9.16: Leaves

Could life exist without the leaf?

A leaf looks so simple. But it is actually a very complex structure. And it may be one of the most important organs in all kingdoms. Life as we know it could not exist without leaves. Why? One word: photosynthesis.

Leaves

Plants have specialized organs that help them survive and reproduce in a great diversity of habitats. Major organs of most plants include roots, stems, and leaves. Leaves are the keys not only to plant life but to all terrestrial life. The primary role of leaves is to collect sunlight and make food by photosynthesis. Despite the fundamental importance of the work they do, there is great diversity in the leaves of plants. However, given the diversity of habitats in which plants live, it’s not surprising that there is no single best way to collect solar energy forphotosynthesis.

Leaf Variation

Leaves may vary in size, shape, and their arrangement on stems. Nonflowering vascular plants have three basic types of leaves: microphylls (“tiny leaves”), fronds, and needles.Figure below describes each type.

Leaf variation in nonflowering plants reflects their evolutionary origins. Can you explain how?

Flowering vascular plants also have diverse leaves. However, the leaves of all flowering plants have two basic parts in common: the blade and petiole. The blade of the leaf is the relatively wide, flat part of the leaf that gathers sunlight and undergoes photosynthesis. Thepetiole is the part that attaches the leaf to a stem of the plant. This occurs at a node.

Flowering plant leaves vary in how the leaves are arranged on the stem and how the blade is divided. This is illustrated in Figure below. Generally, the form and arrangement of leaves maximizes light exposure while conserving water, reducing wind resistance, or benefiting the plant in some other way in its particular habitat.

• Leaves arranged in whorls encircle upright stems at intervals. They collect sunlight from all directions.
• Leaves arranged in basal rosettes take advantage of warm temperatures near the ground.
• Leaves arranged in alternate or opposing pairs collect light from above. They are typically found on plants with a single, upright stem.
• The blades of simple leaves are not divided. This provides the maximum surface area for collecting sunlight.
• The blades of compound leaves are divided into many smaller leaflets. This reduces wind resistance and water loss.

Leaf variation in flowering plants may include variations in the arrangement of leaves and the divisions of the blade.

Seasonal Changes in Leaves

Even if you don’t live in a place where leaves turn color in the fall, no doubt you’ve seen photos of their “fall colors” (see Figure below). The leaves of many plants turn from green to other, glorious colors during autumn each year. The change is triggered by shorter days and cooler temperatures. Leaves respond to these environmental stimuli by producing less chlorophyll. This allows other leaf pigments—such as oranges and yellows—to be seen.

A deciduous tree goes through dramatic seasonal changes each year. Can you identify the seasons in the photo?

After leaves turn color in the fall, they may all fall off the plant for the winter. Plants that shed their leaves seasonally each year are called deciduous plants. Shedding leaves is a strategy for reducing water loss during seasons of extreme dryness. On the downside, the plant must grow new leaves in the spring, and that takes a lot of energy and matter. Some plants may “bank” energy over the winter by storing food. That way, they are ready to grow new leaves as soon as spring arrives.

Evergreen plants have a different strategy for adapting to seasonal dryness. They don’t waste energy and matter growing new leaves each year. Instead, they keep their leaves and stay green year-round. However, to reduce water loss, they have needle-like leaves with very thick cuticle. On the downside, needle-like leaves reduce the surface area for collecting sunlight. This is one reason that needles may be especially rich in chlorophyll, as you can see from the dark green pine needles in Figure below. This is also an important adaptation for low levels of sunlight, allowing evergreens to live far from the equator.

Compare the color of the evergreen needles and the deciduous leaf. Why is the darker color of the needles adaptive?

Summary

• The primary function of leaves is to collect sunlight and make food by photosynthesis.
• In a deciduous plant, leaves seasonally turn color and fall off the plant. They are replaced with new leaves later in the year.
• An evergreen plant keeps its green leaves year-round. It may have needle-like leaves to reduce water loss.

Review

1. Name the two main parts of an angiosperm leaf. What is the function of each part?
2. Identify strategies used by deciduous and evergreen plants to adapt to seasonal dryness.
3. Relate leaf variation to environmental variation.