* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Leaf FAQ
Ornamental bulbous plant wikipedia , lookup
History of botany wikipedia , lookup
Plant breeding wikipedia , lookup
Plant nutrition wikipedia , lookup
Plant defense against herbivory wikipedia , lookup
Flowering plant wikipedia , lookup
Plant secondary metabolism wikipedia , lookup
Plant reproduction wikipedia , lookup
Plant use of endophytic fungi in defense wikipedia , lookup
Photosynthesis wikipedia , lookup
Plant ecology wikipedia , lookup
Plant stress measurement wikipedia , lookup
Evolutionary history of plants wikipedia , lookup
Plant physiology wikipedia , lookup
Sustainable landscaping wikipedia , lookup
Plant morphology wikipedia , lookup
Plant evolutionary developmental biology wikipedia , lookup
Glossary of plant morphology wikipedia , lookup
Leaves from the Outside In This FAQ will answer some questions about the structure of leaves. The main focus is on the leaf surface: how fungi get through it and how we can study it. This will help you understand what is proposed in one of the grant proposals that you will evaluate. What do leaves do? If a plant’s job in life is to help a seed make more seeds (think about that!), then the leaf’s job is to gather the energy needed for the plant to grow, flower, and make more seeds. What do other parts of a plant do? Roots gather water and minerals that the leaves and rest of the plant need, and they keep the plant from blowing away or falling over. It is the job of the stems and braches to hold up the leaves in such a way that they get as much sunlight as possible. Flowers are a plant’s reproductive structures for making seeds and thus more plants. How do leaves gather energy? Sunlight is a form of energy. Photosynthesizing cells (mesophyll) in the middle of the leaf trap that energy. Chemical reactions in the mesophyll use the energy to make sugars from water (from the soil by way of the roots) and carbon dioxide (from the air). All living parts of the plant, including the leaves themselves, can use the sugars to release energy for the work of growth and development. What are leaf veins for? Leaf veins contain cells specialized for transporting water and minerals to the other cells of the leaf, as well as other cells specialized for transporting sugars from the leaf to other parts of the plant. The veins are the circulatory system of the leaf. What makes up the “skin” of a leaf? The outermost layer of cells in a leaf is the epidermis. There is an epidermis on both the upper and the lower surfaces of the leaf. Most epidermal cells are part of the flat surface of the leaf, but some are the guard cells mentioned below, and some form hairs with various functions. The entire outer surface of both epidermises is coated with a waxy cuticle. The outermost layer of the cuticle is almost pure wax, which may be highly sculptured. What do the cuticle and epidermis do? One of the greatest problems faced by land plants is that they lose water by evaporation to the surrounding air. The epidermis protects the leaf against losing too much water; in particular, water and water vapor cannot pass through the cuticle. But the carbon dioxide needed for photosynthesis can’t pass through the cuticle, either—now there’s a problem for you! The plant solves this problem by a compromise made possible the presence of pores called stomata (singular: stoma). The stomata can open to allow carbon dioxide to enter the leaf or close to keep water in. There is no cuticle over the open stomata. Pairs of cells called guard cells control the opening and closing of the stomata. Are the leaf surfaces of all plants the same? That is, if we make something happen in one plant species, will it happen in another? There are a few differences, some of which might matter for our purposes. Wheat and other grasses belong to a group of flowering plants that have long and narrow leaves, with veins running parallel. Most of their epidermal cells are in parallel rows along the length of the leaf. These cells are roughly cylindrical in shape. The leaves of most other flowering plants, including things like roses, carrots, and oak trees, don’t look at all like grass leaves. Their veins are not parallel, and their epidermal cells are flat and very irregular in shape. These cells often look like pieces of jigsaw puzzles, unlike the neat rows of epidermal cells in wheat leaves. How do fungi attack leaves? When a fungal spore starts to grow, the first thing we see is a tiny hypha called a germ tube elongating from the spore. For the hypha to invade the leaf and start an infection it must somehow get through the epidermis. It must either “drill” through the cuticle and epidermis or find a stoma and gain entry through it. Some fungi can drill, but Puccinia graminis (the fungus causing the stem rust of wheat) can only get in by way of a stoma. In either case, the germ tube forms a clinging structure called an appressorium, which sticks tightly to the leaf surface. Another hypha grows out of the appressorium. In some fungi this “peg” can generate enough pressure to burst through the cuticle and epidermis; in others the peg can only reach the interior of the leaf if it is directly over an open stoma. Once a peg is inside the leaf, it branches, sending hyphae spreading through the interior. When these hyphae contact mesophyll cells they can form haustoria that take up sugars and other nutrients that would otherwise serve the plant. In this way the fungus derives the nutrients it needs to grow and reproduce, producing new spores to spread the disease to other plants. How can we study the cuticle and epidermis in the laboratory? The plants are usually grown in plant growth chambers, in which the day length, light intensity, temperature, and (sometimes) humidity are precisely controlled. Growth in such chambers yields uniform plants that can be studied as they undergo experimental treatments. The most useful tool for observing the fine structure of the cuticle and epidermis is the scanning electron microscope. A micromanipulator can be used to place spores at precise locations on the cuticle or to disrupt the structure of the cuticle or epidermis. We use a light microscope to see what we are doing with the micromanipulator.