The author takes two extrapolation methods for the numerical solution of partial differential equations and shows that they can be expressed as semi-implicit Runge-Kutta methods. He points out that they are of lower order for nonlinear equations. He finds other semi-implicit methods including a second order three stage diagonally-implicit Runge-Kutta (DIRK) formula given also by R. Alexander [ibid. 14, 1006-1021 (1977; Zbl 0374.65038)] which is not among his references. (This paper does not distinguish between the terms semi-implicit and diagonally implicit.) His numerical results compare various methods for parabolic problems which are stiff or have rapid decay.