Luke indicated that Jesus explained the parable (ἔστιν δὲ αὕτη ἡ παραβολή) about the seed. Quite simply, the seed is the word of God (ὁ σπόρος ἐστὶν ὁ λόγος τοῦ Θεοῦ). This explanation about the seed in the sower parable, can be found in all 3 synoptic gospels, Mark, chapter 4:14, Matthew, chapter 13:18, and here. Matthew explained that Jesus had asked them to hear this parable about the sower. He said that this was all about hearing the word of the kingdom, since the seeds were the word of the kingdom. Mark said that the seeds were the word, without any further clarification. Luke and the other synoptics had no explanation about the sower himself, just the seed as the word of God or his kingdom. How have you planted the seeds of the word of God?
Mark said the seeds were the word (ὁ σπείρων τὸν λόγον), without any further clarification. Matthew, chapter 13:19 was more explicit with the seeds as the word of the kingdom (τὸν λόγον τῆς βασιλείας). Luke, chapter 8:11, was ever more specific when he explicitly said that seed was the word of God (τὸν λόγον τοῦ Θεοῦ). Mark said that the sower (ὁ σπείρων), either Jesus or his apostles, was sowing the word (τὸν λόγον σπείρει), the word of the kingdom, or the word of God into good ground, so that it might bear fruit.