Essentially, there are two parts to every leaf: the veins and edge, and the petiole, the stem that extends from the leaf. Blade types are categorized by the differences between dozens of physical attributes that the veins, edge and overall shape of the blade may have. There is the formation of the blade, the shape and pattern of the tips and edges, as well as the texture of the leaf to think of as well. All of these combined will describe the type of leaf blade you are looking at.
Each leaf blade has veins that work similarly to the way human veins work. Leaf veins carry water and nutrients through different parts of the plant for its survival. There are four different types of veins: parallel veins in which the veins run parallel to each other; penniparallel veins that run out horizontally from the midrib (main center vein); pinnately veined blades that only radiate from the midrib and are not necessarily parallel, such as the Camellia plant leaves; and palmately veined leaves that have principal veins running from the base of the leaf, like the maple leaf blade.
Many leaf formations exist. If you can imagine the enormous variety plants with leaves, that is the variety of blade shapes you may encounter as well. Some of these shapes include but are not limited to: linear, oblong, elliptic, ovate and sagittate. A linear leaf is narrow and several times longer than it is wide; its width is uniform. An oblong leave is nearly twice as wide as it is long with parallel sides for their length. Elliptically shaped blades are widest in the middle and narrow at each end. Ovate blades are shaped like eggs, the broadest part being at the base. Sagittate leaves are shaped like arrows. They have lobes at their base that point downward while the main body tapers upward to a point.
The edges of leaves are also called margins. Different types of edges include, but are not limited to: entire, serrate, dentate, crenate, undulate, sinuate, incised, lobed and cleft. Entire blade edges have an even line, without teeth or notches. Serrate blades have saw-like teeth pointing forward. Dentate blades have teeth pointing outward and are large. Crenate blades have rounded or scalloped teeth. Undulate blades form a wavy line on their edge that bends slightly inward and outward in succession. Sinuate blades are wavy. Incised blades are sharp, deep irregular teeth. Lobed blades have rounded incisions that are not deeper than halfway between the edge and center of the leaf. Deeply lobed has incisions that are almost to the center of the leaf.
The texture of the blade is important to its description. Some textures are succulent, scabrous, coriaceous, smooth, downy and pubescent. Succulent leaves are juicy, soft and thick. Scabrous are rough to the touch like sandpaper. Coriaceous are leather-like and tough. Smooth are as its name suggests. Downy blades have short, weak and soft hairs. Pubescent blades are hairier. A canescent blade is covered with gray or white hairs such as the Texas sage plant.
- Photo Credit a leaf image by timur1970 from Fotolia.com