Leaf "shape" is a particular definitive term in botanical science. There are many shapes of leaves, from conifer needles to kidney- or wedge-shapes to obovate shapes. One classification of leaves that does have only three categories is classification according to veins/nerves. The classification of leaves according to vein or nerve structure specifies the three categories of:
- Pinnate leaves
- Palmate leaves
In parallel-veined leaves, the veins run along-side each other making the leaves very tough and sturdy, as seen in celery stalks or hemp reeds. It's easy to see that the veins in celery stalks run alongside each other throughout the stalk.
In pinnate leaves, nerves are discussed rather than veins. There is a central nerve running up the middle of the leaf from which secondary, smaller nerves branch out to each side.
In palmate leaves, the discussion is again one about nerves rather than veins. In this classification of leaves, there is a central nerve hub from which secondary nerves radiate out as do spokes of a wheel.
When asking about leaf classification that specifies three categories, vein classification is the most common classification with three categories. Otherwise, it is clear that there are many basic blade and edge shapes by which leaves can be classified.
Identification of the plants can be helped by the shapes of the leaf. There exists several types of shapes, such as ovate, lanceolate, elliptic, cordate, reniform, dissected, reniform, linear or oblong.
It is important to be noticed that some shapes can fall between the existent shapes, such as elliptic-ovate or lanceolate-ovate. It is also possible to exist changes in shape, which are influenced by the sunlight exposure. Hence, it is noticed that the leaves that do not receive the needed sunlight, suffer modification in shape, they becoming thinner and larger, expanding their surface to capture a bigger amount of light. On the other hand, the leaves that do receive the needed amount of light, keep their normal shape.
There are also registered modifications in the shape and size of certain leaves, over their lifespan, they occuring in the heteroblasty process.