Plant Physiology (Biology 327)  - Dr. Stephen G. Saupe;  College of St. Benedict/ St. John's University;  Biology Department; Collegeville, MN  56321; (320) 363 - 2782; (320) 363 - 3202, fax;    ssaupe@csbsju.edu

Ecophysiological Analysis of Leaf Shape

Objectives:  Upon the completion of this lab you should be able to:

1. to describe factors that influence leaf shape

2. measure leaf area using a simple weighing technique

3. use metric measurements

4. plot a graph

5. do simple statistical analyses

PreLab:  Complete the exercises given in the Prelab Assignment.

Introduction: 
    When admiring the fantastic diversity of leaf shapes it is sometimes easy to forget that these are the products of evolution (trends in leaf evolution).  The leaves of each species are adapted for their unique environment.  Further, individuals in a species can show subtle changes in leaf shape that will help improve their survival value.  The purpose of this lab exercise is to make some predictions concerning trends in the evolution of leaf shape, which are ultimately determined by the physiology of the plant. 

    First, let�s review a little terminology.  A leaf, which is the primary photosynthetic organ, is comprised of three parts � the blade, the stalk (petiole), and stipules which are a pair of leafy appendages at the base of the leaf.  Depending on the species, one or more of these parts may be absent in a given species.  The margin of some leaves is entire (smooth edge) while others are lobed (indented) or serrate (toothed).  The area between the lobes of a leaf is called a sinus.  A leaf with very deep and fine lobes that make the leaf almost appear to be compound is called dissected.  In some leaves, which are termed compound, the blade is divided into sections called leaflets.  In contrast, simple leaves are undivided and have only a single blade.  It�s sometimes challenging to distinguish between a leaf and a leaflet but a sure-fire way is to look for a bud, which is an embryonic shoot.  Only leaves have a bud at the base.
 

    The boundary layer is a region of relatively still, non-moving air that is directly adjacent to the surface of any structure.  In order for gases such as carbon dioxide and water vapor to enter or exit a leaf they must pass through the boundary layer that acts something like a moat around a castle.  To get into a castle we can swim across the moat, but it will surely slow us down.  If the moat isn�t continuous or irregularly shaped, we will have an easier time hopping and weaving our way into the castle.  Similarly, the thicker the layer, the more slowly the rate of carbon dioxide uptake or water loss.  One reason for this is that the air in the boundary layer is usually more humid than surrounding air because water lost from the leaf temporarily gets trapped in the boundary layer reducing the rate of additional water loss.  In a similar fashion, the boundary layer can retard carbon dioxide uptake into the leaf.  In still conditions, the boundary layer increases in thickness and reduce water loss and restrict carbon dioxide uptake while a breeze will blow away the layer, much like draining the moat, and increase water loss and carbon dioxide uptake. 

    Since air tends to move more smoothly over a larger surface than a smaller one, the boundary layer tends to be thicker and more intact in larger leaves.  Thus, one advantage of dividing leaves in numerous sections is that it helps to increase the ability of the leaf to absorb carbon dioxide for photosynthesis by decreasing the size of the boundary layer that surrounds the leaf.   To break up the boundary layer, plants divide their leaves into smaller sections (compound & dissected) which prevents the formation of a continuous layer and makes it easier to remove in a breeze. 
 

    On any given individual, leaves typically show modifications to the amount of sunlight to which they are exposed.  These leaves are called sun and shade leaves.  Compared to shade leaves, sun leaves typically are thicker, have a smaller surface area, have a greater specific leaf area (cm g^-1), a thicker cuticle, more chlorophyll per unit area, and less internal air space.
 

    Ultimately, leaf shape is a response and tradeoff to light, water availability and other factors.  In this lab we will explore some of the factors that influence the shape of leave of species that grow in deciduous woods such as those at St. John�s and St. Ben�s.  We will especially focus on differences in leaf shape between the trees and herbs in the forest and also between the herbs that bloom early in the season (spring herbs or wildflowers - SPH) before the tree canopy closes and those (summer herbs or wildflowers (SMH) that bloom afterward.
   
 Methods:

1. You will be given the pressed, dried leaves from the following species:

2. Digitize andor photocopy each leaf (see pre-lab). 
    

3. Collect data from each leaf and complete Table 1.  To do so:  (a) record whether the leaf is derived from a tree (T), summer herb (SRH) or spring herb (SPH); (b) record the type of margin; (c) measure the length (in mm) of the leaf from the base of petiole to the leaf tip; (d) measure the width (mm) of the leaf at the widest/largest point; (e) measure the length of the petiole (mm); and (f) remove the petiole and weigh the leaf blade (gm).
    

