Many plants have a part called a petiole that attaches the leaf to the stem. On some plants, the petiole has changed because of environmental reasons so that it looks just like a leaf and is called a phyllode. Both petioles and phyllodes can develop the ability to photosynthesize and function as foliage. Main stems like branches and flower stalks (known as rachis) can also become phyllodes.
In flowering plants, a petiole is generally slender and chiefly renders support by attaching the leaf blade to the stem. In some plants, the petioles become modified or specialized in a way that they become leaf-like in appearance (expanded) and serve not only to support the leaf but to carry out photosynthesis as well.
Not all leaves have petioles. Some are, instead, attached directly to the plant stem. Leaves that do not have petioles, such as those of the broomrape family, are called sessile leaves. Nearly-petiolate leaves are those that have very short petioles. Grasses do not generally have petioles, but some have petiole-like structures called pseudopetioles. Yet another form of the petiole is the rachis—a continuation of the petiole which attaches compound leaves to their plant stems.
Phyllodes are flatter, wider versions of petioles. They are found on most species of acacia trees and shrubs. Many phyllodes start out with true leaves as seedlings, but they fall off after the phyllodes have developed.The phyllodes of Acacia koa, for instance, are thick and coriaceous helping the plant in surviving the stressful environment.
In mosses, the phyllodes refer to the leaf-like structures of the moss plant in the gametophyte stage. The phyllodes are not regarded as true leaves although similar in appearance and function. It is because phyllodes do not have vascular tissues.
Phyllodes and Phylloclades
A phyllode is a modified petiole or a shoot that connects the stem with leaf; its function resembles that of a leaf. A phylloclade, by contrast, is a modified stem, that is responsible for photosynthesis. Melanoxylone is an example of a plant with phyllodes while cactus and cocoloba have phylloclades.
It is possible to tell a phyllode from a phylloclade by looking for these key differences:
- A phyllode is a modified leaf, while a phylloclade is a modified stem.
- Phyllodes bear an axillary bud while phylloclades do not.
- Phyllodes do not have flowers or buds is while in phylloclades do.
- Phylloclades have axillary buds that look a bit like spines, while phyllodes do not.
- Phylloclades have scaly leaves while phyllodes do not.
- Phylloclades have node and internodes while phyllodes do not.
Uses for Petioles
Petioles, phyllodes, and phylloclades all play important roles in plant respiration and survival. In addition, certain petioles are actually crops. The edible parts of rhubarb, artichoke, and celery are all petioles grown specifically as food.