Abstract:
Preface
This thesis is divided into two parts:
Part I - The Use of Rapid Freezing Techniques to Study Unfixed, Non-Cryoprotected Animal and Plant Tissues by Freeze-Substitution and Freeze-Fracturing .
Part II - The Distribution of P-Protein in Mature Sieve Elements of Phloem, and its Relation to Proposed Translocatory Mechanisms.
The ultrastructure of the sieve elements of the phloem has remained a controversial issue for many years, mainly because of the extreme sensitivity of the cells to manipulation and conventional electron microscopical fixation methods. It was thought that rapid freezing, in association with freeze-substitution or freeze-fracturing, would stabilise the tissue and provide an image of the sieve element as little removed as possible from the in vivo condition. Thus, it was necessary to utilise a technique which froze tissue extremely rapidly, so that the use of cryoprotectants could be avoided.
Therefore, the work reported in Part I of this thesis describes efforts to evaluate the available freezing techniques, and to adapt and test a suitable technique for the investigation of phloem ultrastructure.
This technique, coupled with other conventional electron microscopical methods, is employed in Part II of this thesis to study the ultrastructure of mature sieve elements.