50 MORPHOLOGY OF THE ANGIOSPERMS 



morphologically is doubtful. The term fusiform wood-parenchyma cells 

 perhaps best suggests their nature. 



The more highly specialized types of fibers occur in wood that is in 

 other ways highly specialized — often much simplified — especially that of 

 woody herbs, subshrubs, and shrubs. In herbaceous stems with strong 

 cylinders of secondary wood, as in many Compositae, the wood consists 

 largely of fibers. 



The cells of wood parenchyma stand in vertical rows that extend for 

 long distances in the secondary xylem. The rows are basically uniseriate, 

 consisting of elongate, mostly rectangular cells placed end to end, but 

 two or more rows may stand together, forming clusters or bands of 

 various sizes. Wood-parenchyma cells are formed by the transverse di- 

 vision of daughter cells of fusiform cambium initials, each daughter 

 cell forming a vertical series of a few cells. Each series unites, end to 

 end, with a similar series above and below to form long, continuous 

 columns. Elongate fiberlike parenchyma cells, formed directly by fusi- 

 form initials, are called fusiform wood-parenchyma cells. These living 

 cells, with fiberlike shape and wall, were formerly called "substitute 

 fibers." 



Wood-parenchyma distribution is termed di§use where single columns 

 are isolated and scattered among the other cells of vertical series; 

 terminal where they are present as isolated strands or tangential clusters 

 at the end of a season's growth; metatracheal where the strands are 

 aggregated in clusters or bands but not including scattered, isolated 

 strands and are wholly or largely free of contact with vessels; paratra- 

 cheal where aggregates of columns are associated, wholly or largely, 

 with vessels and typical tracheids; vasicentric where the aggregates are 

 restricted to sheaths about the vessels. No sharp distinction can be made 

 between the diffuse and metatracheal types, and the term apotracheal 

 has been proposed to include paratracheal and vasicentric types. 



In wood-parenchyma distribution, the diffuse type seems to be the 

 most primitive, and the vasicentric the most advanced. Terminal distri- 

 bution appears to be a reduction type derived from diffuse and, there- 

 fore, an advanced type. 



Wood parenchyma is usually absent in stems of herbs that have 

 discrete vascular bundles and in some vines and subshrubs with anoma- 

 lous vascular steles where the bundles are separated by wide primary 

 rays. In some shrubs, wood parenchyma may increase greatly in amount 

 where the rays are reduced in the later-formed wood. In wood where 

 rays are reduced and disappearing, the ray cells are upright, and the 

 disappearing ray is apparently transformed into wood parenchyma by 

 the elongation of the ray initials in the cambium. 



In highly specialized woods, shorter and wider wood-parenchyma 



