Re: how do plants that make seedless fruit reproduce?

Some seedless fruits, such as navel orange and seedless watermelon, do not 
reproduce naturally so depend on humans to propagate them. Navel orange and 
most fruit trees are usually grafted. Triploid seedless watermelon is grown 
from seed obtained by crossing a diploid seeded parent with a seeded tetraploid 
parent.

Other types of seedless fruits, such as bananas and pineapple, have natural 
vegetative methods to propagate themselves. Bananas and pineapple reproduce by 
suckers. The top of a pineapple fruit or crown will also root and form a new 
plant. 

Some types of seedless fruit, such as persimmon, seedless cucumber, and 
pineapple are not always seedless so can reproduce by seed. Persimmon trees are 
either male or female. A female persimmon that is not pollinated may produce 
seedless fruit but it can produce seeded fruit when pollinated. Pineapple and 
seedless cucumber can produce seeded fruit if cross-pollinated, so commercial 
growers do not allow cross-pollination. 'Bartlett' pears normally have seeds, 
but in California's Sacramento Valley, they often are seedless because of the 
favorable climate. 

Search the Mad Scientist site for several other responses on seedless fruit.

Reference

Hartmann, H.T. at al. 1996. Plant Propagation: Principles and Practices, 6th 
edition. Englewood Cliffs, NJ: Prentice Hall