The story of 甲

armor, nail, one

甲 depicts a tortoiseshell, it is as hard as armor. In the past, 甲 also meant detention, since the tortoise's body was 'detained' inside a shell, but such meaning had been replaced by 押. On the other hand, seed is covered by thick shell, and seed is the first stage of a plant, so 甲 is used as the first of TianGan (Heavenly Stems).