Leaves

Leaves are shoot structures that attach to stems and branches at nodes. Leaves are made up of cells that usually contain a high concentration of chloroplasts (cell organelles unique to plants) and are specialized sites for photosynthesis. We will explore photosynthesis in greater detail later; for now, remember that photosynthesis is the process of capturing light energy and converting it into chemical energy that can be stored in plants (like starch and sugar). In some plants, leaves may be modified for nutrient storage (as with onions, where the bulb is made up of fleshy leaves), or for support (as with peas, where some leaves are modified into tendrils that wrap around a trellis).

Leaves are also the surface where water that has moved from the soil into the roots and up through the plant finally evaporates back into the atmosphere in a process called transpiration.

Angiosperms, which are flowering plants whose seeds develop inside an ovary, tend to have flattened leaves. Many perennial angiosperms (flowering plants that can grow for many years) have leaves that senesce, or die, at the end of each growing season and are replaced at the beginning of the next growing season. Gymnosperms, plants whose seeds are produced without the protection of an ovary, tend to have needle-like leaves. Perennial gymnosperms tend to hang on to their leaves for a number of years. This saves energy, since the plant doesn’t need to grow a whole set of new leaves every year. The needle-like form helps retain moisture in harsh, dry climates, including those that are very cold and dry.

Leaf parts and venation

Angiosperm leaves typically have a blade or lamina, a flattened part with high chloroplast concentration. They may also have a petiole, the stalk that attaches the blade to the stem at a node. Stipules, small leaf-like bracts at the point of attachment of the petiole to the stem, may also be present. Some leaves have no petiole at all, and are termed sessile.

In contrast to the blade-petiole structure, grasses have a sheath-type structure in which the blade attaches to an envelope of leaf tissue that wraps around the shoot of the plant and then attaches to a lower node on the stem.

Leaf blades also have characteristic patterns of venation. In grasses, the veins lie parallel to each other and to the long edges of the leaf. We call this parallel venation, and it is typical of monocots. Most other angiosperms have a strong major midrib with veins branching from the midrib, smaller veins branching from those, and so on to form a netted venation throughout the leaf. This type of venation is typical of dicots.

Leaves may also have a palmate venation where several veins radiate from the point where the petiole attaches to the blade. Ginkgo tree leaves have palmate venation. The veins fork, then travel a bit, then fork again, travel, fork, and so on until the veins reach the margin (edge) of the leaf. Sugar maple leaves have a classic palmate venation with five lobes.

Watch this video on leaf veins:

Leaf segmentation

Simple leaves

Simple leaves have uninterrupted leaf margins. The leaf may have lobes like the oak leaf, but the blade has one continuous margin. The venation differs in the two examples below. The oak leaf is pinnate, with a major vein heading down the midrib of the leaf. The maple leaf is palmate, with major veins that radiate from the point of attachment to the petiole.

Compound leaves

The sumac leaf is a good example of how a compound leaf has a blade that is completely interrupted and segmented into separate leaflets. What you see in the picture — the entire thing — is one leaf. The leaf is divided or segmented into leaflets.

The petiole extends from the point of attachment at the node to the first leaflet. The central axis from that point on — from the first leaflet to the tip of the leaf — is called the rachis.

Virginia creeper is an example of a palmately compound leaf. The stalk that connects the leaflet to the top of the petiole is called the petiolule. In this case there is no rachis; all leaflets are attached directly to the top of the petiole.

Below is a compound leaf with three leaflets, called a trifoliate leaf. Soybean, clover, and dry bean all have trifoliate leaves. In contrast to the palmately compound leaf above, there is a rachis to which the central leaflet is attached.

To tell whether a leaf is simple or compound, ook for attachment to a node. If the point of attachment doesn’t appear to be a node, it is likely a leaflet attached to the rachis of a compound leaf.

Watch this video on compound leaves:

Additional optional reading

For more information about leaves, explore this Wikipedia page, starting about halfway down at the heading “Morphology (large-scale features)” and continuing through “Veins.”