Leaf Structures

Leaves come in a wide variety of shapes and sizes and are an important feature used for plant identification. Leaves can be round, straplike, needlelike, or heart-shaped. The broad, flat portion of a leaf, called the blade, is the site of most photosynthesis. The blade is usually attached to the stem by a stalklike petiole. The maple leaf shown in Figure 29-14a is a simple leaf; it has a single blade. In compound leaves, such as the white clover in Figure 29-14b, the blade is divided into leaflets. In some species, the leaflets themselves are divided. The result is a doubly compound leaf, such as that of the honeylocust shown in Figure 29-14c.

Leaves consist of three tissue systems. The dermal tissue system is represented by the epidermis. In most leaves the epidermis is a single layer of cells coated with a nearly impermeable cuticle. Water, oxygen, and carbon dioxide enter and exit the leaf through stomata in the epidermis. Epidermal hairs are often present and usually function to protect the leaf from insects and intense light.

The number of stomata per unit area of leaf varies by species. For example, submerged leaves of aquatic plants have few or no stomata. Corn leaves have up to 10,000 stomata per square centimeter on both upper and lower surfaces. Scarlet oak has over 100,000 stomata per square centimeter on the lower leaf surface and none on the upper surface. Regardless of their exact distribution, stomata are needed to regulate gas exchange.

In most plants, photosynthesis occurs in the leaf mesophyll (MEZ-oh-FIL), a ground tissue composed of chloroplast-rich parenchyma cells. In most plants, the mesophyll is organized into two layers, which are shown in Figure 29-15 on the next page. The palisade mesophyll layer occurs directly beneath the upper epidermis and is the site of most photosynthesis. Palisade cells are columnar and appear to be packed tightly together in one or two layers. However, there are air spaces between the long side walls of palisade cells. Beneath the palisade layer is the spongy mesophyll. It usually consists of irregularly shaped cells surrounded by large air spaces, which allow oxygen, carbon dioxide, and water to diffuse into and out of the leaf.

The vascular tissue system of leaves consists of vascular bundles called veins. Veins are continuous with the vascular tissue of the stem and the petiole, and they lie embedded in the mesophyll. Veins branch repeatedly so that each cell is usually less than 1 mm (0.04 in.) from a vein.

Venation is the arrangement of veins in a leaf. Leaves of most monocots, such as grasses, have parallel venation, meaning that several main veins are roughly parallel to each other. The main veins are connected by small, inconspicuous veins. Leaves of most dicots, such as sycamores, have net venation, meaning that the main vein or veins repeatedly branch to form a conspicuous network of smaller veins.

(b) COMPOUND LEAF

(c) DOUBLY COMPOUND LEAF

figure 29-14

(a) A sugar-maple leaf is called a simple leaf because it has only one blade.

(b) A white clover leaf is called a compound leaf because the leaf blade is divided into distinct leaflets. (c) The honeylocust has a doubly compound leaf because each leaflet is subdivided into smaller leaflets.

(c) DOUBLY COMPOUND LEAF

figure 29-14

(a) A sugar-maple leaf is called a simple leaf because it has only one blade.

(b) A white clover leaf is called a compound leaf because the leaf blade is divided into distinct leaflets. (c) The honeylocust has a doubly compound leaf because each leaflet is subdivided into smaller leaflets.

Cuticle

Upper epidermis

Palisade mesophyll

Vascular bundle (vein)

Spongy mesophyll

Stomata

Was this article helpful?

0 0
Sirens Sleep Solution

Sirens Sleep Solution

Discover How To Sleep In Peace And Harmony In A World Full Of Uncertainty And Dramatically Improve Your Quality Of Life Today! Finally You Can Fully Equip Yourself With These “Must Have” Tools For Achieving Peace And Calmness And Live A Life Of Comfort That You Deserve!

Get My Free Ebook


Post a comment