Unidentified Plant Parts drop from various mobs and herbs in Zangarmarsh. Herbalists can also gather them from the bog giants in The Dead Mire and The Spawning Glen, and Rotting Forest-Ragers in The Bone Wastes of Terokkar Forest.
As a quest objective
This item can be handed in to Lauranna Thar'well at Cenarion Refuge as part of a repeatable quest to gain reputation with Cenarion Expedition. 10 of these can be turned in at a time, and each turn-in rewards 250 reputation (275 for humans) and is repeatable until honored. Each turn-in rewards a , which has a chance to contain an .
This item can be sold to vendors or other players on the auction house. They are a much more cost efficient way to gain Cenarion Expedition reputation than Coilfang Armaments, and it can be well worth it to buy enough plant parts to get you to honored before you start questing in Zangarmarsh. 360 Unidentified Plant Parts are required to get to honored from scratch (330 for humans). The Uncatalogued Species can be turned in past honored, so save them.