Ecology is the study of an organism and how it interacts with its home. In this study, the home will be primarily the terrestrial environment or dry land. The major difficulty that plants on dry land have is, of course, being able to prevent drying and providing water to all its tissues. Thus, in this exercise we will study a number of ways in which plants have adapted to living in dry environments such as deserts. It must be remembered, however, that there are several types of "deserts". The usually type of desert is a physical desert in which water is simply absent. However, there is also a physiological desert in which, while the water is present it is unavailable to the organism. Such a desert would be the ocean, where the hypertonic nature of the water prevents the water from being available to the cell, or anyplace the water is frozen, and thus also unavailable.
As seen in Exercise 2 on Plant Taxonomy, some plants are not well adapted to living on dry land either because they do not have vascular tissue and/or they do not produce seeds. Both of these are related to water since the ability to absorb and transport water throughout the plant is obviously important as is the ability to protect their sperm, eggs and developing embryos. Differences can also be seen in the root systems where plants adapted to arid, dry, environments have root systems that are frequently shallow and very wide spread. This allows them to catch as much of the limited rainfall as possible before it moves down into the ground. Some of these plants are also capable of producing chemicals that flow out into the soil and prevent other plants from growing near them. This reduces competition and increases the amount of rain they can catch.
In terms of traits that are easily studied, one of the most important adaptations that some plants have made to life on land, and particularly to living in dry areas, is by modifying their leaves. Leaves are one of the most adaptable parts of the plant. It is not unusual for one plant to have several types of leaves with shade leaves, sun leaves, young leaves, and mature leaves all present.
Leaves are responsible for the process of photosynthesis and are thus the area where the most water and gas exchange occurs. This occurs through the stomata, the microscopic openings in the epidermis. Therefore, the more stomata a plant has, the more that are present on the top of the leaf and the more exposed they are, the more water will be lost. Thus, a plant may have adaptations that would reduce the number of stomata, move more of them to the bottom of the leaf or lower the stomata under the epidermis. Another microscopic adaptation is the thickness of the waxy layer known as the cuticle. The thicker this layer is the less water loss will occur. This water loss can also be reduced if there is a thick epidermal layer surrounding the plant's leaf.
Besides changes to the stomata, many leaves are actually designed to reduce water loss. The most obvious differences seen between the leaves of plants living in moist areas and those living in dry areas are the size of their leaves. This is related to the surface area of the leaf with the amount of water lost being directly proportional to the surface area. A tree such as a sycamore, which is usually found along a stream, will have large broad leaves while a sage, usually found in the arid hills of Southern California, will have narrow leaves. Evergreen trees are an interesting case where leaf size is also an important factor as the needles have very little surface area. The ultimate adaptation of this type is, of course, the cacti which have reduced their leaves completely until they are no more than thorns. In this case, the process of photosynthesis now takes place in the green stem.
Some plants reduce water loss by reducing heating by the sun. The less sunlight they absorb, the less they heat up and the less water evaporates from them. Some plants have their leaves arranged so that they do not directly face the sun, either by being permanently positioned that way or by rotating throughout the day to present only their edges to the sun. Other plants reduce heating by having shiny or light colored leaves that reflect most of the heat producing sun while others have surface coatings that decrease heat loss.
MATERIALS & METHODS:
Garden Map Plant List
Microscope Assorted botany texts
evergreen leaf c.s.
xerophyte leaf c.s
dicot leaf c.s.
hydrophyte leaf c.s.
Internet connection to http://www,apu.edu/~skinnes/kinnes_garden.htm
1. Microscopic Analysis: Using the texts provided, rank the following types of leaves as to the environment where they are usually found according to the amount of water they would be exposed to. Then examine the microscope slides showing cross sections of these leaves. Diagram the microscopic view with particular reference to the thickness of the epidermis and, if visible, the cuticle. As you move along the cube-shaped cells of the epidermis you will occasionally see the round guard cells that make up the stomata. Note their relative numbers, location and positioning in the epidermal layer.
2. Environmental Analysis:
a. Roam the Interpretative Garden to find at least two plants that, on the basis of leaf structure, seem to fit each of the habitats listed below. Describe the traits that lead you to place them in each category.
If an Internet connection is available, type in the URL:
Select "Garden Maps" and then click on the quadrant of the Garden you found your plant in. Identify which plant is the one you are interested in and click on it. The scientific and common names will appear along with the habitat of the plant. Continue identifying each of the plants you have selected and record the information in the "Results" section.
If an Internet connection is not available, use the Garden Map and Plant Lists to identify the plants you use in much the same manner.
Coastal or Riparian: plenty of moisture
Woodland or Forest: moderates amount of moisture
Sage or Chaparral: reduced amounts of moisture
Desert: lacking moisture
b. Use the Plant Lists to identify plants whose habitat is stated as being in one of the areas listed above. Using the Garden Map, locate each of the plants in the Garden and examine them to determine if their leaves would match the expected structure, size, etc. for a plant in that area. List their name and leaf traits in the "Results" section. Comment on whether of not their anatomy seems to fit their habitat.
1. Microscopic Analysis:
b. Diagram the cross-sections below:
a. evergreen leaf b. xerophyte leaf
c. dicot leaf d. hydrophyte leaf
2. Environmental Analysis:
a. Complete the following table after your walk through the Garden.
AREA REPRESENTED NAME OF PLANT LEAF DESCRIPTION
b. Complete the following table after you identify plants for each area and examine their leaf structure.
AREA REPRESENTED NAME OF PLANT LEAF DESCRIPTION
1. What differences did you note between the internal leaf structure of the different leafs?
2. Describe the relationship seen between the internal anatomy and the habitat of the plant.
3. Of the sixteen plants you studied in the Garden, how often did the habitat of the plant match the leaf adaptations described?
4. Describe the plants that did match their habitat:
5. Why do you think some of the plant leaves did not match their habitat in this manner?