Plants > Leaves

Seed Plant Anatomy: Leaves

This page covers the leaves of both conifers and angiosperms.

Leaves are the key to what plants do: 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.

Gymnosperm Leaves: Pine Needles

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.

Pine needle cross-section

The outer layer of the leaf is called the epidermis. This layer, one cell thick, protects the leaf from damage. The epidermis secretes a cuticle (visible as a pink layer outside the epidermis), a waxy layer that reduces evaporation.

The epidermis and cuticle block the diffusion of carbon dioxide and oxygen as well as water, so the leaf needs openings to allow for diffusion. The openings are called stomata (singular: stoma), and they can be opened or closed by guard cells. Inside the epidermis there is a layer of mesophyll cells. These cells are filled with chloroplasts (dark red in this picture); the chloroplasts perform photosynthesis. The mesophyll cells are surrounded by air spaces, which enables them to perform gas exchange. Air can move slowly in and out of these air spaces through the stomata. Note that the stomata are sunk below the surface of the leaf; this reduces evaporation. Sunken stomata are one of many adaptations that help pine trees thrive in dry environments.

Vascular tissue is also a key component of leaf structure. In the middle of this pine needle there is a single vascular bundle. The bundle contains phloem and xylem.

Phloem transports the sugars that are produced in photosynthesis from the leaves to the rest of the plant. The phloem cells are small and thin-walled; in this slide, as in many others, the phloem cells appear blue.

Xylem transports water and inorganic nutrients from the roots up to the rest of the plant; xylem cells are thick-walled and reinforced with lignin to withstand the necessary pressure. The xylem cells are stained red.

Transfusion tissue surrounds the vascular bundle in pine needles. This tissue apparently helps transport materials into and out of the vascular tissue. This 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.

5-needle pine

Pine needles grow in bundles. The images above show cross-sections of a single-needle pine; each bundle has only one needle. Here is a picture of a more-typical 5-needle pine; the five needles grow together in a tight bundle.

Each of these needles has the same features as the 1-needle example above.

Angiosperm Leaves: Syringa (Lilac, a dicot)

Like pine needles, the leaves of angiosperms must balance the conflicting needs of gas exchange and preventing desiccation. However, angiosperm leaves come in a wide range of morphologies, suited to a wide range of habitats.

Syringa, also known as lilac, has more or less typical dicot leaves. This plant uses the C3 pathway for photosynthesis, a topic that will be covered in Bio 6B.

Syringa leaves don't look like pine needles, but they have most of the same anatomical features.

Epidermis covers the outside of the leaf, top and bottom, and secretes a waxy cuticle that reduces evaporation. (On many slides, the cuticle is difficult or impossible to see.)

Stomata (singular: stoma) are openings in the epidermis, allowing for gas exchange and transpiration. Guard cells can open or close the stomata.

Mesophyll cells fill the middle of the leaf and perform photosynthesis. They are packed with chloroplasts. "Mesophyll" means "middle of the leaf." The mesophyll cells are surrounded by air spaces.

Flowering plant leaves normally have multiple veins, or vascular bundles. The vascular bundles contain xylem and phloem.

Phloem transports the sugars that are produced in photosynthesis from the leaves to the rest of the plant. Phloem cells are small and thin-walled; in many others, the phloem cells appear blue.

The veins in Syringa leaves are not all parallel; they branch and cross one another, forming a netlike structure called reticulate venation (reticulate means netlike). Syringa is a typical dicot.

Angiosperm Leaves: Zea (Corn, a monocot)

Zea (corn) is a monocot and a member of the grass family. Like other monocots, it has leaves with parallel veins. Thus, when Zea leaves are cut in cross section, you see all the veins cut straight across -- not diagonally, like some of the veins in Syringa leaves.

On closer inspection, you may notice another important difference between the veins of Zea and those of Syringa. In Zea, the vascular bundles are surrounded by another type of cells, called bundle sheath cells. These cells are essential for C4 photosynthesis, the specialized photosynthetic pathway performed by corn and many other drought-tolerant plants.

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This page updated September 20, 2011