Vincent's NT Word Studies
8. That sprang up and increased (anabainonta kai auxanomenon). The Rev. literally renders the participles, growing up and increasing, thus describing the process more vividly. These two participles, moreover, explain the use of the imperfect tense ejdidou (yielded), denoting continuance. It began to yield and kept yielding as it increased.Thirty (eiv triakonta). Lit., up to thirty.
Robertson's NT Word Studies
4:8 {Growing up and increasing} (anabainonta kai auxanomena). In Mark alone. A vivid detail enlarging on the continued growth implied in the imperfect "yielded fruit" (edidou karpon). It kept on yielding as it grew. Fruit is what matters.