126 



BOTANY 



In addition to the tracheids, small amounts of wood parenchyma 

 are also produced in Gymnosperms by a transverse division of the 

 cambium cells. It is in the parenchymatous cell rows of the wood of 

 Pines, Spruce-Firs, and Larches that the schizogenous resin-ducts are 

 produced (Fig. 139, h). In other Conifers the wood parenchyma con- 

 sists of simple rows of cells, which afterwards become filled with resin. 



In the structure of their secondary tissue the wood strands of 

 Dicotyledons exhibit a great variety of form. These structural differ- 

 ences may, however, be reduced to a few phylogenetic variations. In 

 fact, it is customary to derive all the elements entering into the forma- 



Fig. 139. — Transverse section of the wood of a Pine at the junction of two annual rings. /, Early 

 wood ; s, late wood ; (, bordered pit ; «, interposition of a new row of tracheids resulting from 

 the radial division of a cambium cell ; H, resin canals ; m, medullary rays ; g, limit of late 

 wood, (x '240.) 



tion of the wood of Dicotyledons from the two classes of tissue already 

 met with in the Gymnosperms, tracheal tissue and the parenchymatous 

 tissue of the wood. To the tracheal tissue belong the TRACHEIDS (Fi°\ 

 143, t) and the vessels (g). Under the parenchymatous tissue of the 

 wood are included wood parenchyma (Fig. 144, hp), with relatively 

 short cells rich in contents ; fibrous cells (ef), of greater length, but 

 with similar contents and not more strongly thickened ; and wood 

 fibres (h), which are usually greatly elongated, pointed at both ends 

 and strongly thickened. 



The tracheal tissue consists of elements which lose their living contents at 

 an early stage, and in their fully - developed condition are in reality only dead 

 cell cavities. In this class are included tracheids having relatively wide lumina 



