The Microscopic Structure of Hardwoods

Vessels are diffieult to visualize in their entirety due to their length and irregular pathways in wood. The eomplexity of these pathways has been investigated using video and cinematographie reeordings of numerous individual transverse seetions of wood so providing the clearest three-dimensional impression of vessel pathways. 

Fibres make up a high proportion of the volume of most hardwoods. Fibres are imperforate, axially-elongated cells, with small lumens and ends that taper into pointed tips. The density of a hardwood is largely determined by the proportion of fibres to other cell types present in the wood. In a low density wood, the vessels occupy a major proportion of the wood volume, whereas denser woods have a larger proportion of thick-walled fibres. The secondary walls of fibres are usually sparsely pitted and the cells lack cell contents at functional maturity. 

Vasicentric tracheids are found close to the vessels in some hardwoods, particularly in the earlywood of ring porous species. They are short tracheid-like cells with profuse sidewall pitting. They are often longitudinally bent and flattened transversely on account of the lateral expansion of the adjacent vessels. 

Axial parenchyma cells (also called longitudinal parenchyma) are generally very abundant in hardwoods. Like vessel elements and fibres, axial parenchyma cells are derived from the axially-elongated fusiform initials of the vascular cambium but, whereas vessel elements and fibres (except septate fibres) remain unsegmented, axial parenchyma cells are formed by the transverse segmentation of the derivatives of fusiform initials. Axial parenchyma cells, therefore, tend to lie in vertical files 

The elassification of various ray types present in hardwoods is complex. Rays are broadly defined as homogeneous or homocellular if they have only procumbent parenchyma cells, or heterogenous or heterocellular if they have axially-elongated parenchyma cells associated with their margins. 

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