Most of the Cassias that I've tried from seed have needed to be soaked in warm water for 24 hrs before sowing them. A heat mat will probably be helpful with most of these--a lot of tropicals need bottom heat to germinate, or at the very least will germinate better with it.
Calif_Sue Northern California United States (Zone 9a)
I have a book I picked up years ago, 'From Seed to Bloom' by Eileen Powell and it had two listed.
Sowing:indoors-5-8 weeks before last frost. Outdoors -early spring, when soil is cool and light frost is still
possible, or late autumn.
Depth: Just cover
Germinating time: 20-25 days
Requirements: Soak seeds for 2 hrs. in warm water, provide 70-75F thereafter.
Sowing: indoors 8 weeks before last frost. Outdoors After last frost
germinating time: 30-365 days
Requirements: Difficult. use only fresh seed. Soak for 12 hrs in warm water, sow in flats and keep at 80 F.
Another Google search 'sowing heliconia seeds' brought up info "The seed requires soaking for two/three days and sown in a well drained damp compost at a temperature of 30°C. The seed may take a couple of months to germinate and the seedlings will need a temperature of 30C by day, 18°C to 22°C by night and good sunlight to bring them on. Sow in late spring when our improving weather conditions will help"
Actually, it was from this site which came up quite often. http://www.jungleseeds.co.uk/ (I love Google!)
I've also heard that Passiflora seeds benefit from an overnight soak in orange juice... In Hawaii, somebody told me they put their fresh passie seeds into a glass of passion juice, allowing them to soak until the juice has fermented for best germination.
We'll see. I just sowed some P. carulea, did the overnight soak in OJ, put them on a seedling heat mat...