BIO 5 GENERAL BIOLOGY

ROOTS, STEMS, AND LEAVES

1. Plants have three major parts or organs, roots, stems, and leaves. In BIO 5, we have examined both the external structures and their internal structure or anatomy.

2. The root system is responsible for anchoring the plant, absorbing water and minerals from the soil, storing food, and conducting water and minerals and food. There are two types of root systems. A tap root system (bio5_lab\roots_stems_and_leaves\Acer_negundo_MC.jpg ) consists of a main root that grows downward vertically into the soil. All branch roots arise from the tap root. The second type of root system is called a fibrous root system. This type lacks a tap root and is comprised of roots that all arise from the base of the stem. Since the roots grow from the stem, these are called adventitious roots. The Coleus cutting in the image (bio5_lab\roots_stems_and_leaves\Cutting _MC.jpg ) shows adventitious roots arising from the stem. A fibrous root system is shallow and does not grow deep into the soil.

3. Roots grow longer only at the root tip. This is called apical growth and results from cell divisions in the root apical meristem and elongation of cells produced by the meristem. The root apical meristem is protected from damage from soil particles by the root cap. Find the root apical meristem, region of elongation, and root cap in the image ( bio5_lab\roots_stems_and_leaves\Root_tip_MC.jpg).

4. Water and mineral absorption takes place in mature areas of the root. The surface area of the root is greatly increased in these mature areas by root hairs ( bio5_lab\roots_stems_and_leaves\Root_hair.jpg). It is the mature region of the root that is the location of the typical root cross section shown in the image ( bio5_lab\roots_stems_and_leaves\Whole_cross_section_MC.jpg). Note that the cortex is larger than the diagrams used in class show. The cortex is the region of the root in which food (starch) is stored. The purple "dots" in the cells of the cortex are starch grains. The center of the root is the location of the vascular tissue, the xylem and the phloem. The partly bright red circle surrounding the xylem and phloem is the innermost layer of the cortex, the endodermis. The function of the endodermis is to filter minerals, allowing some to enter the xylem and excluding others. The outermost cell layer of the root is the epidermis. This cell layer is the boundary between the root environment and the root interior.

5. The very center of the root cross section ( bio5_lab\roots_stems_and_leaves\Vascular_cylinder_MC.jpg) is called the vascular cylinder. The xylem of the vascular cylinder is the red "X" in the very center. The xylem is surrounded by four green-staining areas of phloem. The xylem conducts water and mineral; the phloem conducts food (sucrose). The vascular cylinder is surrounded by the endodermis which consists partly of red-staining cells.

6. If you closely examine plants, like these Coleus plants ( bio5_lab\roots_stems_and_leaves\Coleus_Plant_MF.jpg), you will notice that the leaves get smaller and smaller toward the top of the plant. This is because the youngest (hence, smallest) leaves are at the top and the oldest (and largest) leaves are at the bottom of the stem. Like roots, stems have apical growth. The stems get longer from cell divisions by the stem apical meristem and cell elongation of the cells produced by the meristem. In addition to producing new stem cells, the stem apical meristem also produces the leaves. The image ( bio5_lab\roots_stems_and_leaves\Leaf_primordia_MC.jpg) shows the stem apical meristem in various views. Not that the young leaves (mounds of cells at the sides of the meristem) are produced in a precise pattern. This pattern prevents leaves from shading one another.

7. A cross section through the mature region of a stem ( bio5_lab\roots_stems_and_leaves\Labeled_MC.jpg) shows that there are distinct differences in the arrangement of tissues of roots and stems. The pith is located in the very center of the stem. The pith is a region of food storage in stems. The vascular tissue is arranged in separate vascular bundles forming a ring around the pith. The region between the vascular bundles and the outermost layer, the epidermis, is called the cortex. The cortex is a region in which photosynthesis occurs in green stems and in which food storage takes place. The stem epidermis is covered by a waterproof layer called the cuticle. The cuticle prevents water loss from the stem. Each vascular bundle of the stem ( bio5_lab\roots_stems_and_leaves\Vascular_bundles_MC.jpg) includes both xylem and phloem. The pith is at the top of the image; the epidermis is at the bottom. The xylem (large red-staining cells) is located toward the pith. The phloem (green-staining cells) is located toward the epidermis. Like the root, the stems conduct water and minerals and food. Also like the root, they store food. Some stems are green. These stems carry on photosynthesis. Additionally, stems bear leaves and reproductive structures. Some stems may be modified into twining structures ( bio5_lab\roots_stems_and_leaves\Vitis_tendril_MC.jpg).

8. Leaves differ in their arrangement on stems. If there is one leaf per node or point of attachment to the stem, the arrangement is called alternate ( bio5_lab\roots_stems_and_leaves\Wild_plum_branch_MC.jpg). If there are two leaves per node, the arrangement of leaves is called opposite (bio5_lab\roots_stems_and_leaves\Lonicera_MC.jpg ). If there are more than two leaves per node, this arrangement is called whorled. Leaves also differ in complexity. In some leaves the blade is not subdivided into leaflets (bio5_lab\roots_stems_and_leaves\Wild_plum_branch_MC.jpg ) though it may be lobed ( bio5_lab\roots_stems_and_leaves\White_Oak_Leaf_MC.jpg). These are called simple leaves. In some leaves, the blade is subdivided into leaflets (bio5_lab\roots_stems_and_leaves\White_ash_leaf_MC.jpg ). These are called compound leaf. The leaf shown in the image is a pinnately compound leaf. Some leaves are palmately compound. In trying to decide whether a leaf is simple or compound, it is important to locate the axillary bud ( bio5_lab\roots_stems_and_leaves\Bitternut_hickory_budMC.jpg). Leaves have axillary buds. Leaflets don't.

9. The blade portion of the leaf is thin and rather fragile. The veins, especially the midveins, provide structural support for the leaf ( bio5_lab\roots_stems_and_leaves\Norway_Maple_MC.jpg). The leaf in the image has six midveins. This pattern of veins is called palmate net venation. A cross section through the midvein (or midrib) (bio5_lab\roots_stems_and_leaves\midrib_MC.jpg ) shows that this is a thicker region with many thick-walled support cells. A cross section through the blade portion of the leaf shows several smaller veins (bio5_lab\roots_stems_and_leaves\Cross_section_MC.jpg ). Each one of these includes a vascular bundle of both xylem and phloem cells. The cross section of the leaf blade shows both the upper and lower epidermises. The lower epidermis has numerous pores or stomata. Each stoma is surrounded by a pair of guard cells. The region between the two epidermises is called the mesophyll. The elongated cells are called palisade mesophyll; the irregular-shaped cells are called spongy mesophyll. While photosynthesis occurs in both types of mesophyll cells, most occurs in the palisade mesophyll and there are more chloroplasts in these cells. A thick cuticle covers both epidermises (bio5_lab\roots_stems_and_leaves\cuticle_MC.jpg). The image shows a red-staining cuticle covering the top-most cells in the leaf cross section.