Hi beanhead,
Are you asking about the evolution of sex per se or the evolution of sexual reproduction in plants specifically?
Here is a general theoretical paper
Otto SP. Related Articles, Links
The advantages of segregation and the evolution of sex.
Genetics. 2003 Jul;164(3):1099-118.
and here is one on plants
Holsinger KE. Related Articles, Links
Reproductive systems and evolution in vascular plants
Proc Natl Acad Sci U S A. 2000 Jun 20;97(13):7037-42.
cheers,
M