Stem Anatomy WHAT DO you think of when you think of a stem? Do you think of a flower stalk, the trees in your area, or a soybean stalk? Most people probably visualize something like the flower or the bean stalk. Study the stem and you ll learn how incredibly important it is to the life of a plant. Objective: Describe the structures and functions of a stem. Key Terms: apical meristem bud scale scar bud scales bulb cambium cladophyll climbing stems corm cortex heartwood herbaceous stems internode lateral bud leaf scar lenticels node phloem pith rays rhizome sapwood scape spine stolon terminal bud translocation tuber water potential woody stems xylem Stems Stems have many important jobs in a plant. Stems are responsible for the size and shape of a plant. Stems serve several functions. Stems support the leaves. They hold the leaves in the most efficient position to collect sunlight. This allows the plant to produce as much food as possible. Stems move water, minerals, and manufactured food throughout the plant. Stems that are green in color help produce food through photosynthesis. While this is not usually the Page 1 www.mycaert.com
primary food production, it can be quite important in plants with no leaves or very small leaves. Stems store food that has been manufactured by the plant. Some plants have herbaceous stems, while others have woody stems. Herbaceous stems are usually soft, green, and flexible. Plants with herbaceous stems are annuals, biennials, or perennials that die to the ground at the end of the growing season. Woody stems are generally hard and produce secondary growth. They may go dormant at the end of a growing season and resume growth when conditions are right. EXTERNAL STRUCTURES OF A STEM There are many structures on the stem that serve different functions. They are also very useful in identifying plants. Close observation can reveal the external structures on a stem. The growing point at the tip of the stem, called the apical meristem, is contained inside the bud at the end of the stem, called the terminal bud. The apical meristem has the same type of structure that the tip of the root has and is responsible for growth in length of the plant. The leaf is attached to the stem at the node. The area between leaves is called an internode. At the node, just above where the leaf is attached, there is always a side bud, called the lateral bud. On the outside of both terminal and lateral buds are small protective structures called bud scales. When the leaf falls off the stem, it leaves behind a small scar, called the leaf scar, just below the lateral bud. When the buds sprout each spring, the bud scales fall off, leaving behind a ring of scars called the bud scale scar. The distance between the bud scale scars represents one year s growth of the stem. Lenticels are small openings on the stem that allow a stem to exchange gases with its environment. They appear as small spots or raised bumps. One Year s Growth Lenticel Bud Scale Terminal Bud Axillary or Lateral Bud Internode Node Leaf Scar (Vein Scar) Terminal Bud Scale Scar Pith FIGURE 1. Parts of a typical stem. Page 2 www.mycaert.com
INTERNAL STRUCTURES OF WOODY AND HERBACEOUS STEMS Inside the stem, there are tissues that transport materials throughout the plant. Stem tissues are organized in one of the following ways. The important vascular tissues are either found in small bundles scattered throughout the stem or arranged in rings or a ring of bundles. The first way, scattered bundles, is found in monocots. The second way, in rings, is found in dicots. Three important tissues found inside the stem are xylem, phloem, and cambium. The xylem is tissue that conducts water and minerals throughout the plant. The xylem is made of tube-like cells that grow together to conduct liquids. Xylem tends to be found closer to the center of the stem. The phloem is tissue that conducts food that is produced in the leaf to the rest of the plant. Phloem cells also form tubes. Phloem is found generally toward the outside of the stem. Cambium is tissue that is responsible for the production of new xylem and phloem. Cambium is responsible for growth in girth of the stem. Cambium is generally found between the xylem and the phloem. Monocot Stems and Herbaceous Dicot Stems Phloem Cambium Xylem Monocot stems and herbaceous dicot stems have some similarities and some differences. Monocot stems have an epidermis. The epidermis provides protection. Vascular tissues, xylem and phloem, are grouped in vascular bundles. The vascular bundles extend the length of the stem. A cross section of a stem shows that the vascular bundles are scattered throughout the stem. Xylem grows on the inside portion of a vascular bundle and phloem on the outside. Surrounding the vascular bundles and making up the remaining bulk of the stem is parenchyma tissue or ground tissue. The vascular bundles lack cambium that would give rise to