STRUCTURE OF WOODY STEMS 



201 



of the water and mineral salts supplied by the roots. However, 

 much stored food in trees is transferred to the growing regions 

 through the xylem. This is especially true in the early spring as 

 well known in the case of Maples where the xylem carries a large 

 amount of sugar in the spring sap. In Gymnosperms the xylem 

 consists chiefly of tracheids through which the water ascends by 

 passing from one cell to another through the thin places of the 

 bordered pits which are provided in their cell walls. In woody 



Fig. 180. — Piece of a stem of Scotch Fir four years old, showing the 

 medullary rays as they appear in cross, radial, longitudinal, and tangential 

 longitudinal sections of the stem. Enlarged six times. After Strasburger. 



Dicotyledons the xylem consists of tracheae of the annular, 

 spiral, pitted, and scalariform type, among which are inter- 

 mingled wood fibers, wood parenchyma, and some tracheids. 



The xylem tissues formed in the spring, when there is need for 

 rapid transportation of water and dissolved substances to the ex- 

 panding tips, consist of large cells with large cavities and thus 

 give to the spring wood that open, porous character which con- 

 trasts so much with the compact character of the fall wood that 

 the annual rings result. Annual rings, as their name indicates, 

 are formed usually only one each year and consequent!}' their 

 number indicates quite well the age of the tree. Since the cam- 



