Adaptation+Lab+Student+Manual

Lab 4: Adaptation
**Objectives:** - Learn about plant adaptations, and deduce function from observations - Learn about research proposals - Practice developing hypotheses and predictions, as well as building a research proposal from them - Practice presentation skills Borrowed from Todd Barkman’s course (BIOS 202: General Botany) with his permission. Edited by Tomomi Suwa Plants are constantly faced with changing environmental conditions. Therefore, many plants have developed different morphologies, anatomies, and physiologies to adapt to their environments. Excessive loss of water by transpiration from leaves is perhaps one of the most difficult challenges facing plants in most environments. Because stomata are the primary sites of water loss, plants usually produce them only on the undersides of their leaves. Reduced numbers of stomata may also occur in cases where water loss is extreme. Recall that most plants are faced with an inescapable conundrum: they must keep their stomata open to take in CO2 for photosynthesis but whenever this occurs, water escapes from their leaves. In this lab, we will go to the demonstration greenhouse and observe several plants that are native to different climates throughout the world. During your observations, try to think about which plant is best/poorly suited to periods of drought. Based on your observations and information about the environment in which each plant grows, **formulate hypotheses** about differences in stomatal density as an adaptation to reduce water loss in these plants. Make specific **predictions** that may be tested in relation to the approximate amount of rainfall received by the plants. Although you will not be able to conduct a manipulative experiment, you can still formulate testable predictions and collect data to allow you to either support or reject your hypotheses using a mensurative experiment. The primary data collection you will do is to count stomatal densities of the leaves presented in class/in the greenhouse. (To learn more about manipulative vs. mensurative experiments, see page 142.) The protocol for determining stomatal density is as follows: 1. Apply an **even, thin layer** of clear nail polish on to the leaf surface. 2. Wait till the polish dries completely. 3. Gently lift the sides to peel off the nail polish without tearing it. 4. Place the peeled layer onto a slide (without any creases). 5. Observe under a 10x lens and count the number of stomata. If the number of stomata is too numerous to count, just count a portion of the field of view and multiply the results accordingly. 6. Calculate stomatal densities per cm2.
 * Plant Structural Adaptations to the Environment**
 * Hypothesis**:
 * Predictions**:
 * Methods**: How did you choose where to take measurements (from which plant and where on the leaf)?
 * Results**: Draw a picture of the pattern of stomata, and write down your calculations for


 * Plant 1**
 * Details: Drawing: Calculations:**


 * Plant 2**
 * Details: Drawing: Calculations:**

1) Compare and contrast this method for reducing water loss with another method? What trade-offs might there be for a plant that adopts one strategy versus another?
 * Discussion**:

2) List an adaptation (from your study, internet research, today’s activity, etc.). What experiments would you need to run to show that this is adaptation? 3) Describe how the results in an ecological study could help you to understand an evolutionary process.

SUPPLEMENTAL INFORMATION AND TOPICS

 * A comparison of major constraints on desert and rainforest plants **

One of the striking differences between deserts and rainforest is the difference in the factors that structure the overall community. Competition for light shapes the rainforest community, while competition for water shapes the desert.

In the desert, plants are widely spaced and have extensive root systems designed either to rapidly take up any available surface moisture after rains or to forage deep into the ground for groundwater. For example, roots of a plant in the Kalahari Desert were found at a depth of 68m and may extend even deeper.

In the rainforest, canopy trees manage to intercept most of the light. Other plants are either relegated to the low-light environment in the understory or live as epiphytes. True epiphytes (e.g., many bromeliads and orchids) germinate in the canopy and live their entire life without contact to the ground. Hemi-epiphytes germinate on the ground and grow up to the canopy, but they eventually lose their contact to the ground. Stranglers germinate in the canopy, grow roots down to the ground, and then rapidly expand to wrap around the host tree, eventually killing it. Vines and lianas (woody vines) rely on trees for support, and, consequently, do not have to invest in supportive tissue. Thus vines can get to the light rapidly and with a low investment in metabolically expensive woody tissues.

//Suggested area of investigation://

Contrast the differences between the structural consequences of competition for light and water. Interpret the differences in the structure of the desert and rainforest communities in terms of these trade-offs for light and water.


