THE CACTUS GYNOECIUM: A NEW INTERPRETATION

Although taxonomists interpret the ovaries of most cacti as inferior,3 the inferior position has been derived in a very different manner from that in most flowering plants. In the classical situation, the dorsal sides of connate carpels are invested by a floral tube (hypanthium); in Cactaceae, on the other hand, the dorsal sides of the carpels are mostly free-not invested by a floral tube. The ventral sutures are pulled downward during formation of the hollow receptacle so that they come to lie on the sides of a locule between flanges of tissue which represent the reduced remains of fused margins and lateral walls of adjacent carpels. The morphology of an individual carpel is like that of a pea pod which has been split midway between dorsal and ventral bundles, then opened up so that the dorsal segment projects upward, the ventral segment containing the ovules, downward. In most cacti, the ventral segments are adnate to the walls of a hollow floral receptacle. In all cases observed so far, the floor of the locule appears not to be invested by carpellary tissueit is interpreted here as representing free receptacular tissue. In other words, the morphological bases of cactus carpels are not at the bottom of the locule, but above it, where the dorsal and ventral bundle systems diverge from the recurrent receptacular bundles. This interpretation, like the more complex one offered by Buxbaum, explains the commissural stigmas reported by Tiagi for Opuntia and Mammillaria. IN TWO EXTENSIVE reviews, Douglas (1944, 1957) has summarized the literature on the inferior ovary and the varying opinions that have been held concerning its nature. Both she and, more recently, Eames (1961, p. 245) have pointed out that in most cases the gynoecium is enclosed by the bases of connate outer floral parts-the outer tissues of the ovary are appendicular. Cactaceae, on the other hand, provide one of the few exceptions to this general rule, for there seems to be universal agreement that in this family the gynoecium is sunken in the end of a modified branch-the outer tissues of the ovary are receptacular. In the interpretation of other structural features of the cactus gynoecium, however, there is no general agreement. Is placentation of the conventional parietal type as is generally implied in taxonomic treatments, or is it a highly derived and modified type, which gives a false impression of simplicity? An answer to this question is imperative before the morphology of the cactus carpel and its parts can be understood and compared with carpels of other families. Such an understanding might prove to be of considerable assistance in determining the phylogenetic position of the family, concerning which there is likewise no agreement. The purpose of this article is to present briefly current theories concerning the morphology of the cactus gynoecium and, primarily, to introduce 1 Received for publication November 10, 1963. 2 This investigation was supported by a grant from the National Science Foundation. 3 The term "inferior" is preferable to "epigynous" because it has less definite morphological implications. It is doubtful that cactus flowers are truly epigynous. a new one, which I think has the virtue of simplicity. Furthermore, the new interpretation is supported by 3 somewhat different, yet complementary, approaches to the investigation of structure: developmental anatomy, vascular anatomy, and comparative morphology. Because the article is largely theoretical, there will be no meticulous attention to anatomical detail. This is not a study of the flower of any one species, genus, or other taxon. It is the presentation of a general picture, which I hope will serve as a foundation for more detailed studies in the future. MATERIALS AND METHODS-The materials used in this study have come to a large extent from a cactus collection which I have been accumulating for a decade. The original specimens were collected in the field, purchased from commercial collectors, or sent to me by friends who are engaged either in cactus culture or research. Ladislaus Cutak and Harry Johnson have provided live cuttings of various pereskias; Dr. I. W. Bailey has supplied preserved materials. Each of these persons has obtained his specimens from sources unknown to me. The specific identifications are, to the best of my knowledge, correct. However, misidentifications at the specific level would not alter the conclusions presented in this paper. For the higher cacti (the non-leafy cacti, which belong to the tribes Opuntieae and Cereeae), it has been convenient to use microslides and embedded tissues employed in previous investigations dating back to 1950. All specimens were prepared for histological study by the usual paraffin method. Clearing of floral buds and flowers was done with lactic acid by modification of a technique deseribed bv Bebhnham (1939).