Re: the dark and light lines of the tree trunk are secondary xylem.do all for

Rest of Question:

them still functionable ? and where are the secondary phloem ?


Reply:

The secondary xylem in the center often stops functioning as the tree ages. 
Some trunks have a heartwood, often darker in color, of nonfunctional xylem. 
The outer functional xylem is called sapwood, which is often lighter in color.

Secondary phloem is a thin layer under the bark. It doesn't get very thick 
because it is lost as the trunk increases in diameter. Between the secondary 
xylem and secondary phloem is the vascular cambium, where cells divide to form 
both secondary xylem and secondary phloem.

Reference


Secondary Xylem/Phloem


Cross section of a tree trunk