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B. swaseyi Basal Leaflets and Leaflet Reduction
by Bob Harms
The basal leaflet pair of B. swaseyi, attached at the very base of the
rachis, is always significantly smaller than the leaflet pair above it.
These leaflets are sometimes reduced in size, generally accompanied by a
reduction in the number of spines, with extreme reduction resulting in a
one-spined one-nerved leaflet with virtually no lamina.
4 to 2 spine reductions of basal leaflets.
1 (& 3) spine reductions of basal leaflets.
When counting B. swaseyi leaflets the basal leaflets are commonly
missing or reduced to inconspicuous size. One must thus always note the
basal abscission ring of the rachis. For instance, in the images below
superficial inspection might indicate that the basal leaflets are not
significantly smaller.
On closer examination the bottommost articulation ring reveals a
greatly reduced leaflet (originally similar to the 1-spined leaflets
above).
With extreme reduction or absence of the basal leaflets, it is the basal
articulation ring that provides a clear basis for distinguishing the
hybrid plants, which lack the smaller basal leaflets, and for
distinguishing stipules from minimal basal leaflets.
Minimal basal leaflet (left) & stipule (right) from the
same B. swaseyi.
The images of hybrid leaf bases below show stipules that could
conceivably be mistaken for 'bristles,' if it weren't for the absence
of annular abscission zones.
B. trifoliolata X B. swaseyi leaf bases with stipules.