Matthew 13:8 But other fell into good ground, and brought forth fruit, some an hundredfold, some sixtyfold, some thirtyfold.
Matthew 13:23 But he that received seed into the good ground is he that heareth the word, and understandeth it; which also beareth fruit, and bringeth forth, some an hundredfold, some sixty, some thirty.
Basically its he that heareth the word  and understandeth it, which also beareth fruit and bringeth forth some hundredfold, some sixty and some thirty.
How do they bear fruit? By that the word, that is the gospel remains in them. And so they bear fruit.