The best difference between these two (_D.rotundifolia_ and _D.spatulata_)
is in the seeds. _D.r._ has long fusiform seeds whereas _D.s._ has
ovoid-elliptic ones.
Also, the stigmata of _D.r._ are slightly broadening towards the apex (and
the styles are not thickened basally) whereas those of _D.s._ are +/-
narrowing towards the apex.
_D.r._ *sometimes* has a leaf lamina broader than long. As far as I know
this is *never* the case with _D.s._.
But once ripe seeds are at hand, I do not believe any confusion between at
least these two is possible.
Kind regards
Jan