The thanks I owe for this PhD are rather as many as the words that it consists of, probably equally important and definitely harder to express. I would, nevertheless, like to express my gratitude to everyone, who was, as meant to be, present or absent. I would like to express my deep gratitude to my supervisor Prof. Stylianos Delivopoulos for many things, most of which are difficult to express in words. I thank him for his continuous assistance, guidance, encouragement and friendship during the time I have spent at AUTH, Greece. Moreover, I would like to express my deep gratitude for his kindness and hospitality, for his quantity of enthusiasm and quality of interest. But most of all I thank him for having been both a teacher and a 'father' to me. I would also like to express my gratitude to Dr. Thomas Sawidis for helping me in the subject and scientifical revision of my work. I thank him for his kind heart which has boosted up my enthusiasm and emotions. I will never forget his assistance, discussions and joke especially about the "noble price". I also thank Dr. Barbara-Evelin Diannelidis for her endless assistance and encouragement. I am deeply grateful to the following botanical staff; Prof. Ioannes Tsekos who gave me the opportunity to elaborate my Ph.D at the Aristotle University of Thessaloniki before his retirement. In addition, I would like to express my very deep gratitude for his assistance and critical suggestions that he has made on my research. I also thank Dr. George Nikolaidis, Maria Moustaka and Michael Moustakas for their help and technical support especially in the use of their microscopes. Thanks are also due to Dr. Stylianos Kokkini, Artemios Bosabalidis and Eleftherios Eleftheriou for their moral support and encouragement. I am deep grateful to Dr Emmanuel-Nicholas Panderis for his friendship, jokes and some scientific assistance. Finally, I would like to thank everybody in the Department of Botany for the joyful atmosphere created in the lab during my research work. I also thank the SEM technician Mr. Stavros Oikonomidis for his help in carrying out the SEM protocol. I would like to express my deep gratitude to Mr. Tasos Makrantonakis and Thanasis Nikodimos for their help, moral support and lovely atmosphere created in Sharaf Al-Tardeh Prolegomena vii the TEM laboratory. I also thank Mrs. Soula Sapountzoglou (mama tou kosmou), Pavlos Gaithatzis and the secretary of the Department Mrs Arete Dimopoulou.I express my sincere thanks to my colleagues who, with their behaviour, gave a unique meaning of the concept "team work" and academic ethics. Those folks are Ioannes Akdamakis, Spyros Gelis, Natasa Tsirika, Giorgos Kofidis, Katerina Aligizaki, and Kalopesa Eleni for their constant encouragement and making a friendly atmosphere in the lab. They made sure not let a day pass without joking, smiling and drinking as well.