Which of the following does not lose their nucleus at maturity?

The dead element present in the phloem is

$(a)$ Give reasons for the following:
$(i)$ Multicellular organisms are considered more efficient.
$(ii)$ Sclerenchyma cells have a narrow lumen.
$(iii)$ Epidermis is thicker in desert plants though it is usually single layered.
$(b)$ Name the following:
$(i)$ Tissue that occurs in specific regions of growth.
$(ii)$ Tissue present at the growing tips of stems and roots.
$(iii)$ Tissue at the base of the leaves or internodes on twigs.
$(iv)$ Basic packing tissue in the form of a few layers of cells.

The chemical present in the walls of bark of the tree which makes them impervious to gases and water is

Cork cells are made impervious to water and gases by the presence of

What is the difference between simple permanent tissue and complex permanent tissue?