Q: If chloroplasts are what make a leaf appear green and they are only present in leaves, why does the stem of an herbaceous plant also appear green?

A: Actually, chloroplasts are found in both the leaf and stem cells of herbaceous plants, making these structures appear green. Chloroplasts contain the green pigment chlorophyll, a molecule responsible for absorbing the light energy used in photosynthesis. Chlorophyll pigments are also responsible for the green color of chloroplasts.

Q: Why does the course consistently refer to plants (such as grasses) evolving better methods of, say, growth? Do plants really evolve?

A: The premise of natural selection is that members of a population that possess adaptations to their environment are more likely to survive and reproduce. Although natural selection does act on individuals by determining which of them will survive and reproduce, individuals do not evolve. However, populations can undergo evolutionary change over many generations. Recall that a population consists of individuals of the same species that live in a certain place at the same time. Environmental pressures (competition for water, light, or space) determine which individuals survive to reproduce. In this way, natural selection enables populations to adapt-through modifications in structure, physiology, ecology, and behavior-to different environments. If a population continues to evolve over time it may eventually become a separate species.

Q: What would happen if you put some leaves in bags? Would they make enough carbon dioxide to continue photosynthesis?

A: During respiration, a plant consumes oxygen and produces carbon dioxide by breaking down carbohydrates. During photosynthesis the opposite reaction takes place: the plant consumes carbon dioxide and water to produce oxygen and carbohydrates. If the leaves are still attached to the plant when they are placed in the bag, respiration will produce enough carbon dioxide for photosynthesis to continue. This concept is explored in Activity 26: Plant-in-a-Bag. However, if the leaves are removed from the plant before being placed in the bag, photosynthesis will cease because the water source has been eliminated.

Q: How are pyruvate molecules broken down into carbon dioxide?

A: During respiration, carbohydrates and oxygen are consumed by a plant to produce carbon dioxide, water, and energy. Throughout the first two stages of respiration, the carbohydrate glucose is broken down into carbon dioxide. In the first stage, called glycolysis, each six-carbon glucose molecule is split into two three-carbon molecules of pyruvate. In the Krebs cycle, the second stage, the three-carbon pyruvate molecules are broken down into two smaller acetyl groups. These acetyl groups enter the Krebs cycle where they are oxidized to yield two molecules of carbon dioxide.

Q: What are the advantages and disadvantages associated with vegetative propagation?

A: Another term for vegetative propagation is asexual reproduction. In asexual reproduction, a single individual gives rise to offspring that are genetically identical to the parent. This form of reproduction does not involve the union of gametes. Instead, stems, leaves, and roots give rise to new individuals when they become separated from the rest of the plant. Because they are rooted in one place and often rely on insect pollinators, plants have limitations when it comes to finding suitable "mates" for sexual reproduction. Since asexual reproduction only requires one parent, this is not a concern. However, natural populations have to keep adjusting to a constantly changing environment. The genetically identical offspring of asexually-reproducing plants lack recombination and genetic variability. As a result, these populations are not able to adjust as readily to changing conditions as populations that can reproduce sexually.

Q: In a plant with perfect flowers, is there an advantage to pollinating with another plant over self-pollinating?

A: Perfect flowers have both female and male reproductive structures (carpel and stamen, respectively) and therefore can self-pollinate. Plants are self-pollinated if pollination occurs within the same flower or within a different flower on the same plant. Many angiosperms have adopted self-pollination as a regular mode of reproduction, despite the genetic benefits of outcrossing, which is the cross-pollination between individuals of the same species. Self-pollination, which results in offspring populations with a high proportion of genetically similar individuals, can be advantageous under certain circumstances. For example, it may be desirable where an individual is well-suited to its environment. Another advantage is the lack of dependence on animals or other pollination agents. However, there are also benefits to outcrossing. Primarily, outcrossing provides the greatest opportunity for variability in a species and, therefore, enhanced adaptability and survival over generations.

Q: Are pollen spores more complex versions of moss spores or a completely different type of spore?

A: The spores found in pollen grains and in moss seta capsules are the same type of spore. They are both haploid reproductive cells that give rise directly to individual offspring.

Q: Is there a specific time to collect seeds?

A: At the end of the season, allow the last blooms to form seedheads. Gather seedheads when they look almost dry (crisp, papery, or stiff). As a rule of thumb, this happens about one month after flowers fade. The best time for gathering fruit is in the afternoon on a sunny, dry day. Rain or dew on the fruit promotes mold in storage. Collect fruits in a paper envelope or bag and take them indoors. Let them thoroughly dry and then open the seedheads and shake the seeds out. Make sure they are absolutely dry before storing them in an airtight container in a cool, dark place until planting time next spring.

Q: Where can I find more information about the chilling requirements for specific seeds?

A: Some seeds require a chilling period prior to germination to help them break their natural dormancy. The length of this period depends on the seed in question. There are no general guidelines. If you purchase a package of seeds that have a chilling requirement, specific instructions will be listed on the label. Some good general references for germination requirements are:

Seeds of Woody Plants in the United States published by the US Forest Service. It can be purchased through the Superintendent of Documents, US Government Printing Office, Washington, DC 20402.

American Horticultural Society. 1999. Plant Propagation: The Fully Illustrated Plant-By-Plant Manual of Practical Techniques. DK Publishing. ISBN: 0-7894-4116-0.

Powell, Eileen. 1995. From Seed to Bloom. Storey Books. ISBN: 0-8826-6259-7.

Q: What should one plant after tomatoes, for example, to "clean up" the soil?

A: Plants that are closely related often share pest and disease problems. One way to reduce their spread is to use crop rotation. Crop rotation involves growing families of vegetables in different locations in the garden from year to year so it becomes more difficult for pests and diseases to find the plants they favor. A general rule is to avoid planting crops from the same family in the same location for three years. For example, tomato is in the Solanaceae family, as are potato, pepper, and eggplant. If you have planted tomatoes in a certain location this year, avoid planting these other crops in this location for the next two years.

Q: I want to amend my soil with compost. What is the proper compost-to-soil ratio?

A: There's no hard and fast rule. It's hard to add too much organic matter to garden soil, and every bit helps maintain good soil structure, improves moisture management, and increases nutrient-holding capacity. Generally, gardeners are advised to add a 1- to 2-inch layer of compost each time they till, turn, or replant a bed or garden. It's also helpful to add partially-decomposed organic matter, such as rotting leaves or old straw, and fresh green materials like grass clippings or a green manure crop. This provides a rich array of materials in various stages of decomposition and with different nutrient levels to boost and widen the soil microbe population, and ultimately, soil quality. Your climate (temperature and humidity), soil pH, and soil type also affect the rate at which organic matter is used up by microbes, so check with your County's Cooperative Extension office for regional specifics.