Leaves

Leaves are what plants are all about; they are the sites of photosynthesis. They must perform a delicate compromise between gas exchange and evaporative water loss, taking in enough carbon dioxide for photosynthesis while limiting excessive water loss. Some evaporation is necessary for transpiration, but too much will kill the plant. Leaves also require vascular tissue to transport water and inorganic nutrients into the leaf and to transport the sugars produced through photosynthesis out to the rest of the plant.The leaves of vascular plants contain several different specialized types of tissues, which interact to make the functioning organ we call a leaf. You should become familiar with each type of cell and what it does.

Objectives

• Recognize each of the tissue types and structures found in leaves and explain what they do.
• Recognize the differences between simple and compound leaves.

Specimens: whole

• Whole leaves of various plants, including monocots & eudicots and modified leaves such as onions.

Specimens: Microscope slides

• Pine (Pinus) needle cross section (single needle, two-needle, and 5-needle types)
• Monocot & eudicot leaf Zea & Syringa cross section
• Zea (corn) leaf cross-section and longitudinal section
• Dicot leaf types
• Lilium leaf cross section (monocot)
• Zea leaf cross section (monocot)
• Dianthus leaf cross section (eudicot)
• Ficus leaf cross section (eudicot)
• Ligustrum leaf cross section (eudicot)
• Nerium leaf cross section (eudicot)
• Syringa (Lilac) leaf cross section
• Mnium (moss) leaf. This is a nonvascular plant; see the picture on the Mosses page.
• Any other leaf slides that are out in lab.

Whole leaves

Before you go to the microscope, look at some whole leaves to help you picture what you'll see in the cross sections.

Observe the whole leaves of various land plants. While these leaves show a variety of shapes, they all do more or less the same job. Some of the differences in shape can be understood in terms of the conflicting requirements that leaves face: absorbing light, exchanging gases, avoiding dehydration, avoiding predation. Break off a small piece of a leaf and look at it under the microscope; you should be able to identify dermal, ground, and vascular tissue. Which of the leaves in lab are simple, and which are compound? Within the flowering plants (angiosperms), there are two large groups with different styles of leaves. eudicots (roses, for example) have leaves with a netlike, branching system of veins. Monocots (grasses, for example) have parallel veins. Later in this lab you’ll see more differences between eudicots and monocots. See Campbell, p. 603 for a summary of monocots and eudicots.

Observe the live specimens of Elodea and other aquatic plants. Can you see the cuticle? The stomata? The mesophyll? Why do these leaves look different from the leaves of land plants? What do they have in common with algae or nonvascular plants? Observe the live specimen of an onion. Can you find leaves, stem, and roots?

Syringa (Lilac) leaf cross section

Observe the prepared slides of Syringa (Lilac) leaf cross section. Study Campbell (fig. 35.18, 10th ed.) along with this slide. You should be able to recognize and describe the function of these parts of a leaf cross-section:

• Epidermis (dermal tissue). A single layer of cells on the top and bottom of the leaf. Each cell has a nucleus, which may be visible as a red dot in the cell. However, this is a thin slice of some large cells, and in many cases the slice does not happen to include the nucleus. The epidermis secretes the cuticle, a waxy layer that surrounds the outside of the leaf. The cuticle isn't visible in these slides.
• Stoma (plural: stomata). Stomata are openings in the leaf to allow for gas exchange. Stomata are created by guard cells, which can expand or contract to open or close each stoma. Closing the stomata reduces water loss, but can also slow photosynthesis by preventing CO₂ uptake. The guard cells are another example of dermal tissue.
• Mesophyll (ground tissue). Mesophyll means "middle of the leaf," and these cells fill most of the leaf's volume. They are the primary sites of photosynthesis, and they are filled with chloroplasts, which are visible as small red dots. The mesophyll cells are surrounded by air spaces that allow circulation for gas exchange.
• Vascular bundles, containing xylem and phloem. Xylem forms the system for transporting water and inorganic nutrients from the roots all the way up to the leaves. This transport sometimes involves powerful pressure gradients, so the xylem cells are heavily reinforced. You can recognize them in this slide because they have very thick walls, stained deep red. The phloem cells help do the job of transporting sugars produced in photosynthesis. Phloem transport typically involves weaker pressure gradients than xylem transport. In contrast to xylem cells, the phloem cells are smaller and thinner-walled, and they stain blue in this slide. In a vascular bundle of the leaf, the xylem is usually on top and the phloem on the bottom. Make sure you know which way is up on your slide.

Note that all these cells share some features that mark them as typical plant cells: they are very large compared to animal cells, they are contained in a boxlike cell wall, and most of the volume of the cell is filled with a membrane-bound storage organelle, the central vacuole.

Syringa is a eudicot, and has reticulate venation, meaning that the veins branch to form a netlike arrangement. You should see one large vascular bundle in the midrib of the leaf and other, smaller vascular bundles, which may be sliced at an angle.

Zea (corn) leaf, cross section & longitudinal section

Corn is a monocot, and a member of the grass family. It contains all the features listed above for Syringa leaves, but with two notable differences. First, corn leaves have parallel veins. This means that in a leaf cross-section, you’ll see all the veins cut straight across. (In a leaf with reticulate venation, some of the veins will be cut straight across, and some will be cut at an angle.) The parallel venation of Zea leaves should also be obvious in longitudinal sections.

The other notable way that Zea leaves differ from those of Syringa is that the vascular bundles in Zea are surrounded by bundle sheath cells. These cells aid in corn’s specialized mode of photosynthesis, called C4 photosynthesis. (Most plants perform C3 photosynthesis. The differences between these two photosynthetic pathways are covered in Bio 6B.)

We have two kinds of prepared slides of Zea (corn) leaf: cross-section and longitudinal section. In the longitudinal section, you should see that the vascular bundles run parallel to the plane in which the leaf was sliced.

Pine needle cross sections

Pine trees are gymnosperms, meaning that their reproduction differs in some important ways from angiosperms such as Syringa or Zea. These differences will be covered in Plants III.

The leaves of pine trees are called needles. Though their shape is different from the leaves of most angiosperms, they contain more or less the same tissue types. Pines often live in harsh conditions: hot, dry summers and freezing winters. They are good at withstanding environmental stress. Their needles, with a low surface area-to-volume ratio, help reduce damage due to drying out or heavy snows.

We have microscope slides of three different kinds of pine needles. Five-needle pines have needles that grow in bunches of five, two-needle have two, and single-needle pines grow with one needle per bunch. Inside these needles, you’ll see all the features listed above for Syringa leaves. Each needle has one or two vascular bundles.

Pine needles also have some features not seen in Syringa leaves. The vascular bundles are surrounded by a thin layer of endodermis, which may help control water loss from the leaf. Transfusion tissue surrounds the vascular bundle inside the endodermis, and apparently helps transport materials into and out of the vascular tissue. These tissue is abundant in pine needles, but not in most leaves of flowering plants. Resin ducts carry resin, which is a hydrocarbon-containing substance that may help protect the leaves.

The cuticle may be visible as a faint pink layer around the outside of this pine needle. The stomata are sunk into small pits in the epidermis; this reduces airflow and evaporative water loss.

References & further reading

Campbell Biology, 11th ed. Chapter 35 for tissues and leaf structure. See also fig. 30.16 and 30.17 for a comparison of monocots and eudicots.