Stuppy W., A new combination in Tephrocactus Lem. (Cactaceae), Kew Bulletin, 2001, 56(4):1003-1005, doi: 10.2307/4119314
A comparison of seed structure leading to the transfer of Puna bonnieae to Tephrocactus. The author describes the covering derived from the ovule funiculus and the embryo shape, comparing them with Puna clavarioides, now Maihueniopsis clavarioides.