Re: Why do guard cells in plants have chlorplasts?

There are several references on the web as to the function of chloroplasts 
in the guard cells of the stomatal apparatus.  I had always heard that the 
chloroplasts were non-functional but, this is apparently not the case 
according to these references.  If the chloroplasts are functional, sugars 
would be produced, changing the osmotic potential within the guard cell.  
Water would enter in response to this change in potential and the guard 
cells would open the stoma wider.
 http://www
.microscopy-uk.org.uk/mag/articles/stomata.html
 http://www.le
arn.co.uk/default.asp?WCI=Lesson&WCU=1446
 http
://www.rrz.uni-hamburg.de/biologie/b_online/e32/32f.htm#co2

As to why other epidermal cells lack chloroplasts I can only offer some 
logical thinking.  The epidermis is often covered by a cuticle  making it 
difficult for water to exit the cells and equally difficult for gases, such 
as carbon dioxide, to enter the cells.  If carbon dioxide can't enter the 
cells, photosynthesis does not occur and there would be no need for 
chloroplasts.