For a simple leaf with a pointy apex, is “leaf length” :
A. the distance along the leaf axis from the point of attachment (e.g. to a petiole or directly to the stem) to the leaf apex?
or
B. the distance from the basalmost-edge of the lamina of a leaf to the leaf apex, along an imaginary line parallel to the leaf axis?
or
C. something else (e.g. perhaps including the length of a petiole, particularly if “winged”)?
For cases where A doesn’t equal B, consider a heart-shaped leaf with lobes that extend behind the blade’s attachment point.
I can remember coming across keys that explicitly differentiate between A and B, but what if there is no distinction in the text? Is there a common convention?
In general, if not specified otherwise, leaf length would include all parts of the leaf including the petiole (if any), starting at the point of attachment, but generally not including any lobes that extend “behind” the point of attachment. If the measurement is referring only to a part of the leaf, such as the leaf blade (lamina) or a leaflet, that should be specified as part of the description.
That said, context is important, and sometimes an entire work could adopt a different convention where, for example, the lengths given apply only to the leaf blades. So it’s always good to do a reality-check if in doubt - look through the reference for clues, see if they consistently specify measurements for parts of the leaf differently than the whole leaf, check a few measurements against known taxa, etc.
Thanks for the reply. It’s counterintuitive to me that statements like “leaves 2x longer than broad” would include the petiole, for example. They must mean the blade in at least that instance, no? I guess I’ll have to do the hard work of trying to find examples of plants this author has personally identified, and try to match those against his own descriptions.
People who write keys: This is extremely frustrating to the uninitiated!
EDITED initial question to reflect answer by @jdmore that the petiole length is in fact, generally included:
For a simple leaf with a pointy apex, what is “leaf blade length”: ?
A. The distance along the leaf axis from the point of attachment (e.g. petiole or directly to the stem/twig) to the leaf apex?
or
B. The distance from the basalmost-edge of the leaf blade to the leaf apex, along an imaginary line parallel to the leaf axis? (That is, the leaf blade “height” if it were separated from its petiole and made to stand upright)?
As you most likely already know but is worth mentioning again and again is that leaf size is usually low on the list of plant identifiers— it’s often highly variable depending on a number of factors but especially amount of sunlight, soil nutrients, and water available. Two plants of the same species can have tiny or extremely large leaves depending on where they’re growing. Some species have consistent leaf size, while many don’t.
Agree. Even shape, as I saw in the statement “leaves 2x longer than broad” can be not really true if there is more sunlight, less humidity in the air, a water source near… I see this pretty often as I started noticing varying populations of Rauvolfia tetraphylla in my “happy place” of observations.