Late wood, formed in the latter part of the growing season, indeed has fewer xylary elements with narrower vessels. This is because the cambium, the layer of cells in plants that gives rise to secondary xylem (wood), is less active in colder temperatures, which are typically associated with the later parts of the growing season, such as autumn and winter.

So, both Assertion A and Reason R are true. Moreover, Reason R correctly explains why Assertion A is the case: the decreased activity of the cambium in colder temperatures results in the formation of late wood with fewer, narrower vessels.

Therefore, the correct answer is :

Option D : Both A and R are true and R is the correct explanation of A.