Name: 		__			 	  __     	   Period:		         Living Environment

LAB #          LEAF STRUCTURE

Plants are incredible organisms! They can make all their own food from the simple inputs of:

§     sunlight

§     air (carbon dioxide)

§     water

§     minerals

This biological wizardry is accomplished through the magic of photosynthesis. This process can be summarized by the equations below.

carbon dioxide 	     +      water      +      light energy            sugar      +      oxygen

or the process can be written as:

→

6 CO2         +     6 H2O     +      light energy

C6H12O6       +       6 O2

This means that plants are able to harness the energy of the sun to turn CO2  from the air into the carbon-based molecules of life — carbohydrates, proteins, lipids, and nucleic acids.

Plants capture the sun’s light within their green leaves. Inside a leaf’s cells are green organelles
— chloroplasts — which do all this hard work of producing the food that feeds the plant… and, in fact, the whole rest of  the world, too!

To do this job best, leaves have evolved a specific structure — 3 types of tissue arranged in layers:

§     epidermis

§     mesophyll

§     vascular tissue

The  epidermis is the outer layer of cells that acts like a protective “skin” for the leaf. Covering the epidermis is a waxy coating, called the  cuticle, which stops evaporation of water from the leaves  thereby  helping  plants  conserve  water.  In  the  lower  epidermis  are  openings  called stomates surrounded by two cells called  guard cells. The stomates act like the lungs of the plant  in  that  they  allow  gas  exchange   —  letting  CO2    into  the  inner  plant  tissues  for photosynthesis and then allowing O2 out as a waste product of photosynthesis.

The mesophyll is the main inner leaf tissue making up the blade of the leaf. Most of the photosynthesis of the plant takes place in the mesophyll. The mesophyll in the upper part of the leaf is made up of tightly packed cells, full of chloroplasts, and is called the  palisades layer. The mesophyll in the lower part of the leaves is made up of loosely packed cells and is called the  spongy layer.

The  vascular tissue functions like the circulatory system of the plant. The  xylem carries water from the roots to the leaves and to the other upper parts of the plant. The  phloem carries the sugars produced during photosynthesis in the chloroplasts of the leaves to any place else in the plant that needs the food. Xylem and phloem are found in vascular bundles in the veins of the leaf.

1.   Below is a diagram of a cross section through a leaf.  Label the structures discussed above.

2.   Examine  the  prepared  slide  of  a  cross  section  through  a  leaf  under  the  compound microscope. Draw a neat, clear  diagram of your specimen in the space below. Find THREE of the structures illustrated above and  label them.

3.	Observe a specimen of a Tradescantia leaf, seen below. Locate and label the stomates on the underside of the leaf. Locate and label the  guard cells — two bean-shaped cells on either side of the stomate.

4.   Construct a model of the cross section of a leaf using the model parts provided. Color each leaf structure or tissue a different color using the color key below.

COLOR KEY	
LEAF STRUCTURE	COLOR CODE	
Cuticle	yellow	
Lower epidermis	orange	
Palisades layer	green	
Phloem	brown	
Upper epidermis	orange	
Spongy layer	purple	
Xylem	blue	
Guard cells	red	

5.   Cut out each of the colored cell layers. Be sure to cut along the outer edge of each group or along the dotted line. Do not cut out the inside spaces of each layer.

6.   On a separate sheet of paper, correctly place each of the cell layers together by laying one on top of each other. You may consult your textbook to make sure you get all the structures in their correct places. Review with your teacher before you glue them. Once you receive approval then glue them together and  label each structure or cell layer.

§     cuticle

§     upper epidermis

§     lower epidermis

§     palisades layer

§     spongy layer

§     stomate

§     guard cells

§     xylem

§     phloem

§     vascular bundle

7.   Now  add the sun to your diagram to show which side of the leaf collects the energy from the sun.

8.    Place the symbols  for CO2, O2  and H2O on your model and  draw arrows to show the movement of these gases in and out of the leaf through the stomates. Show the completed diagram to your teacher.

9.   Answer the Summary Questions.

SUMMARY QUESTIONS

1.   What are the 3 functions of stomates.

a.	 		

b.	

c.	

2.   Why are most of the stomates on the bottom of the leaf?

3.   Why are the epidermis layers of the leaf coated in a waxy cuticle?

4.   Why are the cells of the palisades layer packed so tightly together?

5.   Why are the cells of the spongy layer packed so loosely together?

6.   What is the function of the xylem?

7.   What is the function of the phloem?

8.   Describe how the functions of the xylem and phloem specifically relate to photosynthesis.

a.   xylem    	

b.   phloem   	

9.   Why would the cells of the palisades layer have more chloroplasts in them?

10. What is the function of guard cells?

11. Why do stomates need to be open?

12. Under what conditions would guard cells close the stomates?

13. Why do plants need to take in CO2?

14. Write out the equation for photosynthesis.

cut along here

Tissue Layers of a Leaf

epidermis layers –   • cut on the  OUTSIDE of these epidermis layers
• do  NOT cut these epidermis layers apart
• all other pieces will be glued into here

cut along here

cut along dotted line

cut along dotted line

phloem                                                           xylem