THE 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 



xvovi storage cells. F'g. 78. — arrangement of 



Tissues in Two -year - 



1 here are also old stem of moonseed. 



wood fibers which AP'A; /.parenchyma. The fibro- 



, . , n J vascular bundles, or wood 



are iniCK-Waiieu strands, are very prominent, wUh 



and rigid ih Fig. *'" '"'^''""^'■y "y^ between. 

 "jG), and serve to support the sap-canals 

 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. y6). These thin spots 



(Fig. 80) allow the sap to pass to other 



cells or to neighboring vessels. 

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



wall at o, showing 



of cells whose function is growth. These pit borders at o, ». 



Fig. 79. — Markings 

 IN Cell- W^alls 

 OF Wood Fibers. 



sp, spiral ; an^ annular ; 

 sc, scalariform. 



Fig. 80. — Pits in 

 THE Cell Wall. 



