Definition - What does Sinus mean?
In leaves with lobes, the spaces between the lobes are called sinuses. Plant sinuses can be present in large-lobed leaves and shallow-lobed leaves, but not all leaves have lobes or sinuses.
Most plants create carbohydrates in their leaves through photosynthesis. Sinuses and lobes do not serve a purpose, other than defining the leaf’s shape and providing more surface area for photosynthesis to occur than would be possible with an entire leaf of the same physical size.
MaximumYield explains Sinus
There are many different leaf types, including simple lobed and entire leaves, as well as trifoliate, pinnate, fascicle, palmate, bipinnate and tripinnate leaves. Many types of leaves have lobes, or projections that form the leaf’s shape. The gap between each lobe is called a sinus.
It’s easy to recognize sinuses in many common leaves you see every day. For instance, think of a maple leaf with its five lobes. There are four sinuses on the leaf, with each one being located between two lobes. However, not all leaves have multiple lobes or sinuses. For instance, the leaf of the cyclamen perscicum has only two lobes, and a single sinus between them. The leaf of a white oak tree, on the other hand, has many lobes and sinuses.
Some leaves, called entire leaves, have no lobes or sinuses. Rather, the leaf consists of a single blade or lamina. An example of this type of leaf would a dogwood tree leaf. Lobed leaves can have deep or shallow lobes and sinuses, or rounded or pointed lobes and sinuses.