Leaves are a part of the plant's shoot system. They serve as the primary site for photosynthesis and this function is reflected in their structure. Typically, leaves consist of a stalk called a petiole, that attaches the leaf to a stem, and a flattened blade. If the leaf has a single, undivided blade it is called a simple leaf; leaves with the blade divided into several segments are compound leaves. A palmately compound leaf is one in which all the leaf segments attach to the petiole at the same point; in a pinnately compound leaf the segments attach at different points along the length of an extended petiole.
Numerous veins (bundles of vascular tissue) run through the blade of the leaf, serving to bring water to the leaf cells from the roots, and to collect the products of photosynthesis for distribution to other plant organs. In some plants, the larger veins all run the length of the blade with smaller veins branching off and interconnecting them. This is called parallel venation. In other plants, netted venation occurs, in which the large veins branch repeatedly as they enter the blade. These produce smaller, interconnecting branches as well.
The anatomical features of leaves must allow them to efficiently absorb light as an energy source for photosynthesis, to maximize exchange of gases such as carbon dioxide and oxygen with the atmosphere, and to minimize loss of water by evaporation from leaf surfaces. In a typical leaf, the epidermis is a single layer of cells with a significant cuticle. The thickness of the cuticle varies, being very thick in plants adapted to arid environments and very thin or nonexistent in plants adapted to a watery environment. Throughout the epidermis, stomata are present, formed by specialized guard cells. The placement and frequency of occurrence of the stomata are also variable, again usually determined by the environment to which the plant is adapted. For example, in plants adapted to arid environments, the epidermis may have sunken areas within which the stomata are found, and the stomata are most likely to be present on the lower leaf surface.
The majority of the leaf interior is composed of photosynthetic parenchyma cells called mesophyll. In many plants the layers of mesophyll near the upper leaf surface are composed of elongate parenchyma cells packed tightly together. This palisade mesophyll is the most efficient arrangement for collection of light as it hits the leaf surface. The layers of mesophyll near the lower leaf surface are composed of more nearly spherical parenchyma cells which are loosely packed. The arrangement of this spongy mesophyll facilitates the movement of gases through the mesophyll. Directly beneath each stomate at the leaf surface there is usually a small air space, called a stomatal chamber, where no mesophyll cells are present. This also aids in gas exchange.
The vein. of the leaf are embedded in the mesophyll tissue. Each vein consists of xylem and phloem, with the xylem generally closest to the upper leaf surface and the phloem closest to the lower surface. Most veins are supported within the mesophyll by a bundle sheath of smaller, tightly packed parenchyma cells, or collenchyma or sclerenchyma cells
In some monocots, usually grasses, the arrangement of leaf tissues differs from the typical structure described above. In these plants, there is no distinction between palisade and spongy mesophyll, and the bundle sheath surrounding the veins is formed by a group of large parenchyma cells with a special role in photosynthesis.
1. Examine the leaf types on display and briefly sketch a simple leaf, a palmately compound leaf (also example two can be viewed), and a pinnately compound leaf.
2. Examine the examples of leaf venation types on display and briefly sketch a leaf with parallel venation and one with netted venation.
3. Examine a slide of a Syringa leaf c.s. and draw a portion of it. Label the upper epidermis, lower epidermis, stomate, guard cell, palisade mesophyll, spongy mesophyll, xylem, phloem, and bundle sheath.
4. Examine, and draw a portion of, a Zea leaf c.s. Label the upper epidermis, lower epidermis, mesophyll, xylem, phloem, and bundle sheath.
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