by 
saniya
ANSWERS: 1
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Cow parsnip (Heracleum maximum) is one good example, and commonly species of hemlock produce top-flattened clusters. Your question is vague, however, so I'm not sure exactly what you mean by "flat surface" (flat where? How?). A more general (but not fool-proof) answer is that UMBEL and RACEMOSE CORYMB inflorescence produce flat clusters (again, fat on TOP). I hope that helps.
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