What is the chemical substance that makes the cells of the sclerenchyma hard? Mention where this tissue is likely to be present.

(N/A) The chemical substance that makes the cell walls of sclerenchyma hard is $Lignin$. This substance acts as a cement and hardens the cell wall.
Sclerenchyma tissue is typically found in the following locations:
$1$. Around vascular bundles.
$2$. In the veins of leaves.
$3$. In the hard covering of seeds and nuts (e.g.,the shell of a coconut).