How to tell petiole apart from stem? Is it by the rule that there is usually a pair of stipules at the base of a petiole?
Petioles typically have the presence of one or more of the following:
- Stipule at base of the stalk
- Axillary bud between the stalk base and attached stem
- Leaf scar when removed
Source: Miami University
None of these characteristics would be present if the stalk you were looking at is a stem.
However, they are not always each visible on every species...
It is not always easy to find the bud at the base of a petiole, it may not be visible early in the growing seaon and sometimes a mature bud is “hidden”, such as being enclosed by the petiole base.
although in many species the stipules are inconspicuous or entirely absent (and the leaf is then termed exstipulate)
Leaves are usually borne from petioles and rarely from the stem itself -- so presence of a leaf (or compound leaf parts) is usually indicative of a petiole as well. However, sometimes plant leaves lack a petiole altogether, as in the case of sessile leaves.
In other instances, specifically those of compound leaves, the stalk of compound leaves (i.e., the rachis) will lack the properties previously described for a petiole, but following the compound leaf to the nearest stem typically will result in locating the petiole with one or more of the above mentioned characteristics at its base connecting the compound leaf to the stem.