THE TEEE ITS HOOT-SYSTEM 95 



masses being almost undistinguishably pressed into 

 these (pr.pJi). 



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

 around which the microscopic primary xylem groups 

 (pr.x) are arranged, but these, again, are merged between 

 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 considera- 

 tion for the present. 



Mention must now be made of a remarkable biolo- 

 gical phenomenon in connection with the roots of the 

 oak. This is the very common occurrence of young root- 



