Shapes and Bases
The variety of leaf shapes is one of the attractions of having different kinds of plants in the garden. Defining the shape of a leaf is based on the ratio of length to width, the curve of the leaf edge, and the location or position of the widest point in the leaf. Keep in mind that the terms describing the leaf shape can apply to leaflets as well as simple leaves. The following terms are defined with these characteristics in mind.
A triangular leaf, in which the length is essentially the same as the width, is known as a deltoid or deltate leaf. Cottonwood trees have leaves of this shape. Leaves that are one and a half to two times longer than they are wide can be one of three different general shapes. They are the elliptic, like a narrow oval, with the widest point in the middle, and symmetrically curved edges, the oblong leaf, which has parallel edges, not curved, and the ovate, which is like the shape of an egg. Ovate leaves have symmetrically curved edges, but the widest point is closer to where the petiole and leaf blade join, not in the middle as in elliptic leaves. Chestnut trees have elliptic leaves, while oblong ones are found on willow Oak and ovate leaves on the beaked hazelnut.
A familiar leaf shape is the heart shape, or cordate leaf (see thumbnail picture above). They typical cordate leaf has the lobes of the "heart" at the point where the petiole joins the leaf blade. If the lobes are at the opposite, or tip, end, that shape is called obcordate. An egg-shaped leaf can also have the wider point nearest to the petiole junction, or nearer the tip end. If the wider portion is towards the base, that is an ovate leaf. When the wider portion is nearer to the tip end, that shape is called obovate (see examples of both types in the image at left).
A couple more of the commonly seen shapes are the lanceolate and the linear. The lanceolate leaf shape is like the tip of a lance, three to six times longer than wide, and widest towards the base. Linear leaves are more than twelve times longer than they are wide and are characteristic of the leaves found on many grasses.
Back to the Base
Leaf bases have a wide variety of shapes as well, and an equally wide variety of terms for describing them. Some of the more common shapes you are likely to see in your garden are the acute (tapering to a sharp point with convex, or outwardly curved, sides), the oblique (a base that is not symmetrical on both sides), the cordate (having lobes like a heart), the cuneate (tapering to a sharp point, but with straight sides), the hastate (a base with outwardly pointed lobes), the peltate (a leaf like is found on Nasturtium, where the petiole joins the blade in the middle of a rounded leaf rather than at the edge), and the sagittate (a base with a downward pointing lobe on each side, similar to an arrowhead). Look around in your garden and you are likely to find examples of many of the types of shapes I've mentioned in this article.
In The Basics of Leaves III, I'll introduce you to leaf tips, leaf margins, and leaf arrangements on the stem. Some of the same terms used for leaf bases can also be used for leaf tips, and some of the terms for leaf margins are similar to terms used in reference to compound leaves, as I will illustrate for you.
Image credit: Florida Center for Instructional Technology Clipart