Abscission zone. Longitudinal section of Acer stem x40 with leaf petiole coming off to the left. Arrow shows the abscission zone. A vein enters the leaf passing through the abscission zone.
Asclepias leaf abscission x40. The pointer shows the abscission zone in this stem ls. The petiole is on the left with a vein passing through the abscission zone. A group of flower buds is on the right.
Acer sun leaf xs. x400. Note the dense palisade mesophyll in this leaf that was exposed to direct sunlight. Spongy mesophyll is in the lower half of the leaf.
Acer shade leaf xs. x400. The palisade mesophyll is less dense and the spongy mesophyll is more extensive in this shaded leaf compared to leaves on the same tree that were exposed to direct sun.
Stoma and guard cells xs. on Acer lower epidermis. x1000. Two guard cells have a small opening (stoma) between them for gas exchange. Note the large intercellular space, part of the spongy mesophyll, immediately inside the stoma.
Monocot leaf bases x20 frequently form sheaths around the stem that bears them. Such leaves have no petioles and are broadly attached to the stem at the base of the leaf.
Ammophila leaf xs.. x40. This is a beach grass leaf that is curled in the adaxial (upper surface) direction due to temporary drought conditions. Unlike most leaves, most stomata are on the upper surface where they are protected from rapid water loss by the high relative humidity within the curled leaf. White buliform cells can be seen at the base of the V-shaped notches on the adaxial (inner, upper) leaf surface. The leaf uncurls when these buliform cells fill with water, enlarge, and become turgid when drought conditions end.
Buliform cells x400. The large buliform cells are on the right at the base of a V-shaped notch on the upper epidermis of a curled beach grass (Ammophila) leaf. Stomata in xs can also be seen associated with intercellular spaces within the leaf mesophyll. When the buliform cells fill with water and enlarge this causes the leaf to uncurl.
Clivia leaf xs. x40. This is a monocot leaf. Note the mesophyll which is not easily distinguishable as palisade vs spongy. The cuticle is somewhat thicker on the upper surface.
Clivia leaf vein xs. x100. The large red ringed cells are vessels, part of the xylem. The small cells below are phloem. In leaves the xylem of veins is always toward the adaxial (upper) surface and the phloem toward the abaxial (lower) surface. In monocot leaves such as Clivia xylem-phloem placement within a vein is the only way to identify the upper and lower surface of a leaf seen in cross section.
Stoma, guard cells, and cuticle xs. x400. The red material is cuticle covering the lower epidermis of a Clivia leaf. You can see a pair of guard cells, a stoma between them, and cuticular hairs partically covering the stoma. The covering of cuticular hairs helps maintain high relative humidity near the stomatal opening and reduce rapid water loss through the stoma.
Monocot leaf epidermis x40. You can see pairs of guard cells and other cells of this upper epidermis. Dark dots are nuclei. Rectangular cells are over a leaf vein. Stomata are between veins.
Tradescantia upper leaf epidermis x100. Note the lack of stomata on this leaf surface. The black dots are nuclei. Unlike this species, many dicot plants have a few stomata on their upper surface. Most of a dicot leaf's stomata are usually found on the lower surface.
Stoma, surface view x400. This shows a single pair of guard cells surrounding a stoma on the lower epidermis of Tradescantia. The large dark bodies are nuclei. The lighter circular bodies within the guard cells are chloroplasts. In most plants guard cells are the only cells within a leaf's epidermis that contain chloroplasts. The production of sugar by photosynthesis in the daytime within guard cells and not in adjacent epidermal cells causes water to diffuse into the guard cells. This makes the guard cells swell, opening the stoma.
Ligustrum leaf xs. x40. A typical dicot leaf. Note the large central vein as well as the palisade and spongy mesophyll.
Ligustrum leaf mesophyll and small veins x100. Note that one of the small veins is in good cross section and another is very oblique. This shows that the three visible veins are not parallel to each other and thus that the leaf does not have parallel venation. This means that the leaf must have net venation and thus must be a dicot leaf.
Net venation in paradermal view x100. This Ligustrum leaf is sectioned parallel to the leaf surface through the spongy mesophyll. You can see the net-like arrangement of the smallest veins which is the basis for the name "net venation".
Zea mays leaf xs x100. This corn leaf is typical of monocot leaves with their characteristic parallel venation. You can tell that the venation is parallel because the veins are all shown in nice cross section. The only way to identify the true upper surface of the leaf is to look for the xylem in the veins, which is always positioned toward the upper epidermis. There are stomata visible in both the upper and lower epidermis. Some large buliform cells are on the right side of the upper epidermis. Shrinkage of these buliform cells would cause the leaf to curl upward.
Zea mays leaf vein x400. At the top and bottom of the vein are groups of fibers. The secondary cell walls of the xylem cells stain red, including the two very large diameter vessels. Small sieve tubes and companion cells in the phloem stain dark green.
Pinus leaf xs x100. One of a group of two leaves near the leaf base. The outer mesophyll is very dense and contains a resin duct visible on the right. One of several visible stomata can be seen on the right side of the bottom surface.