Plant Anatomy: Practical Course - Panyuta O.O. 2019

Topic 1. The Cell
Laboratory Work No. 2. Plastids

Theoretical Background. Plastids are essential Organelles of plant Cells, with green plastids being of primary importance. In terms of their physicochemical properties, plastids share many features with the Cytoplasm, though they have a denser consistency, which makes them clearly visible under a Microscope. Plastids can be either pigmented or colorless.

Depending on their pigmentation and function, plastids are divided into three groups: green METABOLISM/14.html">Chloroplasts, orange-red chromoplasts, and colorless leucoplasts.

Leucoplasts. Leucoplasts are colorless plastids that are generally rounded in shape. They are most commonly found in young cells. Leucoplasts are capable of transforming into chloroplasts and chromoplasts, which points to the close evolutionary relationship among Different types of plastids.

The transformation of leucoplasts into chloroplasts, and of chloroplasts into chromoplasts, can be observed in numerous organisms. For instance, tomato fruit is initially green; as it ripens, the chloroplasts convert into chromoplasts, changing the fruit's color to red.

Leucoplasts are present in all vascular plants, with monocots tending to have larger leucoplasts than dicots. They occur in both young and mature cells. They are particularly characteristic of plant Organs hidden from light, such as potato tubers, rhizomes, and roots. Leucoplasts are also frequently found in the living cells of perennial tree trunks and develop within surface Tissues, notably in the epidermis of stems and leaves.

Chloroplasts. Chloroplasts vary greatly in shape and size, with an especially wide diversity of forms found in lower plants. They may appear as tiny granules, discs, spirally coiled rings, or plate-like bodies. The external Morphology of chloroplasts is characteristic of specific systematic groups of plants.

Chloroplasts are abundant in the green parts of plants, but they are also present in seeds, bark, wood, pith rays, and other living plant tissues. Their number varies across different tissues and even among different cells within the same tissue. In higher plants, chloroplasts typically form grains with more or less distinct ellipsoidal-lenticular outlines.

Chloroplasts generally measure 4-6 µm, though some plant species possess chloroplasts that reach sizes of 20-25 µm.

Chloroplasts play a vital role in the synthesis of organic matter, a process in which the green pigment chlorophyll is of paramount importance.

To examine the Structure, shape, and size of chloroplasts, any green plant can be used, though leaves of moss, agave, or *Elodea* are particularly well suited for this purpose.

Chromoplasts. Chromoplasts usually develop from chloroplasts, but they can also form directly from leucoplasts. Their derivation from chloroplasts can be observed during the ripening of rosehips, rowan berries, and tomatoes. During this process, the green pigmentation gradually fades, and the fruits turn red.

As is well known, chromoplasts exhibit a range of colors—from yellow to bright orange and red. This coloration depends on the presence of pigments known as carotenoids.

Chromoplasts are found in the petals of nasturtiums and asters, as well as in the mature fruits of rosehips, hawthorns, rowan berries, and tomatoes. They also occur in the Water/115.html">Vegetative organs of certain plants, such as the sporifying shoots of field horsetail and the ROOT crops of carrots.

Each chromoplast consists of a delicate protein matrix that easily dissolves upon contact with water.

The shape of chromoplasts is remarkably diverse. They may appear as granules, polygonal plates, or lobed structures. The carotenoids impregnating the chromoplast crystallize, causing the chromoplasts to typically adopt the shape of the carotene crystals they contain.

Objective: to study the morphological features and structure of plastids.

Materials and Equipment: light microscopes, Glass slides and cover slips, dissecting needles, tweezers, razor blades, glass rods, filter paper, distilled water, 95% ethyl alcohol, plant material.

Preparation. Leucoplasts in epidermal cells of wandering jew leaf (Tradescantia discolor L’Her.)

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Fig. 6. Epidermal cells of *Tradescantia*:

1 - Cell wall, 2 - cytoplasm, 3 - Nucleus, 4 - leucoplasts, 5 - vacuole

To prepare the mount, hold a *Tradescantia* leaf in your left hand, gently tear the upper epidermis with a sharp needle, and peel off a small fragment using tweezers. This Procedure will also capture a small amount of the underlying leaf mesophyll, leaving a thin, uniform film of epidermis at the edges containing leucoplasts. Place the excised fragment into a drop of water on a microscope slide, straighten it out with a needle, and cover with a coverslip.

Examine the preparation first under low magnification, and then under high magnification (Fig. 6). Under high magnification, it becomes clear that The Cell contents primarily consist of a violet-colored cell sap, a thin layer of granular parietal cytoplasm, and a rounded nucleus. In most cells, The Nucleus is surrounded by numerous minute, colorless leucoplasts. Make a biological drawing of the preparation, illustrating the main Structural components of the cell.

Preparation. Chloroplasts in leaf cells of Canadian waterweed (Elodea canadensis L.)

Preparing an *Elodea* slide is very simple. Using tweezers, carefully pluck a small leaf of *Elodea*, place it in a drop of water on a microscope slide, and cover it with a coverslip. Examine the prepared slide initially under low magnification. It is best to position the slide so that the edge of the leaf attached to the stem lies in the center of the field of view. Under high magnification, A large number of rounded or slightly elongated chloroplasts are clearly visible. Sketch individual cells containing chloroplasts.

Next, place the leaves in 95% ethanol and boil them, then examine them under the microscope again. It is easy to notice that after boiling, the green color disappears, and colorless granules can be seen inside the cell. For further study of these granules, draw off the water from under the coverslip using filter paper and add one to two drops of iodine-potassium iodide solution from the opposite side. Upon The addition of iodine, the granules turn a yellowish color. This indicates that the chloroplast consists of two parts, namely: a protein matrix, which is stained yellow by iodine, and the green pigment—chlorophyll.

When the decolorized chloroplasts are treated with iodine, a blue or black dot becomes clearly visible in their center. This is starch, which forms within the plastids As a result of Photosynthesis.

Slide. Chromoplasts in the cells of dog rose fruit (Rosa canina L.)

Fig. 7. Chromoplasts in rose hip cells: 1 - cell wall, 2 - cytoplasm, 3 - nucleus, 4 - chromoplasts

To prepare the slide, carefully cut through the epidermis of a ripe rose hip fruit, gently scrape a small amount of spongy tissue with the tip of a scalpel, and spread it on a microscope slide in a drop of water. Mix the tissue thoroughly so that individual cells separate. To ensure the slide is thin and the cells do not overlap densely, use a very small amount of the spongy tissue.

The preparation is then covered with a coverslip and examined first under low magnification and subsequently under high magnification of the microscope.

The rose hip cells are rounded in shape (Fig. 7). Their cell walls are thin. The cytoplasm is clearly visible within the cell, lining The cell wall as a parietal layer from which cytoplasmic strands extend, crossing the cell in several directions. The nucleus, embedded in the cytoplasm, can also be observed. Orange chromoplasts are distributed more or less evenly throughout the cytoplasm, with a higher concentration clustering around the nucleus. In rose hips, their shape is spherical, similar to that of chloroplasts, though they can occasionally exhibit various forms. After examining the slide, make a drawing of it.

Slide. Chromoplasts in the cells of rowan fruit (Sorbus aucuparia L.)

Take a ripe rowan fruit, the cells of which easily separate from one another, cut through the outer protective tissue with a scalpel, and then use the tip of the scalpel to take a small amount of the pulp, mixing it thoroughly in a drop of water on a microscope slide. Examine the prepared slide under the microscope first at low magnification and then at high magnification.

Chromoplasts of various shapes are present in the cell in large numbers.