The answer is that plants that are the same species can cross fertilize but plants that are of different species cannot cross fertilize.

Here are a few links on Gardenweb where I have addressed this in the past to some degree :

http://forums.gardenweb.com/forums/load/vines/msg0818111210906.html

http://forums.gardenweb.com/forums/load/vines/msg0911453630526.html

http://forums.gardenweb.com/forums/load/vines/msg1123301719599.html

http://forums.gardenweb.com/forums/load/vines/msg031622355367.html

There are a very few exceptions to the species barrier with the vast majority of true inter-specific hybrids (e.g., Ipomoea x leucantha) being in series Batatas , which means they are all very closely related to the Sweet Potato Ipomoea batatas.

There is also Ipomoea sloteri (which took 11 years and an estimated 40,000 to 200,000 attempts to get a stable fertile plant) and the Youjiro Hybrids (Ipomoea nil hybridized with Ipomoea purpurea) brought to us by Dr.Yoshiaki Yoneda.

There is the self-incompatibility factor in which plants of the same species can become both self and / or cross infertile usually due to low genetic diversity and this can happen with highly bred Japanese types or even wild populations of Ipomoea indica or Ipomoea pandurata.

The most important aspect for the beginner to learn 1st , is how to tell the difference between the most common species like Ipomoea hederacea , Ipomoea nil, Ipomoea purpurea and Ipomoea tricolor , then you will know which cultivars which are of the same species will (most likely) cross fertilize.

best,

Ron

It's some good reading material, Desirai. Have you tried crossing varieties in other plant groups?

Yes! Zinnias! My favorite flower :)

The trick is to remove the anthers of the seed parent before the flower opens up and self-fertilizes.

Ipomoea purpurea makes many progeny compared to I. nil, so you would have more material with which to work. Purpurea flowers however are smaller than nil flowers so you would have the challenge of working with smaller plant structures.