$(a)$ State the constituents of phloem.
$(b)$ How does cork act as a protective tissue?

(N/A) The constituents of phloem are:
$(i)$ Sieve tubes
$(ii)$ Companion cells
$(iii)$ Phloem fibres
$(iv)$ Phloem parenchyma
$(b)$ Cork acts as a protective tissue because its cells are dead and compactly arranged without intercellular spaces,which prevents the entry of pathogens. Furthermore,the cell walls of cork contain a chemical substance called suberin,which makes them impervious to gases and water,thereby protecting the plant from mechanical injury and desiccation.