The fibrous composition and the dimensions of the individual types of wood cells in the frame of the whole tree of the fast growing poplar (Populus euramericana cv. I-214 ) and of the spruce (Picea abies) have been compared. The wood of the branches of poplar contains the highest proportion of the libriform fibres which are strongly marked by the highest content of the cell walls compared with the libriform wood fibres of the other parts of the tree. Similarly the wood of the branches of spruce contains the most tracheids with the highest content of wood cells compared with all other parts of the tree. The wood from the roots of poplar has the lowest content of libriform fibres with the lowest proportion of the cell walls. Likewise the wood form the roots of spruce has the lowest appearance of tracheids with the lowest proportion of cell walls in the frame of the whole tree. The wood of the branches of poplar has the lowest content of vessels and wood parenchyma. The low content of parenchyma was determined also in the wood of the branches of spruce, mainly in the wood of the branches with small diameter. In general, the wood from the roots has the highest content of wood parenchyma. The wood from the stem of both species of wood consists mainly of the longest fibres. By some degree shorter are the fibres in the wood of the root, whereas the wood of the branches contains the shortest fibres in the frame of the whole tree. The fibre length in the wood of the branches and of the roots decreases proportionally to the decrease of the diameter. This decrease is more conspicuous in the case of the wood of the branches.