Floral anatomy and vegetative development in Ceratophyllum demersum: A morphological picture of an “unsolved” plant

Abstract

The initiation pattern of organs in the outer whorls of C. demersum flowers is distorted by mechanical pressure, resulting in the phyllotactic variation of staminate flowers. Vegetative buds are the main axillary buds with floral buds as accessory buds, which suggests that the shoot of C. demersum has been modified from a decussate phyllotaxis.

Mechanical Forces in Floral Development

“…Scale bar in ( A ) = 5 mm, in ( B ) = 500 µm, in ( C – F ) = 50 µm. (( A , B , E , F ) reproduced from Iwamoto et al, 2015; ( C , D ) from Iwamoto et al, 2003) [ 31 , 32 ].…”

“…The effect of pressure of the bract can be expressed on either side of the floral primordium along a median gradient, favoring a polarity in flower sequence initiation and development, and eventually also organ loss. Ceratophyllum presents a good example where the effect of the pressure is more pronounced on the adaxial side of the flower primordium ( Figure 7 ) [ 31 , 32 ]. Here, the bract subtending a staminate flower overtopping the floral bud presses against the adaxial side and stamens may fail to develop as organs can only initiate unidirectionally from the abaxial to the adaxial side ( Figure 7 D–F).…”

Refined Interpretation of the Pistillate Flower in Ceratophyllum Sheds Fresh Light on Gynoecium Evolution in Angiosperms

“…The main problem is a long filiform distal extension to the gynoecium ( Figure 14B ), sometimes called the stigma, which is normally attached on the same side as the ovule ( Troll, 1933 ; Shamrov, 1983 ; Endress, 1994 ; Igersheim and Endress, 1998 ; Endress, 2001 ; Iwamoto et al, 2003 ; Endress and Doyle, 2015 ). This extension can be interpreted as median-adaxial in its attachment relative to the main axis on which the flowers develop (but note that flower-subtending bracts are lacking in Ceratophyllum unless all leafy segments in each node are interpreted as parts of the same dissected leaf with a group of collateral axillary buds: Raynal-Roques, 1981 ; Rutishauser and Sattler, 1987 ; Rutishauser, 1999 ; Iwamoto et al, 2015 ). Most commonly, distal carpel extensions are dorsal in basal angiosperms (i.e., angiosperms other than monocots and eudicots).…”

“…A common feature of Pseudoasterophyllites and Montsechia is the absence of leaf stipules ( Kvaček et al, 2016 ; Gomez et al, 2020 ), in contrast with extant Chloranthaceae, indicating strong variation in this diverse lineage. The pseudo-verticillate phyllotaxis of Ceratophyllum is plausibly interpreted as derived from decussate phyllotaxis with interpetiolar stipules, the stipules having been evolutionarily transformed into leaf-like organs ( Iwamoto et al, 2015 ). Among angiosperms other than eudicots and Ceratophyllaceae, interpetiolar stipules are known only in Chloranthaceae.…”

“…Consistent with the aquatic habit, the vegetative body of Ceratophyllum lacks stomata or roots at any stage of development; it also lacks xylem conducting elements. The characteristic “leaves” are dichotomously divided 1–4 times with linear denticulate segments; they appear whorled (verticillate), but could plausibly be interpreted as decussate, with several leaf-like organs at each node ( Schaeppi, 1935 ; Iwamoto et al, 2015 ). The fruits are one-seeded, often with characteristic spine-like appendages.…”