What is a characteristic feature of sieve tubes?

Lignin is the main constituent of

Vessels are not present in the xylem of

$A$: Cells of sclerenchyma have thickened secondary walls.
$R$: Cells have deposition of lignin.

Which tissue is associated with the translocation of food components?

Deciduous plants shed their leaves during hot summer or in autumn. This process of shedding of leaves is called abscission. Apart from physiological changes,what anatomical mechanism is involved in the abscission of leaves?