This thesis investigates the aggregate rays present in the xylem of the New Zealand species of Nothofagus, and compares them with those present in other genera, particularly Fagus and Dracophyllum. The ontogeny of aggregate rays is investigated, and shown to be a feature largely of the 'leaf-gap' in Fagus and Dracophyllum. In Nothofagus, however, aggregate ray ontogeny relates more to the leaf-trace. Neither the 'synthetic' nor the 'dissection' theory is adequate for describing aggregate ray ontogeny. The presence of aggregate rays in Nothofagus is correlated with an irregular (not circular) outline of the pith (TS) and may relate to light and hormonal factors and their effect on cell division and elongation within the elongating internode. Certain regions within the growth increment are dominated by pseudo-transverse anticlinal divisions of cambial fusiform initials, other regions are dominated by fusiform initial elongation. An attempt was made to determine whether these regions could be distinguished based on deviations from the normal distribution recorded for axial element length data. The results are not conclusive, but may merit further investigation. Certain features of the aggregate rays in Nothofagus such as: reduced element length; paucity of vessels; reduction of individual rays; and the formation of 'wound-like' areas, indicate that the fluting may actually be responsible for the presence of these aggregate rays.