Question

A cross-sectional view of a log shows a concentric pattern. Which part of the stem gives rise to this pattern?

Answer 1

the concentric rings on the tree log is due to secondary growth of the tree.
CAMBIUM RING is responsible for formation of this ring! outer side of the ring produces phloem and the inner side produces xylem during the secondary growth!

the distinct rings r formed actually because the secondary growth takes place in two phases every year_--
(i) in summer the layer formed id darker
(ii) in winter the layer is lighter
due to tis alteration of dark and light part we can easily count the rings distingtively!!

Answer 2

Right - to add on to the above, the vascular cambium is the cell layer that produces these rings. In Spring, early wood is formed, which is less dense with wider cells. In Summer and Autumn, late wood with narrower cells is formed (this section appears darker). The resumption of early wood growth the following year produces a contrast between this year's light, early wood and the previous year's denser + darker late wood.