It’s a cold winter day and Ruth Kassinger is eating fresh kumquats that she’s just picked from a tree inside her suburban Washington home.  The kumquat is among a variety of tropical plants in this sunroom that Kassinger calls her conservatory. In place of a worn backyard deck, Kassinger built the room with tall windows.  She installed ceiling skylights, overhead fluorescents and heat lamps.  A small exercise pool could also provide needed humidity. Gradually, Kassinger began filling the room with orange, lime and lemon trees and an unusual yellow citrus with tentacle like fingers called a Buddha’s Hand.