The results reported form part of the series dealing with the timbers of the south-west Pacific area and cover 32 genera of the Myrtaceae. The timbers of this family are perhaps more important, from an economic standpoint, than those of any other in the area under consideration (with the possible exception of the Dipterocarpaceae). Based on the examination of all available species, the anatomical features of each genus have been summarized and the results set out in tabular form. From these a grouping of the genera of the family has been made, which reveals one or two inconsistencies with the botanical classification as proposed by Niedenzu. Particular mention has been made of the anatomical variation in Eugenia (sens, lat.). The affinities of the described genera with those of the New World, and the anatomical differences between the Myrtaceae and the closely alIied Lecythidaceae, are discussed. Anatomical features separating the Myrtaceae from four other families with some similar features have been pointed out.

The cell wall organization, the cell wall texture, and the degree of lignification of tension wood fibres have been investigated in a wide variety of temperate and tropical species. Following earlier work describing the cell wall structure of tension wood fibres, two additional types of cell wall organization have been observed. In one of these, the inner thick "gelatinous" layer which is typical of tension wood fibres exists in addition to the normal three-layered structure of the secondary wall; in the other only the outer layer of the secondary wall and the thick gelatinous layer are present. In all the tension wood examined the micellar orientation in the inner gelatinous layer has been shown to be nearly axial and the cellulose of this layer found to be in a highly crystalline state. A general argument is presented as to the meaning of differences in the degree, of crystallinity of cellulose. The high degree of crystallinity of cellulose in tension wood as compared with normal wood is attributed to a greater degree of lateral order in the crystalline regions of tension wood, whereas the paracrystalline phase is similar in both cases. The degree of lignification in tension wood fibres has been shown to be extremely variable. However, where the degree of tension wood development is marked as revealed by the thickness of the gelatinous layer the lack of lignification is also most marked. Severity of tension wood formation and lack of lignification have also been correlated with the incidence of irreversible collapse in tension wood. Such collapse can occur even when no whole fibres are present, e.g. in thin cross sections. Microscopic examination of collapsed samples of tension wood has led to the conclusion that the appearance of collapse in specimens containing tendon wood can often be attributed in part to excessive shrinkage associated with the development of fissures between cells, although true collapse does also occur. Possible explanations of the irreversible shrinkage and collapse of tension wood fibres are advanced.

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