Exercise 23 - Absorption, Transport, and Transpiration

Students observe the structure of roots and stems to learn the differences between monocots and dicots. Primary and secondary growth are discussed. They see that water rises through xylem by transpiration, and that guard cells open and close stomata.

EQUIPMENT	AMOUNT (Class of 24 with 8 groups)
Compound microscope	1/student
Dissecting microscope	1/student

MATERIALS

SOLUTIONS

0.5% methylene blue
5% sodium chloride (NaCl)

PREPARATION

Two Months before Lab

1. Tomato seeds should be planted and plants cultured in a greenhouse or other acceptable environment. Obtain large geranium and Coleus plants and culture with the tomato plants.

2. The seeds to be germinated and the prepared slides should be ordered to arrive eight days prior to the lab.

3. Woody twigs can be collected and stored for use.

1. The seeds should be germinated four to six days prior to the lab. Moisten the filter paper with warm water and place in the bottom of a petri dish. Sprinkle the paper with 30-40 seeds and place the covered dish in a warm dark area. Check the dishes every day for growth and moisture content.

2. Methylene blue preparation:

   0.5% methylene blue

   0.5 g dye/100 ml distilled water

   Dissolve the dye by stirring constantly. Store the dye in a stoppered Erlenmeyer flask.
   Shelf life: six months

3. Sodium chloride preparation:

   5% NaCl

   5 g NaCl/95 ml distilled water

   Completely dissolve the salt in water and store in eight dropper bottles.
   Shelf life: one month

4. Obtain plants with fibrous root, taproot, and adventitious root systems. Wash soil from roots and place in a beaker of water as demonstration.

CLASSROOM SUGGESTIONS

1. Slides of plant anatomy can be used to summarize the laboratory.

2. Minimum homework would be a short essay on water entrance, movement, and transpiration.

ANSWERS TO CRITICAL THINKING QUESTIONS

1. Several things could limit the height of trees, the most obvious being water transport to the top of the tree. Movement of water up the xylem involves transpirational pull, cohesion of water molecules exerted by their hydrogen bonds and adhesion of water molecules to the xylem walls. However, one needs to consider whether cohesion is strong enough to support a water column higher than 100 m. Tests have shown water cohesion is strong enough to support a water column 150 m high. The transpirational pull (tension) in xylem has been measured that would pull water up 200 m. Theoretically, when only considering water transport in xylem, trees could grow taller, perhaps another 50 m taller, above which height the water column may not support itself. It's probable, though, that there are other limiting factors.

2. Favorable growing conditions during a year will be indicated by a relatively wider annual ring. As well, the relative distance between terminal bud scale cars is indicative of growing conditions at the time, i.e., a relatively short distance between scars could indicate unfavorable climatic conditions during that period of growth.

SUPPLEMENTAL MATERIALS

Bio Sci II, videodisc contains many images applicable to this exercise. Dubuque, IA: Wm. C. Brown Publishers. See appendix.

Growth of Plants, 21-minute film. Chicago, IL: Encyclopaedia Britannica Educational Corp.

Monocot Plant Anatomy, audio filmstrip. Burlington, NC: Carolina Biological Supply. #528702

Root Structure and Function, slide set. Rochester, NY: Ward's. #171W9885

Stem Structure and Function, slide set. Rochester, NY: Ward's. #171W9886