MORPHOLOGICAL STUDIES OF THE NYMPHAEACEAE (SENSU LATO). XIII. CONTRIBUTIONS TO THE VEGETATIVE AND FLORAL STRUCTURE OF CABOMBA

Six species of Cabomba have been examined although the anatomy of the vegetative axes is based on the study of only C. caroliniana and C. palaeformis. A plant consists of an erect short shoot with decussate leaves which bears axillary flowering shoots and rhizomes. A rhizome bears decussate leaves and may also form axillary flowering shoots or turn upward and become a new short shoot. The phyllotaxies of the flowering shoots are proximately decussate or ternate (C. piauhyensis). The flowering shoots with decussate phyllotaxy change to 1/3 phyllotaxy distally; they bear axillary flowers proximally, and extra-axillary flowers distally. Flowering shoots with ternate phyllotaxy do not change distally but each produces first axillary and then extra-axillary flowers. Decussate vegetative axes and flowering shoots have four vascular bundles; ternate vegetative axes and flowering shoots have six vascular bundles, distantly paired into two or three vascular bundle-pairs, respectively. An elliptical vascular plexus occurs at each node. Each leaf receives one bundle-pair from one trace and each flower three bundle-pairs. A two-level receptacular vascular plexus occurs in flowers; the proximal, larger portion provides traces to perianth and stamens and the distal, smaller portion becomes carpellary traces. Each of the three sepals typically receives five branch traces from a basal principal trace, and each of the three petals receives, typically, three branch traces from a basal principal trace. Sepals and petals generally occur in a single, basally connate whorl. Each stamen receives one trace. Each stamen of three-stamen flowers is opposite a petal; each stamen of six-stamen flowers is aligned with an interval between a petal and adjacent sepal. Each staminal trace, which is just above the principal petal trace, in a three-petal flower, is frequently adnate to the latter trace. Each carpel receives one principal trace from the distal, small extension of the receptacular plexus, and each principal trace becomes three conventional veins of a carpel. Ovules may be borne directly over one of the veins or in any position between veins and are supplied by branches of the nearest vein or nearest two veins. All traces, ovular supply veins and the proximal portions of all veins are amphicribral. The several anatomical and morphological differences in vegetative axes and flowers between Cabomba and Brasenia suggest a greater taxonomic distance between the two genera than commonly supposed. It is suggested that extra-axillary flowers in 1/3 helical and ternate flowering shoots of Cabomba might be advantageous in preventing anthesis of flowers beneath peltate leaves. The aberrant position might be the initial evolutionary step toward what, in other nymphaeaceous genera, has shifted each flower to an adjacent helix. It is proposed that the zigzag stem accompanying the trigonal and sympodial flowering shoots may offer greater stability and floatability in water than the monopodial form. Several suggestions are offered for the variability of ovular positions: 1) the variability is a vestige of former laminar placentation in conduplicate carpels; 2) it is a vestige of a primitive condition antedating the current close association of ovules with ventral carpellary veins; 3) it is an early stage of evolution which might have terminated in laminar placentation and cantharophily, but which was replaced by a trend toward myophily.