The phloem in vascular plants transports photoassimilates and other macromolecules from source organs to distant sink organs for growth, storage and development. Mature sieve elements, which are the transporting cells, are void of major organelles, such as nuclei and vacuoles. The unobstructed sieve elements allow the rapid, long distance transport of photosynthates at a low resistance. Despite the essential role of phloem in plants, little is known about the function of the remaining sieve element components, such as the structural phloem proteins (P-proteins), sieve element plastids, endoplasmic reticulum (ER) and mitochondria. The work presented in this dissertation is a compilation on the investigations of sieve element components. One of the few sieve element organelles with known function are forisomes, which are spindle-shaped P-protein bodies present exclusively in Fabaceae species. Forisomes are capable of occluding sieve tubes due to their unique, reversible Ca2+-dependent but ATP- independent contractility. Using bioinformatic tools and site-directed mutagenesis, we studied the structure and function of the forisome-like bodies in yeast, and more importantly, identified three aspartate residues critical for Ca2+-responsiveness, and two more that were required for forisome-like bodies assembly (Chapter Three). We have investigated non-dispersive P-protein bodies in Populus trichocarpa, the homologous proteins of forisomes, to evaluate if they have a similar function. The non-dispersive P-protein bodies have been characterized at a molecular level and we found that the P-protein bodies in P. trichocarpa did not respond to wound of the sieve tube or Ca2+ (Chapter Two). In addition, to investigate other components in sieve elements, we have developed a novel methodology to isolate intact sieve elements in Nicotiana tabacum and performed proteomics analysis. We discovered three novel sieve element specific proteins that are likely localized in a new, undescribed compartment in sieve elements (Chapter Four).