THE TREE—ITS ROOT-SYSTEM 95 
masses being almost undistinguishably pressed into 
these (pr.ph). 
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.z). The 
thin cambium ring can be distinguished running between 
the xylem and phloém 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- 
