Carpels in Brasenia (Cabombaceae) are completely ascidiate despite a long stigmatic crest

Abstract

Background and aims: The morphological structure of anthetic carpels of Brasenia (Cabombaceae), a member of the phylogenetically basal ANITA grade, has not been studied before. The carpel has a long stigmatic crest on the ventral side and could give the impression of a conduplicate structure. This is in contrast to the carpel structure in other genera of the ANITA grade. Therefore, a study of carpel development and carpel structure at anthesis was carried out.

Methods: Carpels of Brasenia schreberi were studied at different developmental stages up to anthesis by means of microtome section series and SEM to analyse and reconstruct the outer and inner carpel morphology.

Key results: Carpels of Brasenia are extremely ascidiate up to anthesis. The elongate stigma originates around the mouth of the young carpel, which is slightly curved toward the centre of the flower. Subsequently, the stigmatic zone below the mouth expands by massive intercalary elongation.

Conclusions: In their ascidiate shape, carpels of Brasenia are similar to carpels of Cabomba, the other genus of Cabombaceae, which, in contrast, has a short stigma restricted to the tip of the carpel. Thus, the morphological structure is independent of the extent (and one-sidedness) of the stigma. The outer shape of carpels at anthesis does not allow the inference of the inner morphological surface. If an angiosperm carpel has a one-sided stigma it can be extremely conduplicate or extremely ascidiate. Therefore, caution has to be used in the interpretation of the structure of fossil carpels.

Fig. 1.

Brasenia schreberi. (A–C) Carpel from young floral bud. Stage before beginning of stigma differentiation. (A) From the side, ventral on the right. (B) From ventral, two hairs marked with arrows; only large apical cell of each hair visible, the two basal cells are much smaller and hidden. (C) Close-up of tip of B, extent of mouth of carpel indicated by arrows. (D and E) Carpels from slightly older bud, stigma differentiation beginning. (D) Group of carpels of a bud, dorsal parts covered by secretory hairs and secretion (arrow points to secretion). (E) Close-up of tip of one of the carpels of (D), incipient stigmatic papillae visible (arrowheads), extent of mouth of carpel indicated by arrows. Scale bars: A and B = 200 μm; C and E = 100 μm; D = 1 m.

Fig. 2.

Brasenia schreberi. (A–C) Carpel from advanced floral bud. (A) From the side, ventral on the right. (B) From ventral. (C) Close-up of tip of (B), extent of mouth of carpel indicated by arrows. (D and E) Carpel in early female phase of anthesis. (D) From the side, ventral on the right. (E) From dorsal. Scale bars: A, B, D and E = 1 mm; C = 100 μm.

Fig. 3.

Brasenia schreberi. (A) Schematic median longitudinal section of carpel in early female phase of anthesis, ventral on the left side. Primary morphological surface drawn with line, receptive surface indicated by dots. (B–V) Transverse section series of carpel in early female phase of anthesis. (B–K) Stigmatic zone. (B) Tip above mouth. (C–E) Mouth. (F–K) Stigmatic zone below mouth. (L and M) Transition from stigmatic zone to ovary. (N–Q) Zone of upper ovule. (R–T) Zone of lower ovule. (U) Ovary base. (V) Stalk. (W) Schematic transverse section of anthetic carpel in stigmatic region. Arrows indicate the direction of pollen tube growth inward and downward, receptive surface indicated by dots. d, Dorsal vascular bundle; l, lateral bundle; l_l, lower lateral bundle; o, ovule bundle. Scale bars: A = 1 mm, B–V = 500 μm.

Fig. 4.

Brasenia schreberi. Transverse sections of a carpel at anthesis. (A) Upper stigmatic region. (B) Base of the stigmatic region. Red arrows in A and B indicate the border of the receptive surface. (C) More or less median longitudinal section of a carpel at anthesis in stigmatic zone (vascular bundle not in the section). The oblique cell files that are directed inward and downward and determine the direction of pollen tube growth are indicated with red arrows. The inner space of the carpel, delimited by the inner part of the primary morphological surface, is filled with mucilaginous secretion (dark). The mucilage is secreted by the inner epidermis, which consists of cytoplasm-rich cells. The cell layers between the oblique cell files and the inner epidermis consist of much elongated cells. d, Dorsal vascular bundle. Scale bars = 200 μ m.