Q.In which season the vessels of Angiosperms are larger in size, why?

Angiosperm vessels are widest during spring, when active growth and abundant soil moisture favour the vascular cambium producing large-diameter, thin-walled vessels for efficient water transport; as the growing season progresses toward autumn, vessels become progressively narrower.

In temperate and seasonal climates, the vascular cambium of a dicot stem does not produce uniform secondary xylem throughout the year — its activity fluctuates with the seasons, and this is what produces the visible growth rings (annual rings) in wood.

• In spring, when temperatures rise and water is plentiful, the cambium is highly active and divides rapidly. The xylem it produces at this time — called spring wood or early wood — consists of vessels and tracheids with large diameters and relatively thin walls, since the plant's priority is fast, high-volume water conduction to support the new season's growth (leafing out, flowering). This wood is lighter in colour and less dense. …

Spring wood (early wood) vessels are wide and thin-walled because the vascular cambium is highly active and water is abundant in spring, favouring rapid conduction; autumn wood (late wood) vessels formed later in the season are narrow and thick-walled because growth and water availability slow down.

In temperate woody dicots, secondary xylem is not produced uniformly through the year — its character changes with the seasons, giving rise to visible annual growth rings:

• Spring wood (early wood): produced at the start of the growing season, when soil water is plentiful (e.g. from melting snow/spring rain) and the cambium is very active. To meet the plant's high water demand as new leaves and shoots resume growth, the cambium produces xylem vessel elements with large diameters (wide lumens) and relatively thin walls, allowing rapid, high-volume water conduction. Spring wood is lighter in colour and lower in density. …