woody, secondary growth. Herbaceous dicot stems have an epidermis. Just inside the epidermis is a layer of cells called the cortex. The cortex is composed of parenchyma, collenchyma, and sclerenchyma cells. Inside the cortex are the vascular bundles. In a cross section of a dicot stem, the vascular bundles are seen to form a ring. The xylem is on the inner portion of the vascular bundle, and the phloem makes up the outside portion of the vascular bundle. Between the two is a single layer of cells, called the vascular cambium. The center of the stem consists of the pith, which is made up of large, thin-walled parenchyma cells. The vascular bundles are separated by masses of pith cells that extend into areas between the bundles. These regions of pith cells are known as pith rays. Pith FIGURE 2. Arrangement of tissues in stems. Page 3 www.mycaert.com
Woody Stems When one views the cross section of a tree trunk, two different colors of wood are evident. The darker wood to the center of the tree is called the heartwood. The xylem cells of the heartwood have filled with gums, resins, pigments, and tannins. They provide strength Pith Ray and no longer function in conducting materials. The lighter wood circling the heart- Inner bark Outer bark wood is called the sapwood. The younger sapwood actively conducts water and dissolved minerals. The age of a tree can be determined by counting annual growth rings. The rate of growth impacts the growth rings. During rapid growth, the cells of the wood are thin-walled and large in diameter. As growth slows during mid-to late summer, the wood cells produced by the cambium become Bark smaller and have thicker walls. The differences in the cells made give the appearance Heartwood Cambium of rings. Each ring is the growth during one Sapwood growing season. Translocation The movement of materials through vascular tissues is known as translocation. A dilute solution of water and dissolved minerals moves through the xylem. Sugars move through the phloem. Water and dissolved minerals move in only one direction in the xylem. The flow of sugars in the phloem can move in different directions. Water Potential Water potential is involved in the movement of materials through the vascular tissues. Water potential is described as the free energy of water. The water potential of pure water is measured at 0 bars. Water potential is lowered (made negative) with dissolved substances. Water moves from an area of higher water potential (less negative) to an area of lower water potential (more negative). Water potential is a measure of a cell s ability to absorb water. It is also the measure of the ease at which water evaporates from the leaves. Translocation Through Xylem FIGURE 3. Cross section of a tree. The movement of water and dissolved minerals begins with absorption into the plant. Under moist soil conditions, the water potential in the root is more negative than that of the soil, so water moves into the root. The solution passes through several tissues to reach the tracheids and vessel elements. Once in the xylem, the water and dissolved minerals are carried Page 4 www.mycaert.com
upward. Interestingly, the plant uses no energy to move the solution. The solution is pushed up from the bottom and pulled from the top. Root pressure moves water up the xylem. Water and dissolved minerals move into the root by osmosis. The root tissues, full of water, exert pressure that forces water up the xylem. Root pressure is observed when there is plenty of soil moisture. The height at which solutions can be forced up the stem is limited. It certainly does not account for the movement of water to the tops of tall trees. Translocation of water also is made possible by a cohesion-adhesion mechanism. As water transpires, tension is created between water molecules at the top of the plant. This tension creates a pull on water molecules of the entire length of the stem. Thus, the water is pulled up the stem from tension caused by the evaporative pull. The cohesion-adhesion theory helps to explain how water reaches the tops of trees hundreds of feet tall. Translocation Through Phloem Sugars are translocated by the phloem. The sugars may be moved from the leaves to actively growing regions of the plant where they are used up. In other cases, they may be moved to storage in roots, stems, fruits, or seeds. The movement of materials through the phloem is a complex operation not fully understood. It can be explained by the pressure flow hypothesis. Sugars move from the cells where they are produced to companion cells in the phloem. ATP energy is used to make the movement of sugar to the companion cells possible. Once in the companion cells, the sugar moves easily to the sieve tube members. The water potential in the sieve tube cells is lowered with the increased concentration of dissolved sugars. Water then moves into the cells by osmosis. The osmotic pressure created