The income arising from any personal property held in a trust created as part of a stock bonus plan, pension plan, disability or death benefit plan, or profit-sharing plan, or retirement plan, for the exclusive benefit of employees, or self-employed persons, to which contributions are made by an employer, or employees, or both, or by such self-employed persons, for the purpose of distributing in accordance with such plan to such employees or self-employed persons, the earnings or the principal, or both earnings and principal of the trust fund, may be permitted to accumulate until the fund shall be sufficient to accomplish the purposes of such plan.