Now, we will compare the word unto a seed. Now, if ye give place, that a seed may be planted in your heart, behold, if it be a true seed, or a good seed, if ye do not cast it out by your unbelief, that ye will resist the Spirit of the Lord, behold, it will begin to swell within your breasts; and when you feel these swelling motions, ye will begin to say within yourselves—It must needs be that this is a good seed, or that the word is good, for it beginneth to enlarge my soul; yea, it beginneth to enlighten my understanding, yea, it beginneth to be delicious to me.
And now, my brethren, I desire that ye shall plant this word in your hearts, and as it beginneth to swell even so nourish it by your faith.
What is the word?
Cast about your eyes and begin to believe in the Son of God, that he will come to redeem his people, and that he shall suffer and die to atone for their sins; and that he shall rise again from the dead, which shall bring to pass the resurrection, that all men shall stand before him, to be judged at the last and judgment day, according to their works.
When I've studied the scriptures, I've always related the seed to faith. As your faith is nourished it will grow from a seed to a big tree, is how I understood it.
Truth is, faith is just a part. The actual seed is compared to the word. The word of the gospel is Christ. Christ is the central piece. That He created worlds. That He descended to earth to a mortal mother. That He taught us by example. That He suffered for me and you (and I believe He still feels that pain and anguish because of me and you). That He died for me and you. And that He was resurrected and lives today.
That is the word. Christ is the word.
Christ is and always will be the central piece to God's plan. When we nourish this seed of believing in Christ and His mission, that testimony, including faith in Christ, will grow into a huge tree.