Examining the Role of Leaf Stomata in Gas Exchange and Transport in Plants

Abstract

The role of leaf stomata in gas exchange and transport within plants is examined in this unit. Two methods of examining stomata are used to collect data on stomatal density in four different plant species.

Objectives
Upon completion of this unit you should be able to:
1. Identify the arrangement of tissues in a plant leaf.
2. Describe the structure and function of a stoma.
3. Relate differences in leaf characteristics and stomatal distribution to habitat.
4. Learn how to prepare slides of leaf epidermis using whole mounts and surface casts.
5. Present experimental data by drawing a bar graph.
6. Interpret experimental results presented in a bar graph.

Readings
Campbell, 2nd Canadian ed.
Tissue Organization of Leaves (subsection only) including Figures 35.17 and 35.18
The Rate of Transpiration is Regulated by Stomata (all of concept 36.4) including Figures 36.12, 36.13, and Making Bar Graphs and Histograms

Activities
To be completed during this week’s lab period:
1. Part II. Distribution of Stomata including:
a. Data collection from the Distribution of Stomata Photographs posted on Moodle.
b. Submit your data to the pooled class data set (your instructor will give directions for submission).
c. Analysis of pooled class data and completion of Table 3.
d. Generation of a bar graph summarizing the results.
e. Review the Distribution of Stomata – Plant Descriptions PDF on Moodle to revise Table 2.

2. Part III. Leaf Structure including
a. Review Lilac and Oleander Habitat and Leaf Anatomy PDF on Moodle.
b. Answer all questions 1-11 while waiting for the pooled class data.

Assignment
Submit Table 3, your bar graph, and answers to the assignment questions at the end of this lab handout

Leaf Orientation
The orientation of a plant’s leaves relative to sunlight can affect both the rate of photosynthesis and the rate of evaporation of water from the surface of the leaves (i.e. the rate of transpiration). Leaves that are horizontal will receive more direct light to power photosynthesis. Exposure to direct sunlight can also cause the leaves to heat up, increasing the rate of water loss due to evaporation. Leaf orientation represents a trade-off between maximizing light capture and minimizing water loss. 

How do you think the light conditions experienced by a plant would affect the orientation of its leaves? What
orientation would be most beneficial in shaded environments? What orientation would be most beneficial in full sun?

Cuticle
The thickness of the leaf cuticle can provide general information about a plant’s ability to retain water, its native habitat, and the orientation of the leaves relative to sunlight. A leaf with a thick cuticle will feel waxy and thicker compared to a leaf with a thin or absent cuticle. Depending on the environment, the cuticle may be thicker on one side of the leaf than the other. 

Under what environmental conditions would you expect a plant to have a thick cuticle? In what conditions would you expect the cuticle to be thin or absent? 

Opening of Stomata During the Day or Night

Most plants open their stomata during the day and close them at night. Some plants close their stomata during the day
and open them only at night. 

Consider the function of the stomata. Why do you think most plants open their stomata during the day? I.e., what is the benefit to the plant of keeping its stomata open during the daytime? 

Even plants that typically keep their stomata open during the day will sometimes close them for an hour or more midday. What abiotic condition do you think causes this to happen? Why would closing stomata be beneficial in this case?

In what type of environment would you expect to see plants that typically close their stomata during the day and open them at night? Explain.Ncluding How They Are To Be Completed And Submitted. Therefore, It Is Important That You Fully Understand The Assessment Instructions Given By Your Trainer To Avoid Issues Such As Academic Misconduct,