Complex sentence: Achenes are "monocarpellate" (formed from one carpel) and "indehiscent" (do not open at maturity) and contain a single seed that nearly fills the pericarp, but does not adhere to it.
Split into simpler sentences: Achenes are "monocarpellate", (formed from one carpel) and indehiscent (they do not open at maturity). Achenes contain a single seed that nearly fills the pericarp, but does not adhere to it.