H. F. Linskens - Böcker

123 phase and hence have no direct bearing on the retention time of solutes. However in gas-solid chromatography, a considerable quantity of the mobile phase may be adsorbed on the surface of the stationary adsorbent which diminishes the column's effective length and ability to retain solutes. In this respect helium has been found to be preferable to most other gases (GREENE and Roy, 1957) because it is adsorbed to the least extent. 3. Packed columns offer a considerable resistance to flow, which may create a pressure differential between inlet and outlet of sufficient magnitude to cause an unfavorable flow rate through a significant length of the column. A reduced inlet/outlet pressure ratio can be obtained by using light molecular weight gases toward which the column packing shows the greatest permeability. The flow rate of the mobile phase is normally adjusted by altering the column inlet pressure, for which purpose commercial pressure regulators of sufficient accuracy are available. Quantitative measurements of the flow rate can be made by a number of methods, including rotameters, orifice meters, soapfilm flow meters and displacement of water. The former two methods are the most con­ venient but the least accurate; moreover they create a back pressure and are temperature dependent whereas although the moving soap bubble is cumbersome to employ and unusable for continuous readings, it is preferred when the highest accuracy is required.

H. F. LINSKENS and J. HESLOP-HARRISON The chapters of this volume deal with intercellular interaction phenomena in plants. Collectively they provide a broad conspectus of a highly active, if greatly fragmented, research field. Certain limitations have been imposed on the subject matter, the most impor­ tant being the exclusion of long-range interactions within the plant body. It is true that pervasive hormonal control systems cannot readily be demarcated from controls mediated by pheromones or information-carrying molecules with more limited spheres of action, but consideration is given in this volume to the main classes of plant hormones and their functions only incidentally, since these are treated adequately in other volumes of this Encyclopedia series (V - ume 9-11) and in numerous other texts and reviews. Similarly, certain other effects, such as those associated with nutrients and ions, are not considered in any detail. Furthermore, we have excluded intracellular interactions, and also consideration of transport phenomena, which are treated in detail in Vol­ ume 3 of this Series. Other aspects of inter-cellular interaction, such as cell surface phenomena and implications of lectin-carbohydrate interactions, and plant-virus inter-relationships, are treated in other sections of this Encyclopedia (Volumes 13B and 14B, respectively). In the volume on physiological plant pathology (Volume 4 of this series) special attention has been given to host­ pathogen interaction. These aspects of our subject will therefore be excluded in the present treatise.