 * Plant Defenses **

Unlike most animals, plants are not able to move and thus cannot evade predators or pathogens directly. Instead, they use a complex system of defenses. For the most part, plants make a trade-off between defending themselves from as many predators as possible and adapting to the damage caused by herbivores. Both of these actions require considerable energy expenditure. Most plants use some combination of the two methods. Grasses combine mechanical deterrence (e.g., positioning of the leaves, silica crystals in the epidermis which wears down the teeth of grazers), chemical defenses, and a high tolerance of grazing. Grass leaves also possess intercalary meristems, which are stimulated to elongate by grazing.

Myrmecophytes represent another very important strategy that plants use for defense in that ants are used as defense against other organisms. One of the most famous of these mutualisms is between ants of the genus //Azteca// and //Cecropia// trees. //Cecropia// is a genus of fast-growing pioneer species whose hollow trunks provide nesting chambers for the ant colonies and branches provide two types of food: Müllerian bodies, which are rich in lipids, carbohydrates, proteins, and amino acids; and Pearl bodies, which are less well known but are believed to be rich in lipids. Sagers //et al.// (2000) found that adult worker ants receive ~19% of their carbon from the tree, while about 43% of the carbon intake of the ant larvae comes from the host tree. In addition, ant corpses and waste products supply 93% of the nitrogen used by the plant. Ants, on the other hand, attack any animals that land on the plant, clip away any vines that attach to the tree, and remove epiphytes that germinate on the tree.

For the most part, plant defensive compounds are called “secondary metabolites,” compounds that are derived from biochemical pathways other than the normal “primary” metabolic pathways. Plant secondary metabolites play a large role in the manufacture of drugs and pharmaceuticals. One of the most famous of these is the Rosy Periwinkle (//Catharanthus roseus//); see the note card in the greenhouse describing this species, which is the source of two drugs that have changed childhood leukemia from a usually fatal disease to a usually curable disease. Two recently isolated secondary chemicals with anti-HIV activity are shown below (Figure 1).

Magnoflorine: Nitidine:


 * Figure 1. ** Two anti-HIV alkaloids, magnoflorine and nitidine, were found in //Toddalia asiatica// (L.) Lam. originally collected in the Mufindi District of Iringa Region of Tanzania in November of 1988. Nitidine inhibited human lymphoblastoid cell killing by HIV-1, and magnoflorine revealed less robust anti-HIV activity with a maximum cytoprotection of only 50%. Nitidine was previously known from //Toddalia//. Magnoflorine, although previously unknown in //Toddalia//, was known from three other genera of Rutaceae, //Fagara//, //Phebalium//, and //Zanthoxylum//. Information and figures from Missouri Botanical Garden (2001).

//Suggested areas of investigation://

§ Identify (at least) three species with conspicuous mechanical defenses in the desert house. What types of herbivores or pathogens would these adaptations defend against? What are some alternative reasons why the plants might have these structures //other than// as defensive structures? Can you design an experiment that would allow you to verify whether these are truly defensive in their function?

§ Identify (at least) three species with obvious chemical defenses. How might you identify plants that appear to have chemical defenses? (Hint: Citronella oil, which is used in citronella candles, is a defensive compound produced by Citronella Grass.)

§ How might you verify that a chemical produced by one of these plants (or at least the crude leaf extract) is effective against a certain herbivore or pathogen? Would demonstrating that a plant extract was effective against a certain herbivore be enough to establish that the chemical plays this role in nature?

§ Producing Müllerian bodies to feed ants entails a considerable expenditure of energy for a //Cecropia// tree. In addition, there is the danger that the ants might take the reward without providing any protection for the tree. How might you determine whether //Cecropia// trees actually receive a net advantage from //Azteca// ants?


 * CAM Plants **

CAM (Crassulacean Acid Metabolism) is a photosynthetic pathway that allows plants to take in and store carbon dioxide (CO2) at night when temperatures are lower. CAM plants open their stomata at night and chemically fix CO2, which is then stored in the vacuole sap. During the day, these plants close their stomata to minimize water loss and release the stored CO2, which is then fixed photosynthetically. In extreme drought conditions, CAM plants keep their stomata closed permanently and recycle the CO2 that they produce metabolically. Also, CAM plants tend to have succulent stems or leaves.

