THE STEM ITS GENERAL STRUCTURE 



larger, even if the tree should live a century. It is not 

 quite inclosed, however, for the narrow layers of soft cells 

 separating the bundles remain be- 

 tween them (Fig. 78), forming ra- 

 diating lines called medullary rays 

 or pith rays. 



The Several Plant Cells and their 

 Functions. In the wood there are 

 some parenchyma cells that are 

 still with thin walls, but have lost 

 the power of di- 

 vision. They are 





an 



so 



now storage cells. FlG - 78. -ARRANGEMENT OF 



TISSUES IN TWO- YEAR - 



There are also 



OLD STEM OF MOONSEED. 



WOOd fibers Which /, pith; /.parenchyma. The fibro- 



, . . n J vascular bundles, or wood 



are thlCk-Wailed strands, are very prominent, with 



FIG. 79. MARKINGS and rigid (/ Fig thin medullary rays between. 



IN CELL WALLS 



OF WOOD FIBERS. 7), and serve to support the sap-canals 



s p, spiral ; an, annular ; or wood vessels (or tracheids) that are 



sc, scalariform. r , , . , . / , i i 



formed by the absorption of the end 

 walls of upright rows of cells ; the canals 

 pass from the roots to the twigs and even 

 to ribs of the leaves and serve to transport 

 the root water. They are recognized (Fig. 

 79) by the peculiar thickening of the wall 

 on the inner surface of the tubes, occur- 

 ring in the form of spirals. Sometimes the 

 whole wall is thickened except in spots 

 called pits (g, Fig. 76). These thin spots 

 (Fig. 80) allow the sap to pass to other FlG 8o _p nsiN 

 cells or to neighboring vessels. THE CELL WALL. 



The cambium, as we have seen, consists Longitudinal section of 



wall at 6, showing 



These 



of cells whose function is growth. 



F 



pit borders at a, a. 



