STEM— ITS GENERAL STRUCTURE 



65 



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 

 now storage cells. 

 There are also 



P 



s 



an 



Fig. 78. — Arrangement of 

 Tissues in Two- year - 

 old Stem of Moonseed. 



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

 , vascular bundles, or wood 



are LniCK-WaiieCl strands, are very prominent, with 



Fig. 79. — MARKINGS and ri^id (7/ Fl°". thin medullary rays between. 



^ ITT OV'O" 



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 

 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 

 cells or to neighboring vessels. 



Fig. 80. — Pits in 

 the Cell Wall. 



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



wall at b, showing 



of cells whose function is groivth. These pu borders at 0, 0. 



F 



