Objectives: Upon completion of this lab, students should be able to:
This lab will cover several aspects of angiosperm (flowering plants) growth and development. The multicellular body of a flowering plant is composed of 3 types of vegetative organs: roots, stems and leaves. The flower is considered a greatly modified stem bearing greatly modified leaves. The following topics will be covered in this lab: seed structure, root structure, stems structure, and leaf structure.
The process of sexual reproduction in angiosperms produces seeds, which are used to propagate the species. Each angiosperm seed contains a dormant embryonic sporophyte and some endosperm. As the embryo germinates, it utilizes the endosperm as a source of stored food. In the following exercises we will examine basic seed structure and seed germination in monocot and dicot seeds. Monocots have one cotyledon (embryonic seed leaf) and dicots have two cotyledons.
The pinto beans provided in the lab have been soaking in water for more than 2 hours. Take one of the beans back to your table and examine the entire bean. Compare the size and texture of the wet bean to the dry beans provided.
Note the kidney shape of the seed and identify the following parts of the seed: the tough outer covering (seed coat), the elliptical scar (hilium) on the concave side of the seed, and the small opening (micropyle) near one end of the hilium. The hilium is a scar left when the bean was harvested from the pod. The micropyle is an opening in the seed coat to the ovule. The remainder of the seed inside of the seed coat is the embryo.
Use a razor blade to separate the seed into 2 halves. The 2 fleshy halves, which make up most of the embryo, are the cotyledons. The smaller part of the embryo may be viewed as a miniature plant. It consists of a hypocotyl, which is below the point of attachment of the cotyledons, and an epicotyl, which is above the point of attachment of the cotyledons. The tip of the hypocotyl is called the radicle and will give rise to the primary root of the bean plant. The epicotyl consists of the 2 small foliage leaves (plumule) and a minute shoot tip. Examine closely the small part of the embryo identify the hypocotyl, cotyledons, and the plumule. Make drawings of what you observe below.
The corn seeds in the beaker have been soaking in water for about 12 hours. Take one of the corn seeds and examine the entire seed. Note the tough outer covering (seed coat), and the peduncle (point of attachment of the seed).
Lay the corn seed flat on your desk and use a razor blade to cut the seed into 2 equal halves. Add a drop of iodine solution to one of the cut surfaces. Wait a couple of minutes and examine the cut surface to which iodine has been applied. Iodine turns starch black.
In corn seeds a relatively large endosperm is still present in the seed. The endosperm is high in starch content, so the endosperm is the portion of the seed that is stained black by the iodine. The embryo is located in the portion of the seed that is not stained by the iodine. The embryo in corn seeds consists of a single cotyledon, a hypocotyl, and an epicotyl. However, it will be difficult to distinguish the parts of the embryo because of its small size. Examine the cut ends of the corn seed and identify the endosperm and the embryo. Make a drawing of what you observe below.
3-day-old bean seedling: Obtain a 3-day-old pinto bean seedling from the germination tray and return it to your table. Identify the following structures on the germinating plant. The hypocotyl is the first portion of the embryo to elongate and rupture the seed coat. This portion of the embryo will give rise to the primary root. The primary root will turn downward regardless of the position of the seed in the soil. The epicotyl has not increased in size enough by 3 days to emerge from the seed coat.
6-day-old bean seedling: Obtain a 6-day-old pinto bean seedling from the germination tray and return it to your table. Identify the following structures on the plant. The upper portion of the hypocotyl has increased in length and has formed an arch, which is evident above the surface of the soil. Examine the primary root and note the presence of branch roots. The foliage leaves of the epicotyl may now be seen emerging between the cotyledons. The cotyledons in 6-day-old seedlings are green and plump. The seed coat has fallen away or is only loosely attached to the cotyledons. The arch in the stem will disappear as the stem grows.
10-day-old plant: Examine the 10-day-old plant and note these changes. By 10 days the hypocotyl arch has straightened, the cotyledons have lost the seed coat and may be beginning to shrivel, the epicotyl has elongated, and the foliage leaves have increased in size. The epicotyl will continue to grow and will eventually give rise to stems, leaves, and flowers. The lower portion of the shoot of the bean plant up to the cotyledons is derived from the hypocotyl. The portion of the shoot above the cotyledons is derived from the epicotyl.Make sure you are able to identify the different parts of bean seedling anatomy.
This slide, which was used in the lab on cell division, will be used to demonstrate primary growth (increase in length) from the apical meristem of the root. Growth of roots involves cell division (mitosis) and cell elongation. Primary growth is usually restricted to an area called the apical meristem, near the tip of the root. The cells making up the apical meristem remain capable of cell division throughout the life of the plant.
Examine the longitudinal section of an onion root tip under scanning power and identify the following regions with the help of the figure.
Examine the buttercup root cross-section under scanning power. Identify the outer epidermis, the cortex, and the central vascular cylinder (stele). The cortex forms the bulk of the root. The cells of the cortex are characterized as being thin walled and not fitting close together.
Examine the cross-section under low and high power. Note the loose arrangement of the cortex cells and the starch grains (stained purple) found in many of the cells. Identify the following cells or cell layers making up the vascular cylinder: the large, thick-walled xylem cells (stained red) arranged in the shape of a cross; and the small compact phloem cells between the arms of the xylem. Make a drawing of what you observe below.
Corn is a monocot and in monocots the vascular bundles are scattered throughout the stem.
Examine the cross-section of a corn stem under scanning power and note the outer epidermis, the large, thin walled cells of the cortex, and the scattered vascular bundles.
Identify the following structures that are in bold. Examine the vascular bundles under low and high power and note the outer supportive cells of the bundle, which have thick walls and are stained red. The xylem region of the vascular bundle is composed of large, open xylem vessels (stained red) and smaller tracheids (also stained red). A large air space is also usually present in the xylem. The xylem is always oriented toward the center of the stem. The phloem is composed of thin walled cells located inside the vascular bundle, but oriented toward the outside of the stem. The two types of cells making up the phloem are sieve tube cells and small companion cells adjacent to the sieve tubes. Make a drawing of what you observe below.
Examine the sunflower stem cross-section under scanning power and note the outer epidermis and the ring of vascular bundles in the outer region of the stem. Examine the vascular bundles under low and high power. Identify the large xylem cells and the smaller phloem cells. Outside of the phloem is a conspicuous mass of supportive fibers called collenchyma (stained red). The cortex lies between the vascular bundles and the collenchyma. Make a drawing of what you observe below.
Examine the slide of Lilac (Syringa) under scanning power and note the midrib and the veins of the leaf. Examine a vein of the leaf more closely under low and high power. The veins are composed of a vein sheath, xylem (stained red) located toward the upper epidermis, and phloem located below the xylem. The same arrangement of the xylem and phloem occurs in the midrib.
Examine a thin portion of the leaf under low or high power and note it is only 8 to 15 cells thick. Identify the following features of the leaf:
Make a drawing of what you observe below.
Now examine the pine leaf cross-section. Note how different the pine needle appears from the lilac leaf. The pine leaf has special adaptations for a dry (xeric) climate. Note the thick cuticle on the outside of epidermis and the lack of air spaces (relative to the lilac leaf). The outer layer of tissue is composed of both epidermis and hypodermis. Examine the stomata on high power. The stomata are sunken. Mesophyll is located inside the hypodermis and lacks the two differentiated layers, palisades and spongy mesophyll. Two types of tissue surround the vascular bundle, a one cell layer endodermis and transfusion tissue inside the endodermal layer. Make a drawing of what you observe below.