Although the CAM pathway is primarily found among desert plants (e.g., Cactaceae, Crassulaceae), this photosynthetic system is also found among rainforest epiphytes, especially among members of the Bromeliaceae (bromeliads). Some species are facultatively CAM in that they only use CAM metabolism when water is limiting, while other species are obligately CAM even when water is present in abundance.

//Suggested areas of investigation://

§ What is the benefit of using this metabolic system in an environment where water is present often in excess? Why would only a small subset of rainforest plants use this photosynthetic system?

§ Look for similarities among CAM plants in the Desert House and in the Rainforest House. What environmental similarities can you imagine between the canopy of a rainforest and a desert?

§ What are some other reasons why desert plants and rainforest canopy plants may share this system? What reason, other than similar environments, could explain the sharing of ecological characters between two groups of plants?


 * Adaptations to drought **

Desert plants are often broadly classed into “drought avoiders” and “drought tolerators.” These plants have mechanisms that allow them to deal with the extreme water stress, or they find some way of ensuring that they do not experience such extremes. In reality, the ways that plants deal with drought are somewhat more varied. Some of the main ways are by using CAM photosynthesis, rooting deeply (e.g., phreatophytes), going dormant in seed form (e.g., annuals), shedding expendable organs (e.g., drought deciduous species), and storing water.


 * CAM plants** open their stomata at night and chemically fix carbon dioxide (CO2), which is then stored in the vacuole sap. In the day they close their stomata to minimize water loss and release the stored CO2, which is then fixed photosynthetically. In extreme drought conditions, CAM plants keep their stomata closed permanently and recycle the CO2 that they produce metabolically.


 * Phreatophytes** (“well plants”) are deep rooting and manage to gain access to groundwater. Thus they avoid drought.


 * Annuals** germinate, flower, set seed, and then die within a single wet period. Some arid areas have regular and reliable wet seasons. In others, rains come only rarely and unpredictably. Thus, some annuals have relatively long growing seasons and have no reason to make especially long-lived seeds. Others face very short, erratic growing seasons and need to produce seeds that can remain viable for very long periods of time.


 * Drought deciduous species** shed their leaves during the dry season, much like deciduous trees do in Michigan in autumn. However, unlike temperate deciduous trees, drought-deciduous trees tend to leaf-up just before the rains come, and some species only remain leafless for short periods of time.


 * Water storage plants** have low-density tissue in their stems that allow them to store water. Some of these plants may shed their leaves during the dry season and photosynthesize through their bark.

//Suggested areas of investigation://

§ What might be some of the advantages and disadvantages of these different strategies? Are you able to find plants that appear to have more than one of these? Which species do you think could be called drought avoiders as opposed to drought tolerators? Why do you classify them this way?

§ Right now the drought deciduous species have leaves and the annuals (e.g., //Argemone mexicana//, Mexican Thistle) are alive and flowering. How could you determine the advantages of some of these strategies?


 * Rainforest Understory Plants **

At the north end of the Rainforest House are a number of plants that do not participate in the intense struggle for light resources characteristic of the humid tropics. The understory light environment is heterogeneous and highly variable both spatially and temporally. The understory has a mixture of seedlings of shade-tolerant canopy trees and true understory specialists. Most available light is in the form of sunflecks, small patches of bright sunlight that penetrate the canopy as the sun moves across the sky.

Understory plants require a unique set of adaptations to survive using the unpredictable and short-term sunflecks. Although the understory has much less light than the canopy, understory plants are well adapted to using sunflecks. Pearcy (1987) found that the understory light environment had only 3% as much light as the canopy, but found that understory plants were able to fix 10% as much carbon as the canopy trees. Sunflecks, lasting between a few seconds and a few minutes, have been shown to account for between 10 and 52% of the total carbon gain by understory species (Chazdon 1986, Vierling and Wessman 2000).

The presence of large canopy trees overhead has other impacts on the understory environment. For example, understory plants experience much less wind stress than canopy trees.

//Suggested areas of investigation://

§ What probable adaptations to this environment do you see among these plants? What trade-offs might they be making?

§ How could you test whether some of these adaptations might actually convey some advantage to the plant in this environment?


