Laboratory Notes for BIO 1003

© 30 August 1999, John H. Wahlert & Mary Jean Holland

PLANT STRUCTURE

Domain Eukarya
Kingdom Plantae
Division Anthophyta

Vascular plants have distinctive cell types, all of which are surrounded by a cell wall of cellulose fibers and other molecules secreted by the cells. Just as in animals, cells are organized into tissues that perform different functions, but plants do not have organ systems like those of animals. The tissues of plants are grouped into 3 basic kinds: ground, vascular, and dermal. Meristem is a special embryonic tissue.

Plants differ from animals in that the tips of roots and stems, called apical meristem, remain embryonic and retain the ability to form new structures, e.g., leaves, stems, flowers, and roots. Hormones secreted by meristem cells are transported elsewhere in the plant; meristem is in part analogous to the endocrine system in animals.

Ground tissues include parenchyma, collenchyma, and sclerenchyma. Thin walled parenchyma cells have a variety of functions such as photosynthesis, starch storage, and secretion; they retain the capacity to divide and are important in repair of damage. Pliable collenchyma cells help strengthen the plant; their walls are thickened and made strong with cellulose and pectin; celery strings are an example. Sclerenchyma cells have very thick secondary walls that are commonly impregnated with lignin, which makes them quite rigid. The lignified sclerenchyma of flax plants is made into linen threads for weaving, sewing, and paper making. Wood is made of lignified xylem cells. The hardness a coconut shell or a peach pit is caused by lignified cells. Ground tissues are analogous to the supporting connective tissue and skeletal elements in animals.

Vascular tissues include xylem and phloem; this is the plant's circulatory system. At maturity xylem cells are dead and form interconnected tubes throughout the plant. They conduct water and dissolved nutrients that the plant absorbed from the soil; their thick, sclerified walls allow them to give mechanical support to the plant. Wood is made of xylem cells. Phloem cells are tubular and have interconnected cytoplasm; they conduct other solutes, chiefly nutrients (e.g., carbohydrates) from areas of food production such as leaves to areas of food storage such as tubers.

Dermal tissues include epidermis and cuticle. The epidermis is a continuous layer of tightly packed cells. It is usually coated with a cuticle of waxes embedded in a fatty substance; this is analogous to keratinized outer layer of skin, including your own, in animals that live on land. Leaf epidermis is perforated by stomata for gas exchange between the photosynthetic mesophyll (parenchyma) and the surrounding atmosphere. Thus leaves function in part like lungs.

Dicot leaves are much like fern leaves in general structure. The upper and lower epidermis are covered with a waxy cuticle to prevent water loss. The mesophyll consists of a palisade layer of tall cells just under the upper epidermis, and chains of spongy mesophyll cells with air spaces between them in the lower part of the leaf. The tall palisade cells are packed with chloroplasts and are the site of most of the photosynthesis in the leaf; they are located in the upper part of the leaf where sunlight penetrates most fully. Spongy mesophyll contains air for gas exchange with cells of the leaf. Carbon dioxide from air is the source of carbon for synthesis of carbohydrates, and oxygen gas is a waste product of photosynthesis. Slides:

Among the Anthophyta, monocots and dicots are easily distinguished. In monocots the leaf veins (vascular bundles) are parallel; in dicots they are branched. Flower parts in monocots are based on multiples of three; in most dicots on multiples of 5.

Slides

Syringa leaf x.s. (dicot). Identify the upper and lower epidermis, the palisade mesophyll, and the spongy mesophyll. Pairs of small cells in the lower epidermis that have a space between them control the stomata, the openings between air outside the leaf and air in the spaces in the spongy mesophyll. The central rib of a leaf is its main vascular bundle, and it gives rise to many side branches. Identify xylem and phloem, the kinds of elongated cells that transport water and dissolved nutrients respectively throughout the plant. Xylem cells in cross section are large with thick walls that are probably stained pink. Phloem consists of tiny, thin-walled cells below the xylem; it is probably stained green. These vascular cells are surrounded by supporting tissue, some of which is hardened as sclerenchyma to give rigidity.

Leaf, lower epidermis. In this view the many openings that communicate between surrounding air and air in the spongy mesophyll can be seen. The singular for each slit is stoma, the plural stomata. Each stoma is surrounded by a pair of guard cells which have a thicker wall facing the opening than on the side that is continuous with the surrounding epidermal cells. When water is available, the guard cells swell and bend away form each other (the stoma opens); bending occurs because the thick cell wall on one side is not as stretchy as the rest. The entire epidermis looks like a jig-saw puzzle of cells dotted with stomata.

Coleus stem tip. l.s. (dicot). This is a longitudinal section through a common ornamental house plant. You can distinguish the central stem and the leaves branching off it. The very tip of the stem is embryonic tissue called apical meristem; as the plant grows, it differentiates into stem, leaves, and flowers. Meristem present in the axils of the leaves (between the leaf and stem, above the leaf) forms axial nodes; these will grow into new stems and leaves if the apical meristem is snipped out. Cells of the apical meristem secrete a hormone that suppresses growth of the axial nodes; when the meristem is removed, the hormonal control is removed. In some areas you can distinguish xylem cells that are supported by a coil of thickened wall that is stained pink; this coil around the cell is somewhat like the cartilage rings that hold your trachea (windpipe) open.

Ranunculus acris: mature root x.s. (dicot). Xylem forms a cross in middle of the root with phloem in its angles. The group of xylem and phloem cells is a vascular bundle. The vascular bundle is located in the center of the root, and it is surrounded by a ring of endodermis cells.

[center of Ranunculus root]

Stem. Monocot and dicot. Dicot stems have vascular bundles around the periphery and not in the center, as the root does. The vascular bundle of the root must divide and spread out into the many bundles of the stem. Broad, thick-walled xylem cells are toward the inside of the stem, and phloem is toward the outside; the outermost part is a bundle cap of sclerenchyma cells which give support. The center of the stem is called pith. The monocot stem has xylem and phloem in bundles that look like monkey faces. There are two large xylem cells making the eyes; an air space makes the nose; thin-walled phloem makes the forehead, and above it is thick walled sclerenchyma. These are scattered throughout the stem cross section. Cells other than these kinds are parenchyma. Both stem examples are of herbaceous stems, which lack woody tissue and growth rings.

Tilia. 1, 2, 3, yr. stem c.s. (dicot). Successive years growth of vascular tissue adds layers to the outside of the stem. These are referred to as growth rings, and the stem is woody. The outermost layer is cortex or bark; just deep to it is the phloem in which you can see triangular wedges called rays. The inner part consists of successive layers of lignified xylem.

[woody dicot stem, years 1, 2, 3]

Grass root tip w.m. (monocot). The root tips of plants are covered with fine root hairs that are like rhizoids--cells that extend out from the root. These maximize water absorption. When you pull up a plant, you shear off the root hairs and the plant may not be able to get enough water when replanted. It is best to dig up a plant in a ball of earth. Beneficial associated fungi (micorrhizae) are also transferred.

Zea mays prop. root tip. (monocot). This propagating or growing root tip contains the root meristem or embryonic tissue. The blunt cap of the root is tough and is pushed through the soil by the dividing and elongating cells. The root meristem is just behind the cap. Here cells undergo mitosis as you saw in previous lab observations of onion root tip. Cells farther up the root are in the extension zone and will elongate but not divide again.

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Last updated 30 August 1999 (JHW)