If you mean "Duckbill platypus" by Platypus in your question, then the fertilization in platypus is internal. They are egg laying mammals, and lay amniotic egg after its fertilization by male inside the female body.
Do Platypus' Have Internal Or External Fertilization?
Platypus is a name of a plant. And that plant's fertilization is a form of external fertilization. It's not an internal fertilization and this plant is also tolerant of drought, humidity and dryness. To learn more about it & about it's different features click here