 * Vines, lianas, and pioneer trees **

Although vines and lianas (woody vines) are present in most biomes, they are especially prevalent in the humid tropics. In disturbed sites, vines can completely take over the canopy and even suppress tree growth. Vines and lianas are “mechanical parasites” by relying on trees for support instead of investing in its own supportive woody tissue. Vines and lianas have large vessels in their xylem, which produce little resistance to the flow of water but also provide little support to the stem. Their stems are usually twisted which makes them less susceptible to snapping if their host tree falls.

Fast-growing early successional trees like //Cecropia// have hollow stems and are usually shade-intolerant (Figure 2). Since they are freestanding trees (i.e., they grow in open areas, such as where the trees have been cleared), they need to be able to support their own weight. On the other hand, they need to grow rapidly so that other trees do not shade them out. Instead of investing a lot of energy into woody stems, these trees have hollow stems or very low-density wood.



Figure 2. Cross-section of the hollow stem of //Cecropia schreberiana// (from Janzen 1973).

//Suggested areas of investigation://

§ Examine 3 or 4 of the vines/lianas and the //Cecropia// tree in the rainforest house. What trade-offs are these plants making? How would you design an experiment to test the contribution or value of these trade-offs?


 * Leaf Morphology **

Leaf morphology is a function of the light environment, nutrient budgets, energy budgets, and seasonality of the climate. Thick leaves can be an adaptation to water stress (i.e., thick cuticles protect leaves from desiccation) or nutrient stress. Whether a plant is evergreen or deciduous can be a function of the severity of the dry season, but there are also many trees in dry environments that remain evergreen. This appears in part to be an adaptation to nutrient limitations in that it costs less in terms of nutrients and energy to keep leaves than it does to make new ones, but the plant will lose more water through the dry season if it keeps its leaves.

Examine the following plants:

//Clusia// – This plant is a strangler that starts its life up in the canopy without contact to the ground, but eventually grows roots down to the ground. //Leucaena// – This leguminous tree is a Nitrogen-fixer and will drop its compound leaves in the dry season if the drought is sufficiently severe. //Citrus// – Although this is a cultivated tree, its leaf morphology is characteristic of many rainforest trees. The leaf tips are elongate to help water run off quickly after rain, and the leaves are very long-lived. //Cecropia// – This tree is a fast-growing pioneer and is common in disturbed areas. This genus does not keep its leaves very long because the rest of the fast-growing pioneer community quickly shades old leaves. Since disturbance tends to produce a pulse of nutrients, this genus is usually not nutrient limited. In addition, //Cecropia// has been shown to get 93% of its nitrogen from its ant mutualists (Seger //et al.// 2000) //Crassula argentea// – These succulent desert plants are primarily subject to drought stress.

//Suggested areas of investigation://

§ Contrast the various leaf-morphologies in these and other plants. How could you test the causal relationship between life-style and leaf morphology?


 * ACTIVITY (Alternate)**

//PROBLEM:// Allen’s Rule (1877) states that warm-blooded animals found in colder climates (i.e. farther from the equator), tend to have shorter limbs than similar warm-blooded animals found in warmer climates (i.e. closer to the equator). Does this rule apply to plants as well?

The task before you today is to discover an answer to the problem presented to you above. To do this, your group will be asked to do the following:

1) Develop a research hypothesis 2) Translate your research hypothesis into a testable prediction 3) Determine the appropriate methods to investigate your hypothesis (see details below) 4) Carryout your investigation 5) Analyze the collected data 6) Summarize, and integrate your results to answer the question proposed and state it in the context of adaptation 7) Explain the results to your peers

We will be using the Teaching Greenhouses, located just west of the Natural Science Building, to investigate this problem. There are two main houses that may be of use to you: 1) a Desert House and 2) a Tropical Rainforest House. There is also a third house, the Evolution House, which showcases plants along evolutionary time progressing from the first primitive aquatic plants, to the most advanced angiosperm plants. You will also have rulers, calipers, hand lenses and paper rulers available for use.

Keep in mind issues of replication, sample size and how you will statistically determine whether or not these data collected actually support your hypothesis!

Your lab grade for this activity will be determined by the work you do in the lab (i.e. did your group go through all 7 steps successfully.

Allen, J.A. 1877. The influence of physical conditions in the genesis of species. Radical Review 1: 108-140.
 * Reference:**


 * || ===ADAPTATION LAB=== ||


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 * Data Sheet:**
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