Anatomy of a Young Dicot Root

1. The epidermis. This is the outermost tissue.

a.) How many cell layers make up the epidermis?

The plant epidermis consists of three main cell types: pavement cells, guard cells and their subsidiary cells that surround the stomata and trichomes, otherwise known as leaf hairs.

b.) Are the cells cutnized?

The walls of the epidermal cells of the above ground parts of plants contain cutin, and are covered with a cuticle. The cuticle reduces water loss to the atmosphere; it is sometimes covered with wax in smooth sheets or long filaments.

2. The cortex. This is the region after the epidermis.

a.) What is the inner boundary of the cortex?

Endodermis is the innermost layer of cortex represented by compactly arranged barrel shaped cells, without any intercellular spaces. The endodermis is wavy in appearance. The cells are richly deposited with starch grains and hence, endodermis is commonly described as starch sheath.

b.) What types of cells make up the cortical region?

It is composed mostly of undifferentiated cells, usually large thin-walled parenchyma cells of the ground tissue system. The outer cortical cells often acquire irregularly thickened cell walls, and are called collenchyma cells. Some of the outer cortical cells may contain chloroplasts. It is responsible for the transportation of materials into the central cylinder of the root through diffusion and may also be used for food storage in the form of starch.

c.) How do the cells of the endodermis differ from other cortical parenchyma cells?

The endodermis prevents water, and any solutes dissolved in the water, from passing through this layer via the apoplast pathway. Water can only pass through the endodermis by crossing the membrane of endodermal cells twice (once to enter and a second time to exit). Water moving into or out of the xylem, which is part of the apoplast, can thereby be regulated since it must enter the symplast in the endodermis. This allows the plant to control to some degree the movement of water and to selectively uptake or prevent the passage of ions or other molecules. The other cortical parenchyma cells pretty much stores any molecules that is absorb; such as, starch, protein, fats, oil and water.

d.) Locate the endodermal cell opposite the xylem arm. It is called passage cell. How does it differ from the rest of the endodermal cells?

Passage cells are endodermal cells of older roots which have retained thin walls and Casparian strips rather than becoming subarized and waterproof like the other cells around them, to continue to allow some symplastic flow to the inside. For the most part, however, old roots seal themselves off at the endodermis, and only serve as a passageway for water and minerals taken up by younger roots "downstream". It is believed that passage cells provide an easier route for water and dissolved solutes to pass through from the cortex to the protoxylem.

3.) The Stele or Central Cylinder is the tissue occupying the central portion. It consists of the pericycle, vascular tissues (xylem and phloem) and the stellar parenchyma.

a.) Near the center of the section locate the xylem elements. How many xylem arms do you find?

There are 3 xylem arms (triarch).

b.) Look for the phloem near, adjacent or between the xylem arms. Are the cells thick or thin-walled?

Phloem is composed of thin-walled cells, namely as follow; sieve tubes and companion cells.

c.) Locate the pericycle, the layer or layers of thin-walled cells just outside the vascular tissues (readily seen at the xylem arm). How many layers of cells do you see in your specimen?

1 layer of cells basing on the picture on the left .

d.) What kind of cells are in the exact center? Are they thick or thin-walled?

Xylem are the cells at the exact center of the specimen. It appears to be thick walled. These are thick-walled tubes that can extend vertically through several feet of xylem tissue. Their diameter may be as large as 0.7 mm. Their walls are thickened with secondary deposits of cellulose and are usually further strengthened by impregnation with lignin. The secondary walls of the xylem vessels are deposited in spirals and rings and are usually perforated by pits.

e.) Is there a pith in your specimen?

No, there is no pith basing on the picture above. Pith is mostly found in the stem at the center of dicotyledons than on the roots.

f.) Setch and label fully the diagrammatic X-section of a young dicot root. Indicate correct magnification.

g.) Study Plate VII showing the detailed X-section of a young dicot root and label the parts. Indicate correct magnification.