Leaf Blade

The leaf blade is the part of the leaf organ that is flat and expanded; typically the part that is associated with leaves. In a two-part leaf structure, the blade attaches to the petiole of the leaf. However, in some cases, the blade is attached directly to the stem.

Veins extend from the petiole across the length of the blade in equal diameters. A midrib vein runs through the center of the blade and connects the minor and major veins to the rest of the vascular tissue.

Leaf blades can vary depending on the necessary adaptations for the plant, but all contain three tissue systems: the dermal, vascular, and ground tissue systems. The dermal tissue system will always contain epidermis tissue that has a single layer of normal epidermal cells. In many leaf blades, this tissue is protected by glandular and eglandular trichomes. The underside of typical leaf blades may also have several stomata, or more than the upper side will. The veins contain layers of vascular tissue with both xylem and phloem tissue, and they are surrounded by the remainder of the leaf blade, which is made of multiple layers of ground tissue. These layers contain vast amounts of mesophyll alongside other cell types.

Green leaf blades contain chloroplasts to maximize photosynthesis after sunlight absorption. Most of this chloroplast is in the underlying layers of mesophyll. Though there are chloroplasts on the layer of epidermis tissue, they develop in the guard cells and are used to produce energy to facilitate the opening of stoma. In some cases, leaf blades do not contain chloroplasts, but other plastids such as chromoplast which tint the leaf yellow, orange, or red.

Leaf blades can be modified alongside the leaves. Mature cactus spines lack nearly all tissue cell types found in leaves: they lack guard cells, vascular tissue, spongy mesophyll, and parenchyma. Showy, attractive flower petals, which are developed from leaf blades, contain different pigments like carotenoids that dye the blades non-green. And the leaves of plants that must adapt to non-mesophytic conditions may have various modifications to the cells present in the tissue layers, such as multiple epidermis, large air chambers, or the presence of the Kranz Anatomy in the vascular and ground tissue systems.