30.4 Leaves  (Page 43/22)

Leaves are the main sites for photosynthesis: the process by which plants synthesize food. Most leaves are usually green, due to the presence of chlorophyll in the leaf cells. However, some leaves may have different colors, caused by other plant pigments that mask the green chlorophyll.

The thickness, shape, and size of leaves are adapted to the environment. Each variation helps a plant species maximize its chances of survival in a particular habitat. Usually, the leaves of plants growing in tropical rainforests have larger surface areas than those of plants growing in deserts or very cold conditions, which are likely to have a smaller surface area to minimize water loss.

Structure of a typical leaf

Each leaf typically has a leaf blade called the lamina    , which is also the widest part of the leaf. Some leaves are attached to the plant stem by a petiole    . Leaves that do not have a petiole and are directly attached to the plant stem are called sessile    leaves. Small green appendages usually found at the base of the petiole are known as stipules . Most leaves have a midrib, which travels the length of the leaf and branches to each side to produce veins of vascular tissue. The edge of the leaf is called the margin. [link] shows the structure of a typical eudicot leaf.

Within each leaf, the vascular tissue forms veins. The arrangement of veins in a leaf is called the venation    pattern. Monocots and dicots differ in their patterns of venation ( [link] ). Monocots have parallel venation; the veins run in straight lines across the length of the leaf without converging at a point. In dicots, however, the veins of the leaf have a net-like appearance, forming a pattern known as reticulate venation. One extant plant, the Ginkgo biloba , has dichotomous venation where the veins fork.

Leaf arrangement

The arrangement of leaves on a stem is known as phyllotaxy    . The number and placement of a plant’s leaves will vary depending on the species, with each species exhibiting a characteristic leaf arrangement. Leaves are classified as either alternate, spiral, or opposite. Plants that have only one leaf per node have leaves that are said to be either alternate—meaning the leaves alternate on each side of the stem in a flat plane—or spiral, meaning the leaves are arrayed in a spiral along the stem. In an opposite leaf arrangement, two leaves arise at the same point, with the leaves connecting opposite each other along the branch. If there are three or more leaves connected at a node, the leaf arrangement is classified as whorled    .