Cell wall differentiation and stages involved with intercellular gas space opening.

The development of the intercellular gas system has been followed during the growth of Pisum sativum root and Phaseolus aureus hypocotyl by means of ultrastructural cytochemistry. The extension of the system is sequential and takes place according to a defined programme of cell wall reconstruction. Contrary to current views, the actual pectic middle lamella does not seem to be directly engaged in the initiation of the aerating system, which conversely appears subordinate to the presence of specialized substructures within the wall. The process is characterized by the early differentiation of a particular layer of wall called the 'splitting layer'. The splitting layer differs from the pectic middle lamella particularly in its insolubility in an incubating medium which removes the wall subunits (EDTA, DMSO, pectinases, cellulases) and its non-reactivity to polysaccharide test involving periodic oxidation (PATAg staining). With ultracryotomy, it displays a distinctive beta-glycerophosphatase activity. The layer gradually splits apart from lateral sites in a manner which somewhat evokes the opening of a zip fastener. The primordial opening, and later the intercellular space, keeps a thin (10-20 nm) extramural coat which is apparently non-glucidic and derives from the splitting layer. Ultimately, local shifts and resorptions of the wall lead to fusion of the early intercellular channels. One of the peculiarities of the opening of the air-space is that because of the mechanism involved the polysaccharides of the wall are not left naked. The processes observed are compared with other cases of cell wall separation.