ECONOMIC WOODS OF THE UNITED STATES 21 



WOOD PARENCHYMA 



Parenchyma occurs in the secondary xylem of all woody 

 plants, and, with few exceptions, is disposed in two systems: 

 (1) the vertical, composed of more or less scattered rows of cells 

 forming the wood parenchyma; and (2) the horizontal, made up of 

 plates of cells extending radially and at right angles to the axis — 

 the medullary rays or pith rays. Its chief function is the distribu- 

 tion and storage of elaborated food materials. 



Typical wood-parenchjona fibres (Fig. 2, F; Plate IV, Figs. 

 5, 6) of Dicotyledons resemble septate wood fibres, but have (1) 

 thinner walls, (2) invariably simple, rounded pits instead of 

 oblique, slit-like simple or bordered pits, and (3) cross walls equal 

 in thickness to the lateral walls. The individual cells of a wood- 

 parenchyma fibre are mostly short and prismatic, pitted with 

 simple pits and (with the exception of the terminal ones, which 

 are pointed) with transverse or oblique end walls. Between 

 wood fibres and wood-parenchyma fibres are intermediate forms 

 without septa — substitute fibres or intermediate wood fibres. 



Where wood parenchyma borders on large vessels it is usually 

 much flattened as a result of the pressure of the expanding vessel 

 segments. In such locations also are sometimes special forms 

 termed conjugate cells because of flatly tubular processes extending 

 from one to another slightly distant, thus uniting them (Fig. 2, H). 



There are special forms of wood parenchyraa in which the 

 individual cells are divided by cross walls into small chambers 

 of approximately even size which contain solitary crystals, usually 

 of calcium oxalate (Fig. 2, G; Plate IV, Fig. 6). Such crystals 

 are only slightly soluble even in the strongest acids, and are very 

 plainly visible under high magnification in both cross and longi- 

 tudinal sections. Crystals occur in all species of Quercus, though 

 they are commonly more abundant in the live oaks than in decid- 

 uous species. In Juglans (Plate IV, Fig. 6), Hicoria (Plate IV, 

 Fig. 3), and I)iospyr OS, ^crystals are often quite conspicuous. 

 Calcium-oxalate crystals are also common in ray-parenchyma 

 cells. 



The distribution and arrangement of wood-parenchyma fibres 

 in different species are subject to considerable variation. As seen 

 on cross sections of woody Dicotyledons the fibres may be (a) 

 scattered irregularly throughout the growth ring (Plate V, Figs. 

 3, 5), (6) arranged in tangential lines or bands (Frontispiece, 



