The seed is the word of God that is proclaimed or taught.  It is the Scripture with its life giving message.  The seed is always sown on the heart. The heart is the center of your person.  It is the unique combination of what one believes/thinks and what one wants/desires.  One who is hardened ground is one whose heart is not receptive to the seed that is sown and this primarily means that one does not understand the meaning or import of the Word that was spoken or heard.