PHOTO STUDY 4-1 Allium (Onion): Surface view of epidermis

A strip of epidermis from the outer surface of one of the inner scales of an onion bulb has been peeled and stained. Note their orderly arrangement. Are there inter-cellular spaces? See the thin strands of cytoplasm that separates vacuoles. Discover the nucleus to be enclosed by a sheath of cytoplasm and suspended by cytoplasmic strands. Look for small pits in the side-walls. Minute bodies are plastids containing oil (elaioplasts).

PHOTO STUDY 4-2 Allium (Onion): Bulb scale, t.s. (epidermis, bulb scale)

Which wall of a cell is the thickest? Though anticlinal walls are actually thin, they appeared to be thick when you viewed them from the top.

PHOTO STUDY 4-3 Erythronium (dog-tooth violet): Surface view of leaf epidermis.

The onion bulb is an underground organ, exposed to soil, moisture, and darkness. The leaf, however, being an aerial organ, is exposed to drying air and sunlight. See how features of the epidermis differ under these two environments. Does this leaf epidermis resemble, or differ from, that of the bulb with respect to regularity in cell shape or in cell alignment? Are all cells alike here? Note the frequency of stomata, each one a narrow slit between two crescent-shaped guard cells. Are stomata distributed in any regularity? Such regularity is characteristic of monocotyledon leaves that have parallel veins. Were these epidermal cells living when the leaf was collected? How can you tell?

PHOTO STUDY 4-4 Erythronium (dog-tooth violet): Leaf surface (x 980)

Examine a pair of guard cells in detail. Note the colorless ridge of cutin along the inner side of each guard cell, extending slightly over the margin of the stomatal opening. Distinguish several chloroplasts in a guard cell. The dark elongated area in each guard cell is a nucleus. Do other epidermal cells have nuclei also? Do they have chloroplasts? Each pair of guard cells resulted from a longitudinal division of a single mother cell. A slight separation of the two daughter cells then resulted in the stoma between them.

PHOTO STUDY 4-5 Capsella (shepherd’s purse): Epidermis of stem, t.s.

How do cell walls in the epidermis compare with those of underlying cells? Are all the walls of an epidermal cell equal in thickness? Against the outer surface of an epidermal cell wall, identify a thin deposit of cutin. A layer of cutin is called a cuticle. Of what significance is this layer?

PHOTO STUDY 4-6 Peperomia (peperomia): Leaf epidermis, t.s. (2 photos)

Photo 1: Although the epidermis of most plants is but one cell deep, in this exceptional case it is several cells deep, and it is called a multiple epidermis. This type arises from the uniseriate epidermis of the very young leaf by repeated periclinal divisions of the original epidermal cells. This being true, one would expect the cells of a multiple epidermis to be in regular alignment; but you see that they are not. What growth features would account for such irregularity?

PHOTO STUDY 4-6 Peperomia (peperomia): Leaf epidermis, t.s. (2 photos)

Photo 2: Is the thickness of the epidermis on the upper side of the leaf the same as that on the lower side? Is there a cuticle? What function, other than protection, might such an epidermis have? Note pits in the wall of the cell in the center, just under the outer epidermal layer.

PHOTO STUDY 4-7 Pelargonium (geranium): Leaf surface. (epidermis) (x100)

Compare the shape of theses cells with that of epidermal cells in Allium and in Erythronium. With all the irregularity of undulating walls, note the total absence of intercellular spaces (except for guarded stomata). Are stomata here in regular alignment, as you found them to be in Erythronium? See large hairs growing out from the epidermis. Hairs are of two kinds: one is a long, stout, pointed hair; the other is stout in its basal half, but slender in its terminal half, and is terminated by a globular gland (shorter center hair). It is the oil from such glands that gives the pungent odor to crushed geranium foliage. Are epidermal cells around the base of a hair as irregular in shape as those elsewhere? Are hairs single cells? Why do you suppose the hairs have taken one stain while epidermal cells have taken another?

PHOTO STUDY 4-8 Pisum (pea): Seed coat epidermis, t.s.

On most plant organs, epidermal cells are flat and tabular, but here you find them to be column-like. Note thickness of outermost walls and of anticlinal walls. See that the cell lumen is very narrow toward the outer end of a cell, but wider at the base where the nucleus is usually situated. Cell walls are neither cutinized nor lignified. Distorted cells and cell fragments below the epidermis are part of the deeper tissue of the seed coat.