That for the faithful and prompt payment of all interest and principal of said bonds as and when the same shall mature according to the tenor thereof, the issue thereof shall constitute an irrevocable pledge by said board of so much of each year’s income from the permanent funds of such state institution, so issuing bonds hereunder, in the hands of the treasurer as shall be needed to provide the “interest and retirement fund” herein mentioned, for the ensuing year, and at all times fully and faithfully to keep the same in not less than the amount necessary to pay the interest and principal maturing as aforesaid; and in addition thereto the issue of said bonds shall constitute an irrevocable pledge by said board of so much of each year’s income from the income and current fund derived from the lease of such of said institution’s lands as remain unsold, as may be necessary to fully protect the “interest and retirement fund” for the ensuing year, and keep the same at all times in proper amount as herein provided.