Forensics Introduction
The plant body is usually composed of three kinds of vegetative organs: leaves, stems,
and roots. In this lab you will learn about the various cells and tissues that make up
these organs.
A tissue is a group of cells that are structurally and/or functionally distinct.
As you study the material in this lab, always start at the lowest magnification so
that you get an overview of the tissues. Then, use higher magnifications to
examine the tissues and cells in detail.
Remember to make drawings of the different structures that you see.
Plant Tissues
Parenchyma
Parenchyma cells are widely distributed throughout the plant body. Parenchyma cells
are living at maturity, have uniformly thin cell walls, and come in a variety of shapes
(see page 341). They may contain chloroplasts and be capable of photosynthesis.
Parenchyma cells often function as storage cells for water and/or food reserves or as
secretory cells.
Prepare a slide of potato tissue. Scrape a small amount of tissue onto a slide. Add a
drop of water and a cover slip.
Sclerenchyma
Sclerenchyma can be found in two different cell types, sclereids and fibers. These cells
provide mechanical support or serve to make plant tissues hard. Fibers and sclereids
differ greatly in shape but share two common features: a.) they always have a thick cell
wall and b.) they are always dead at functional maturity. These cells die before the start
serving their function in the plant.
a.) Sclereids. Sclereids (stone cells;see p. 341).) are usually found in the hardest parts
of a plant [e.g., seed coats, nut shells, and the tissue surrounding a pit in a certain fruits
(cherry pit)]. Their primary function is to make plant tissue hard. They are also scattered
through the flesh of a pear; sclereids give pears a gritty texture. Sclereids vary greatly in
shape, but the ones in pears are roughly spherical
* Make a wet mount of pear tissue. Place a bit of tissue on a slide and cover with a
cover slip. Push gently on the cover slip with the butt of your pen or pencil to spread the
tissue. Note the thick secondary cell walls with conspicuous pits. The pits appear as
deep channels due to the thickness of the wall. The rest of the tissue you see in the
slide is parenchyma used to store energy in the fruit.
b.) Fibers. Fibers are another type of sclerenchyma cell. Fibers are long and narrow
and are used for support in plants. Make a wet mount of the fibers provided.
Plant reproduction
Flowers are the sexual reproductive structures of plants. When pollen from the male
component of a flower reaches the female structure, fertilization may occur. The pollen
carries genetic information that will be combined with the female’s genetic information
that is found in an egg.
Pollen may reach the female through various methods. For example, pollen can be
carried by the wind or a pollinator such as an insect or bird. Pollen shape varies among
different plants. For example, ragweed has pollen with many sharp spines that irritate
some people and lead to allergies.
Examine the various types of pollen that are available for your viewing pleasure.
Fruits and seeds
Fruits develop after fertilization of the female flower parts. Seeds simultaneously
develop inside (normally) the fruits. Fruits are designed to aid in distribution of the
seeds. For example, if an animal eats a berry, it simultaneously eats the seeds. When
the animal deficates, the seeds are deposited far from the mother plant in a batch of
fertilizer!
A fruit consists of several different layers. These layers often include a skin, fleshy
center, and a harder internal protective layer. Look at the apple on display and note the
different layers.
Also, make a wet mount of the skin of the various fruits. Note the distribution of the
pigments in the skin. Remember that the pigments are found in unique cellular
structures.
A seed consists of an embryo, a seed coat, and nutritive tissues. The seed coat serves
as a protective barrier for the young and fragile embryo. The seed coat protects against
the embryo drying out. Closely examine the seed coat of the various seeds provided.
Make a table describing the various features you observe on the different seeds.
Examine a soaked bean seed and try to find the three part of a seed. Draw them.
Part 2. Collenchyma
Collenchyma cells, like parenchyma, are living at maturity and have only primary cell
walls. However, their walls are unevenly thickened (they tend to be particularly thick at
the corners), and the cells are often long. Collenchyma cells have a much more
restricted distribution within the plant, and usually they are aggregated into strands just
beneath the epidermis or along the veins of some leaves. They provide support in
young tissues.
* Use a razor blade to cut several thin cross section from a celery leaf petiole, and
examine them with your compound microscope (see p. 574). The tissue that makes up
the ribs of celery (strings that get stuck between your teeth) is collenchyma. Each rib is
formed of many strands of collenchyma cells stacked end to end. The collenchyma cells
look dark in the center, and the lighter colored walls appear to glisten. The larger cells
near the collenchyma, i.e., those that compose most of the celery stalk, are parenchyma
cells. Identify the parenchyma and collenchyma cells in the celery cross section.
* Now look beneath the epidermis on the Medicago stem you viewed earlier. Locate the
collenchyma. Where is it located?