The term 'seed casing' refers to the protective outer layer of a seed. The two crossword solutions for this term are ARIL (4 letters) and TESTA (5 letters). An ARIL is a fleshy or waxy coating that surrounds some seeds, such as those of the yew tree. A testa is the hard outer layer of a seed, which is composed of several layers of cells. Both the ARIL and the testa protect the seed from damage and help it to survive until it can germinate.