سَعْدَانٌ denotes a plant species in the book. It lacks proper-name status and is therefore fully declinable.
Nom.: هَذَا سَعْدَانٌ — predicate, ḍamma with tanwīn.
Acc.: رَأَيْتُ سَعْدَانًا — object, fatḥa with tanwīn.
Gen.: نَظَرْتُ إِلَى سَعْدَانٍ — after ilā, kasra with tanwīn.