An electron microscopical examination of the influence of low temperature and chemical inhibitors on phloem structure, with supporting physiological work
After a general introduction to previous work in this field of research the thesis divides naturally into two parts. Sieve elements are notoriously difficult to prepare for ultra-structural examination and considerable time was spent in somewhat novel attempts to overcome the problems of turgor pressure release which have bedevilled phloem microscopists for many years. The methods tried are described and evaluated. Working on the principle that artefacts of preparation are less likely to mislead where several methods are employed, the ultrastructure of the normal sieve elements was thoroughly investigated, and it is hoped, established. One novel method which emerged was a procedure designed to facilitate correlation of micrographs of thin sections made by transmission electron microscopy with others taken in the scanning electron microscope. This proved especially useful for identification of structures, such as sieve plates, presenting an unfamiliar appearance under the scanning microscope. This method appears worthy of further application. Having thus established the normal appearance of sieve elements from the stolons the second part of the programme was based on a firm foundation. Administration of inhibitory treatments was carried out as simply as possible, and emphasis was laid on physiological, confirmation that active translocation was proceeding normally before the treatment, and more slowly (generally) subsequently. Electron microscopic examination showed that certain differences of ultrastructure were present in the variously inhibited materials. These differences are described. However, it seemed unlikely that the features found were responsible for the inhibition observed. There is obviously a continuing field for further investigation into the reasons for the effect of inhibitors on phloem transport.