Studies in the Cactaceae.

Abstract

Five taxa representing the three tribes of the Cactaceae have similar patterns of stamen and carpel initiation but display differences in early receptacle development. The first ring of stamens and the carpels arise simultaneously from subsurface layers. The bases of carpels are congenitally connate. Additional stamens are initiated centrifugally. The shape of the floral meristem within the ring created by the first stamens varies. In Pereskia corrugata it remains broadly convex; in Opuntia engelmannii it forms a depression with a small convex central region; in Epiphyllum strictum it forms a broad shallow depression; in Echinocereus reichenbachii var. albispinus it develops a deep depression; and in Mammillaria compressa it develops a depression prior to stamen and carpel initiation. Changes in receptacle shape result from cessation of apical growth and activation of an intercalary ring meristem. These two processes occur earlier in ontogeny in the more advanced of these five taxa.

Chromosome counts and observations of reproduction for 55 taxa of Cactaceae indicate that polyploidy is correlated with self-fertility, adventive embryony, profuse branching, and vegetative reproduction. Six genera (Blossfeldia, Cleistocactus, Frailea, Pelecyphora, Rebutia, and Strombocactus) and 35 species or varieties are reported here for the first time. Preliminary observations of pachytene and diplotene indicate that these stages may be more useful in chromosome recognition than mitotic stages. Secondary association at metaphase I and II is interpreted as a retention of homologue association at interphase I and II (interkinesis). During meiosis of certain species, Feulgen negative bodies are present. The production of an abnormal premeiotic division is suggested as a mechanism for polyploid origin.