4. Using the photocopy or print (black & white is fine) of the digital image determine the total blade area and total sinus area for each leaf.  On the photocopy draw a straight line connecting the lobes, if any, of each leaf.  Then, cut out separately the areas representing the sinuses and the blade.  To do this, first cut out the entire leaf including sinus areas.  Then, cut out the separate sinus areas. 
    

5. Weigh the collective cuttings from the sinuses and the leaf.  Record data in Table 1.  Convert the weight of the cuttings to area using the tracing technique (use Table 2 to record your conversion factor data).  Alternately, you can digitize the leaves with a flatbed scanner and perform the remainder of the operations using a program such as ImageJ or Scion Image.
    

6. Complete Table 1 by calculating petiole/leaf length ratio, the area of the sinuses and the blades, and the sinus area /blade area ratio.
    

7. To estimate leaf thickness, divide the divide the leaf weight (g) by its area (in m²) to calculate the leaf specific weight (g m^-2).  Thicker leaves will have a higher specific weight.
    

8. Share your  data with another member of the class
    

9. Complete the calculations in Table 1, Table 2, and the summary table (Table 3) by entering your data into an Excel (or other) spreadsheet. 

Results/Analysis:   Test each of the following hypotheses by performing suitable statistical tests (t test or ANOVA).  For each, restate the hypothesis as a null hypothesis (H_o), record the probability value, and state the conclusion concerning the hypothesis (do your data support or refute your hypotheses?).

• Summer herbs have larger leaves (blade area) than spring herbs. 
      H_o = 
      p = ______ 
      conclusion: 
   

• Leaf petioles are longer (petiole/leaf length ratio) in spring herbs than summer herbs
      H_o = 
      p = ______ 
      conclusion:
   

• Trees will have thicker leaves (leaf specific area) than herbs
      H_o = 
      p = ______ 
      conclusion:
   

• Spring herbs will have thicker leaves (leaf specific area) than summer herbs
      H_o = 
      p = ______ 
      conclusion:
   

• Herbs will have a great frequency of compound and dissected leaves (sinus area / blade area ratio)than trees
      H_o = 
      p = ______ 
      conclusion:
   

• Spring herbs will have a greater frequency of compound and dissected leaves (sinus area / blade area ratio) than summer herbs
      H_o = 
      p = ______ 
      conclusion:
   

• Test one other hypothesis/prediction of your own
      H_o = 
      p = ______ 
      conclusion:

Post Lab:  (due next cycle in lab.  Please turn in, typed):

• Table 1

• Table 2

• Table 3

• An abstract summarizing the experiment and addressing each of the SEVEN hypotheses/conclusions listed in the Results/Analysis Section.   The abstract should be typed and be approximately one page in length.  The questions scattered throughout the exercises provide a guide to the types of items to include in the abstract.  I recommend starting by saying;  "The purpose of this lab was to....."  Then, describe briefly each major thing we did and the outcome and whether your data were expected or not? These could be mini-paragraphs (two or so sentences each) or part of one larger single paragraph. 

References:

• Aston TJ, Robinson G (1986) Teaching light compensation point: A new practical approach.  J. Biol Educ 20: 189 - 194.

• Burnham RJ,  Pitman NCA,  Johnson KR, Wilf P  (2001) Habitat-related error in estimating temperature from leaf margins in a humid tropical forest.  American Journal of Botany 88: 1096 - 1102.

• Givnish, TJ (1987) Comparative studies of leaf form:  Assessing the relative roles of selective pressures and phylogenetic constraints. New Phytologist 106 (Suppl): 131 - 160.

• Harding, Lynda.  Exploration of leaf shapes.  From Biology Lab Clearinghouse.  Accessed Jan 2004.

• Westmoreland, D (1989) Leaf morphology & light microenvironments: A field exercise.  American Biology Teacher 51: 303 - 306  

• Weyers JDB, Hoglund HO, McEwen (1998) Teaching botany on the sunny side of the tree:  promoting investigative studies of plant ecophysiology through observations and experiments on sun and shade leaves. J. Biol Educ 32: 181 -190.

Last updated:  01/07/2009     � Copyright  by SG Saupe

Last updated:  01/07/2009                  � Copyright  by SG Saupe