Leaves are the major photosynthetic organ for most plants.  Keep this very important function in mind as you study the anatomy of the various leaf types described below.  This week’s lab is not just to teach you about leaf anatomy but also to help you gain an understanding of the role each leaf tissue or structure plays in furthering this photosynthetic function.

I.  THE “TYPICAL” (=MESOPHYTIC) DICOT LEAF STRUCTURE

 Following the directions from your instructor, peel a small piece of the lower epidermis from one of the leaves provided for that purpose.  Make a wet mount of this peeled epidermis and study the surface character of the epidermis and its component cells (compare Figs. 23-18e & 23-22).  Be sure to locate stomates and guard cells.

Which epidermal cells have chloroplasts?
 

What function do the guard cells serve and how does that function relate to the overall function of the epidermis?
 
 
 
 

 Study the prepared slide of ‘Syringa (lilac) leaf cross sections’ (Fig. 26-20).  Locate the three major tissue systems, dermal (epidermis, upper and lower), ground (mesophyll), and vascular.  Locate and observe the following structures in Syringa as well as each of the different types of leaf cross sections to be studied in parts II thru VI below.  Also answer the questions for each of the various leaf types.

cuticle

Note the relative thickness in the different leaf types!
What is the function of the cuticle?
 

Are all the cells of similar size and shape?
Stomates present or absent?
Which (if any) epidermal cells contain chloroplasts?
Is the upper epidermis continuous with the lower epidermis?
How can you tell which is the upper and which is the lower surface?

palisade mesophyll

How many cell layers?
Chloroplasts present or absent?
Are there intercellular air spaces?

spongy mesophyll

How thick relative to the palisade layer?
Chloroplasts present?
Function of the mesophyll?
What is chlorenchyma?
Is there any mesophyll that is not chlorenchyma?
 

Be sure to study the large central midvein (when present) and the smaller vascular bundles as well!
Why are some veins seen in end view while others are in longitudinal view?
How does the position of the xylem and phloem in leaf veins relate to their position in the stem?
What is the function of the xylem and phloem in relation to the photosynthetic function of the leaf?

bundle sheaths

Function?
Are chloroplasts present or absent?

bundle sheath extension

Function?
What cell types are they composed of?

lower epidermis

Stomates present or absent?
Cuticle thickness?
Note the small glandular trichomes scattered on Syringa lower epidermis.

  [Not all the different leaf types to be studied below have all the parts mentioned, while some will have additional structures; nevertheless use the above list of questions as a guide to study each type of leaf.]
 
 
 
 
 
 
 
 
 
 
 

II.  A “LESS TYPICAL” DICOT LEAF STRUCTURE

 Study one of the prepared slides labeled ‘Ficus: leaf c.s. (cystoliths)’.  The basic plan is similar to Syringa but differs in several ways.  Ficus elastica (a species of fig, a common horticultural plant often misnamed as the “rubber plant”) is a tropical plant with some xeromorphic characters.  It has a rather thick cuticle and a multiple epidermis 3-5 cell payers thick which is apparently a water storage tissue.  Also notice that the stomates are sunken in small crypts.  One very unusual feature of Ficus is the calcium carbonate crystals (called cystoliths) that are suspended on cellulose stalks within scattered specialized epidermal cells.
 
 
 
 
 
 

III.  THE “TYPICAL” MONOCOT LEAF STRUCTURE

 Obtain for study prepared slides of ‘Triticum (wheat) leaf’ and ‘Zea (corn) leaf’ cross sections.  These typical grasses are basically mesophytes, although like most grasses, they do have a few xeromorphic characters.  Compare the characteristics of these two grasses with Syringa, using the above list of parts and questions to guide your study.  Pay particularly close attention to the vascular tissues.  Wheat (Fig. 26-27) is a C3 plant and corn and other grasses (Fig. 26-28) are C4 plants.  See the discussion in your book (p. 632) detailing the anatomical differences in the leaves of these two physiological types of plant, and be sure you understand their significance and can recognize each type.
 
 
 
 
 
 
 
 

IV.  A LEAF SUCCULENT XEROPHYTE LEAF

 Study a prepared slide of ‘Yucca leaf c. s.’ and compare it with what you observed in the above mesophytes (Compare to Fig. 26-22).  Identify the various tissues and cell types and locate the stomates, etc.
 What are the characters possessed by Yucca that make it a xerophyte?
 
V.  AN AQUATIC HYDROPHYTE LEAF

 Study the prepared slide of ‘Nymphaea (water lily) leaf cross section’, and compare it to the mesophytes and xerophyte you have examined (see Fig. 26-21).  Identify all the tissues and cell types; be sure to see the unusual elongate and often branched sclereids (see also Fig. 24-10) scattered through the leaf tissue.
 Assuming you had never seen these leaves floating, what are the characters that identify them as belonging to a hydrophyte?
 What is aerenchyma?