The hard parts of the seeds is primarily due to the presence of:

Explanation

Sclerieds, also known as stone cells, are specialized sclerenchyma cells found in some plant tissues. They are particularly abundant in fruits, giving them a gritty or stony texture. They function in giving hardness to seeds. They are shorter and have more varied shapes than sclerenchyma fibers, and are found in various parts of plants like seed coats, fruit pulp, and nutshells.