pushes the sugar solution through the phloem. When it reaches its destination, the sugar is transferred out of the phloem with energy provided by ATP. The concentration of sugar drops in the sieve tube members. In the process, the water potential becomes less negative. Water moves to surrounding cells with more negative water potentials. The movement of sugar through the sieve tube cells does not require energy. However, energy is needed to load and unload sugar from the sieve tube cells. UNDER INVESTIGATION LAB CONNECTION: Monocot and Dicot Plant Tissues Conduct a laboratory activity to locate the vascular tissues and specialized tissues found in monocot and dicot plant specimens. Obtain a variety of plant materials for study. Examples include small woody branches, a corn stalk, a stem from a dracaena, tomato stem, and so on. Carefully cut the specimens straight across with a knife. Study the freshly-cut cross section. Use a magnifying lens to improve the view. Identify the vascular tissues. Draw cross sections of the two stems. Label the major parts. Based on the internal structures of the stems, classify them as monocot or dicot. Page 5 www.mycaert.com
Specialized Stems Generally, it is expected that stems will be upright and above ground. Although we are often right, there are many stems that do not fit into this mold. Some stems are modified to store food or help the plant reproduce. Some stems grow beneath the soil instead of above it. Some types of specialized stems are bulbs, corms, rhizomes, stolons, tubers, cladophylls, scapes, spines, and climbing stems. RHIZOME Typical Leaf TUBER Each Eye is a Node Scalelike Leaf at Each Node Adventitious Roots STEM TENDRIL STOLON Axillary Bud Fleshy Leaves Papery Leaves Flattened Stem CLADOPHYLL Stem Stem BULB CORM FIGURE 4. Specialized stems. Page 6 www.mycaert.com
A bulb is a very short, flattened stem that has several fleshy leaves attached to it. Bulbs tend to be found beneath the soil. An onion is a bulb. A corm is a spherical structure, much like a bulb. The entire structure, however, is a stem as opposed to a stem and leaves. A gladiolus is a corm. A rhizome is a thick underground stem that lies horizontally. Hostas and mother-in-law s tongue are rhizomes. A stolon is a horizontal stem that lies above the ground. A stolon is sometimes called a runner and tends to be involved with the spreading of the plant. Strawberries spread by stolon. A tuber is a rhizome with a tip that is swollen with stored food. Irish potatoes are tubers. A cladophyll is a flattened, modified stem that resembles a leaf. Asparagus and many cacti have cladophylls. A scape is a flowering stem. It is usually leafless and emerges from the crown or roots of a plant. A scape can hold a single flower or many flowers. A spine is a structure at the ends of branches or leaves that have been modified into cylindrical, hard structures with sharp ends. A spine may also be called a thorn, which is a reduced, sharp-pointed stem. The purpose of a spine is to protect plants from herbivores. Climbing stems are stems that cling or wrap around other plants or structures for support. Summary: The stem supports the leaves and flowers; moves water, minerals, and manufactured food throughout the whole plant; and plays a role in the manufacture and storage of food. Some external features of a stem are the apical meristem, terminal bud, node, internode, lateral bud, bud scales, leaf scar, bud scale scar, and lenticels. Three important tissues found inside the stem are xylem, phloem, and cambium. Cambium is tissue that is responsible for the production of new xylem and phloem. Monocot stems have vascular bundles scattered throughout the stem. Herbaceous dicot stems have vascular bundles that form a ring. Woody stems have heartwood and sapwood. The movement of materials through vascular tissues is known as translocation. A dilute solution of water and dissolved minerals moves through the xylem. Sugars move through the phloem. Some types of specialized stems are bulbs, corms, rhizomes, stolons, tubers, cladophylls, scapes, spines, and climbing stems. Page 7 www.mycaert.com
Checking Your Knowledge: 1. What are the main functions of stems? 2. What are the external structures of a stem? 3. What are the internal structures of a stem? 4. What is the process of translocation? 5. What are some types of specialized stems? Expanding Your Knowledge: Collect stems from woody plants that you have identified. Study the external features and note the differences. Practice identifying plants using only the twigs. Web Links: Plant Shoot System http://facweb.furman.edu/~lthompson/bgy34/plantanatomy/plant_shoot.htm Stems http://plantphys.info/plants_human/stems.html Transpiration & Translocation http://hcs.osu.edu/hcs300/pstrans.htm Plant Stem http://en.wikipedia.org/wiki/plant_stem Agricultural Career Profiles http://www.mycaert.com/career-profiles Page 8 www.mycaert.com