THE PLANT BODY 51 



cells accompany shorter fusiform cambium cells. This shortening is 

 closely correlated with a similar shortening in the vessel element. 



The types of wood-parenchyma distribution merge to some extent 

 and may differ in seedlings and mature plants. The presence of both 

 apotracheal and paratracheal parenchyma in the same wood is rare; it 

 is known in only six families — among them Tiliaceae, Myrtaceae, 

 Bombacaceae. 



Within a family, the amount of wood parenchyma may be fairly con- 

 stant or may range from little to much, but type of distribution is 

 fairly constant except in families with considerable range in floral struc- 

 ture. 



There is little information about the occurrence of fusiform wood- 

 parenchyma cells ("substitute fibers"). They are found in the first 

 annual rings of some genera, especially in tropical families, in woody 

 vines, and in suffrutescent herbs. They are, perhaps, most abundant in 

 woody plants that appear to have been derived secondarily from herbs, 

 as in the Berberidaceae; in these plants, strong-walled, elongate paren- 

 chyma cells may furnish the support given by fibers in typical woody 

 plants. 



In contrast with the vertical system, the transverse system of cells in 

 the secondary wood of angiosperms — xijlem rays or wood rays — is made 

 up of a single basic cell type, the parenchyma cell. But the rays, radial 

 sheets of these cells, though simple in cellular make-up, show much 

 diversity in form and cell arrangement. As diey elongate, they may 

 increase or decrease in size; they may divide or fuse with other rays; 

 they may have different form close to the primary body than 

 farther out in tlie secondary wood. In tangential section, rays 

 have various shapes — linear, oblong, fusiform, elliptical, oval; the larger 

 types — as seen in section — may have parallel sides with extended, 

 uniseriate wings at top and bottom. 



Two major types of wood rays are distinguished: uniseriate and 

 multiseriate, made up, respectively, of one and of more than one series 

 of cells (Figs. 22 and 25). Uniseriate rays consist of one type of cell 

 only; multiseriate rays may be homogeneous, consisting of one type of 

 cells only, or heterogeneous, consisting of two types of cells: upright, 

 vertically elongate cells, which form an outer, limiting, enclosing layer, 

 and prostrate inner cells, with their longest diameter usually radial. 

 These two kinds of cells seem to differ cytologically also. The term 

 aggregate is applied to clusters of rays that lie more closely together 

 than other rays in the same wood; "aggregate rays" are not a kind of 

 ray. Clustering of rays is characteristic of genera in widely scattered 

 families. The term primary ray is applied to rays that are connected 

 with the pith. 



