Add to ORKG'.(Y~ ~...~-lk.. gj;7f9'1 I would like to express my sincere appreciation to Dr. Ronald M. Anderson for his invaluable guidance and direction on this thesis. I would also like to thank the other members in my thesis committee, Dr. Edward Allen and Dr. John White for their help and for serving on my committee. I would like to extend my thanks to Elvia Gomez for drawing several of the pictures included in this thesis. I would also like to thank my husband Digala Kulawansa for his loving support. I would like to dedicate this thesis to my late husband Jagath Chandrawansa who always wanted me to pursue higher studies