THE TREE ITS ROOT-SYSTEM. 95 



masses being almost undistinguishably pressed into 

 (pr.ph). 



In the center of the section will be a small speck, 

 around which the microscopic primary xylem groups 

 (pr.x) are arranged ; but these, again, are merged be- 

 tween the relatively huge masses of secondary xylem 

 which makes up by far the major part of the whole 

 (se.x). The thin cambium ring can be distinguished 

 running between the xylem and phloem as a fine line. 

 Certain concentric annular lines may be seen on the 

 section, and each of these marks the position in which 

 the cambium rested during the winter of some previous 

 year. They are the boundaries of concentric zones, 

 termed annual rings, and the thickness of wood which 

 makes up any one annual ring represents the activity of 

 the cambium during that particular year. 



Traversing these annual rings at right angles are fine 

 medullary rays. About five broader ones may be found 

 corresponding to the radii on which the primary xylem 

 groups were formed, but these are not developed by the 

 cambium as the finer ones are. As I shall have to speak 

 of annual rings and secondary medullary rays at greater 

 length when describing the thickening processes in the 

 stem, and as they are formed in the same way in both 

 cases, we may defer their consideration for the present. 



Mention must now be made of a remarkable biologi- 

 cal phenomenon in connection with the roots of the oak. 

 This is the very common occurrence of young rootlets 

 clothed by a fungus mycelium ; the mycelium is found 



