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Enlargement of red area to show funicular attachment of the seed to the replum.  |
A mimosa pod consists of two thin walls (valves) connected at their margins by a distinct edge or replum. The maturing seed is attached to the replum, the fruit's placenta, by an umbilicle-cord-like funiculus. As the pod matures and dries, the valves detach from the replum, thus releasing the seeds. For the photo above the valves were forced open, resulting in some tearing. The blue arrows indicate points of attachment before the dissection. The position of seeds was not preserved. Note that all prickles are along the replum, although they occasionally occur on the valves (with M. texana).
The pods of legumes frequently develop a supporting stalk (stipe) at the base of the ovary as the pod matures. With legumes lacking a replum the distinction is generally clearer (as with Sophora affinis above).
Barnaby 1991 indicates that the base of M. borealis is contracted into a stipe some 4-9 mm long. With our mimosas the distinction between stipitate (having a stipe) and sessile (without a stalk - with simple narrowing of the replum toward the base) is at times a more difficult, relative judgment - especially with older, badly weathered pods. In the photos below the two edges of the M. borealis pod narrow significantly well before the base (prior to reaching the remnants of the corolla. With the others, the two edges appear to meet only at the very base.
 M. aculeaticarpa |
 M. texana |
 M. borealis |