SYNOPSISGelatinous fibres ofSalix fragilisL. are characterized by an inner layer of cellulose which is relatively unlignified when compared with normal wood fibres. The gelatinous layers of mature gelatinous fibres sometimes show a reaction with benzidine or with ruthenium red. Fibres may sometimes have a terminal lamella which borders the lumen. It is suggested that the irregular distribution of organelles within differentiating fusiform cells is responsible for the spasmodic histochemical reactions and presence of a terminal lamella.The Coppick and Fowler method may be used for the localization of lignin in ultra‐thin sections.Although starch grains have often been seen within fibres, their presence has not been clearly related to the presence of gelatinous fibres alone.From light, polarizing and electron microscopical evidence, the walls of normal wood fibres have been found to be composed of S1+ S2+ S3layers or of S1+ S2layers only. The gelatinous fibres invariably have a gelatinous layer which may replace either the S3or both the S3+ S2layers.Microfibrillar disorientations of the gelatinous layer as seen in carbon replicas appear to be caused by the presence of simple pits or incipient slip planes. It appears that the pits through the gelatinous layer may become distended during processing for light microscopy.The convoluted appearance of the gelatinous layer in some sections is now considered to be an artifact brought about by processing and sectioning of fibres.No evidence has been found that the gelatinous layer is formed other than by the gradual deposition of wall material, whether or not there may be periods of faster or slower wall synthesi