if it is not blocked by your specimen.

Move the thin part of the leaf into the light path.

Focus up and down until you see details of the Epidermal surface.

You may switch to the 10X objective to see more details.

You should be able to see Trichomes & Pigmented Epidermal Cells & Guard Cells. The latter may be a little hard to find.

Do NOT spend too much time on this. The goal is to give you an appreciation of Epidermal complexity

Dicot Root

Observe Intact Water Hyacinth Roots. The hair-like projections are actually Lateral Roots. Also note the prominent Root Cap. This protects the delicate Root Apical Meristem. The latter is the source for the Root Cap and the Root Body (everything except the cap)

Observe Commercial Slides of Root Cross Sections.

Locate the Epidermis, Ground Tissue and Vascular Tissues.

The Ground Tissue is composed of Parenchyma cells. These have Thin Cell Walls and a round shape. They may have numerous, starch-storing Amyloplasts.

Carefully examine the Vascular Tissues and Identify Phloem & Xylem.

The Xylem Stains Red due to the presence of Lignin and has a star-like shape.

The Phloem lies between the radiating arms of the xylem. Its cells are much smaller than those in the xylem and they have very thin walls.

The Endodermis is the innermost part of the Ground Tissue and forms a boundary layer between the Vascular Tissues (Stele) and the Ground Tissue. The Endodermis regulates the movements of water and solute between the Stele and the rest of the root. Some Endodermal Cells have Thick, Lignified Walls while others have relatively thin walls. The Endodermis is one of the most important adaptations of  land plants. We will explore this topic later.

Dicot Stem

Observe a Commercial Slide of a Typical Dicot Stem and locate the Epidermis, Vascular Bundles & Ground Tissues.

The Ground Tissue in the center of the stem is called Pith. It contains Parenchyma Cells.

Examine the Vascular Bundles at higher magnification and locate the Xylem, Fibers and Phloem. Phloem lies between the fibers and the xylem and may not have prominent features.

The Ground Tissue between the Fibers and the Epidermis is called the Cortex.

Examine Commercial Slides of Cucumber Stem and locate the Vascular Bundles.

Locate the Phloem which occurs on both sides of the Xylem in Cucumber. Examine the Phloem carefully and try to locate the Sieve Elements & Companion Cells.

The Companion Cells are small and stain darkly while the Sieve Elements have a larger diameter and have pink contents.

Try to locate Sieve Plates. These occur on the endwalls of the Sieve Elements and have relatively large openings called Sieve Pores.   Sieve Pores facilitate the movement of sugar solution through the Sieve Elements.

Monocot Stem

Examine Commercial Cross Sections of Corn, Sugarcane or Bamboo.

The Vascular Bundles are easy to identify because of the Large Xylem Tracheary Elements. 

Note the Distribution of Vascular Bundles throughout the Ground Tissue. There is no Pith in most monocots.

Examine a large Vascular Bundle at higher magnification and locate the Xylem, Phloem and  Fibers which surround the vascular tissues.

The Phloem has a geometric organization which makes it easy to identify the Companion Cells and Sieve Elements.

Monocot Leaves

Given the number of Vascular Bundles in monocot stems, it should not be a surprise to see the numerous veins in their leaves.

Examine the leaves of a typical monocot like Sugarcane or ti & note the many veins which run the length of the leaves. Each vein contains one Vascular Bundle. You may see that there are large and small veins which alternate in a regular pattern.

Examine Commercial Cross Sections of Sugarcane Leaf and note the following, Upper and Lower Epidermis, Vascular Bundles and Ground Tissue.  

The Ground Tissue in a leaf can be called Mesophyll (Middle Leaf). Cells which contain Chloroplasts are called Chlorenchyma or Photosynthetic Parenchyma.

Dicot Leaves

Dicot Leaves usually have a large central Midrib which contains a large Midvein   (Vascular Bundle). Minor Veins branch from the midvein at oblique angles. Consequently, the minor veins may be asymmetrical in cross sections.

Examine a typical Dicot leaf like Kukui and note the Vein Pattern (Venation).  You may need to scrape of some hairs to see the lower surface. Mount the hairs in water and examine them under the microscope. Layteral veins get progressively smaller and smaller. This is called Reticulate or Net Venation.

Monocots have "Parallel" Venation because the Vascular Bundles appear parallel to one another and look  unbranched. However, close scrutiny shows that they produce Lateral Veins at angles approaching 90 degrees. Thus, they have a form of reticulate venation called Striate. The Important Point to remember is that Monocots and Dicots have Reticulate Venation.

They are both MEGAPHYLLS  (big leaf) because they have more than one Vascular Bundle.

Examine a Commercial Cross Section of a typical Dicot Leaf like Pear (Pyrus).

Examine the Midrib and locate the Xylem & Phloem.

Dicot Leaves Typically have two kinds of Mesophyll (Palisade & Spongy). The Palisade cells are like columns and are tightly packed. Spongy cells may be highly branched and are loosely packed. The Palisade Cells intercept most of the light and perform most of the Photosynthesis. The Spongy Cells also contain chloroplasts but mainly serve as a gas reservoir. Note the Lateral Vein.