132 ECONOMIC WOODS OF THE UNITED STATES 



eludes all of the vertical elements of the wood except vessels and 

 parenchyma, namely, libriform fibres, septate fibres, intermediate 

 or substitute fibres, fibre-tracheids, and tracheids. 



Libriform fibres are the cells referred to on p. 18 as typical wood 

 fibres. Fibre-tracheids are fibrous cells with distinctly bordered 

 pits and are intermediate between libriform fibres and vessel-like 

 tracheids; they do not occur in Gymnosperms though the tracheids 

 of the late wood might with some justification be so-called. 



The following diagram shows the relationships of the various 

 elements. In this the tracheid appears as the dominant element. 

 Vessels are composed of segments which were originally tracheids 

 before fusion; intermediate forms occur. Fibre-tracheids and 

 libriform fibres may be considered as modifications of the tracheid 

 in which the mechanical function of strength is emphasized at the 

 expense of water conduction. Intermediate forms between these 

 cells and parenchyma are shown in the diagram which were not 

 brought out in the other classifications. Epithelial cells of resin 

 ducts are shown as specialized forms of parenchyma. 



TRACHETDS Ray tracheitis Ray parenchyma 



^" \ ^^ t\ 



^-^ ! ^ e *a, £■ > Epithelial celU 



, T - , Fibre-tracheids ^s-^ *•• / 



Vessels I - — ^ 5 ' 



'Substitute fibres ' "*""*""^,l^' 



I !" * I Septate fibres [" Wood parenchyma 



l i 



Libriform fibres 



VESSELS 



Vessels are compound elements; they are composed of segments 

 which have become fused at the ends (and sometimes at the sides 

 as well) into vertical series. Each segment normally arises from a 

 single cambial cell and when first formed is completely inclosed by 

 the middle lamella and is morphologically a tracheid. After fusion 

 the cells function, not as a series of individuals, but as a continuous 

 tube. 



The segments may abut on each other squarely at the ends 

 or overlap more or less. Both forms may occur in the same vessel, 

 though decidedly elongated tips are characteristic of certain 

 species. Such tips are usually provided with bordered pits and in 

 some instances exhibit spiral thickenings even though the body of 



