花式図は、図の上側が花序軸側、下側が苞葉側となるように描く (De Crane 2010)。ツユクサ科の花式図は上の左図に示すようになる (De Crane 2010, Evans et al. 2000)。右の写真はムラサキツユクサで花の構造を説明した。
Floral diagrams are drawn such that the upper side of the figure corresponds to the inflorescence axis and the lower side to the bract side (De Craene 2010). The floral diagram of Commelinaceae is as shown in the left panel (De Craene 2010; Evans et al. 2000). The photograph on the right shows Tradescantia ohiensis, in which the floral structure is illustrated.
Cochliostema odoratissimumは、中米南部から南米北部に分布し、タンク型（葉腋に水を溜める）着生植物で、直径5 cmに達するツユクサ科最大の花を付ける（Hardy and Stevenson 2000)。
Cochliostema odoratissimum is distributed from southern Central America to northern South America. It is a tank-forming epiphytic plant (retaining water in the leaf axils) and produces the largest flowers in Commelinaceae, reaching up to 5 cm in diameter (Hardy and Stevenson 2000).
Cochliostema odoratissimum-1: Photo by Kabir Montesinos, https://www.inaturalist.org/photos/596297084, CC BY-NC-ND 4.0
Cochliostema odoratissimum-2: Photo by Diego cabrera, https://www.inaturalist.org/photos/466361117, CC BY-NC
観察には名古屋市東山植物園の栽培株を用いた。観察を許可いただくとともに、音叉を用いた実験にご協力いただいた同植物園に感謝申し上げます。
Cultivated plants at the Higashiyama Botanical Garden, Nagoya, were used for the observations. We thank the Garden for granting permission to conduct the observations and for assistance with the tuning-fork experiments.
Comparison between the flower of Cochliostema odoratissimum and the ancestral flower of Commelinaceae
Cochliostema odoratissimumの花（右上の花式図と下方の写真）は、ツユクサ科の祖先的な花（左上の花式図）と比べて、１）１本の外側雄ずいと２本の内側雄ずい、計３本の雄ずいが癒合して、フード状雄ずい束（staminal cuculli：Hardy and Stevenson 2000）を形成すること、２）外側雄ずいのうち花序軸側の２本が長毛で覆われた外側仮雄ずいとなること、3) 花序軸側の内側雄ずいの原基はできるが成長しないこと、の３点で大きく異なる (Hardy and Stevenson 2000)。また、フード状雄ずい束の外側雄ずいの基部に橙色の長毛が密生する。さらに、花弁の縁に多細胞の数珠状毛が密生する形質は(右下写真）、コクリオステマ亜連 Cochliostematinaeの共有派生形質である (Hardy and Faden 2004)。
The flowers of Cochliostema odoratissimum (floral diagram at upper right and photographs below) differ markedly from the ancestral flowers of Commelinaceae (floral diagram at upper left) in three respects: (1) one outer stamen and two inner stamens (three in total) are fused to form a hood-like staminal fascicle (staminal cuculli; Hardy and Stevenson 2000); (2) of the outer stamens, the two on the inflorescence-axis side become outer staminodes covered with long hairs; and (3) the primordium of the inner stamen on the inflorescence-axis side is formed but does not develop (Hardy and Stevenson 2000). In addition, dense orange long hairs occur at the base of the outer stamens of the hood-like staminal fascicle. Furthermore, the presence of dense multicellular moniliform hairs along the petal margins (lower right photograph) is a synapomorphic character of Cochliostematinae (Hardy and Faden 2004).
Cochliostema odoratissimum の花は、左図に示すように、フード状雄ずい束が雌ずいよりも花序軸の基部側に位置して形成される。この状態で花序が直線的に伸長した場合、フード状雄ずい束は花の下側、雌ずいは上側に配置されることになる。しかし実際には、さそり状花序において花序軸がねじれるため、開花時には中央図および右写真に示すように、フード状雄ずい束が花の上側、雌ずいが下側に位置する。花序が横に伸びだす場合も（右下写真）、花序が上方に伸びだす場合（右上写真）と同じように、雌ずいが下側、フード状雄ずい束が上側に配置するので、花梗に重力感知し方向制御する仕組みがあるのかもしれない。
The flowers of Cochliostema odoratissimum, as shown in the left figure, are formed with the staminal cuculli positioned more toward the basal side of the inflorescence axis than the pistil. If the inflorescence were to elongate linearly in this configuration, the staminal cuculli would be positioned on the lower side of the flower and the pistil on the upper side. In reality, however, the inflorescence axis twists in the scorpioid cyme, so that at anthesis the staminal cuculli are positioned on the upper side of the flower and the pistil on the lower side, as shown in the middle figure and the right photographs. When the inflorescence extends laterally (lower right photograph), as well as when it extends upward (upper right photograph), the pistil is consistently positioned on the lower side and the staminal cuculli on the upper side. This suggests that the pedicel may possess a mechanism for sensing gravity and controlling orientation.
フード状雄ずい束は、花の正面から見て、左側にねじれている（左上写真）。フード状雄ずい束を雌しべ側（左上写真の青色矢印、図B1から図B3）とその反対側（同緑色矢印、図G1から図G4）から観察した。（B1）雌しべ側では、２本の内側雄ずいが密に接しているが、組織としては合着していない。ピンセットを差し込むことで、こじ開けることができる。ただし、長毛が密生しており、フード状雄ずい束の内外で空気の移動はおこりうるものの、内側から外側へ花粉が放出されることは困難であるように見える。（G1、G2）雌しべから遠い側では、外側雄ずいの花糸と２本の内側雄ずいの花糸（G2の黒色星印）が癒合しており、G2の橙色点線部分に切れ目を入れなければ雄ずいを分離できない。分離した３本の雄ずいを、雌しべ側（B2)および反対側（G3、G4）から観察すると、フード状雄ずい束の内部は螺旋状の葯と白色の粉状の花粉（B3の黒色矢印）で満たされている。Cochliostemaは「螺旋状の雄ずい」を意味し、発生過程においてで葯の先端側が伸長し、４〜５回螺旋状に巻く（Hardy and Stevenson 2000、S1、S2)。S2の橙色矢印は、フード状雄ずい束内側に伸びだした花糸を示す。葯は全長にわたって縦裂開する。橙色の毛は外側雄ずいの基部から生じる。
The staminal cuculli are twisted to the left when viewed from the front of the flower (upper left photograph). The staminal cuculli were observed from the pistil side (blue arrow in the upper left photograph; Figs. B1–B3) and from the opposite side (green arrow; Figs. G1–G4). (B1) On the pistil side, the two inner stamens are closely appressed but are not fused as tissues. They can be pried apart by inserting forceps. However, although dense long hairs are present and air movement may occur between the inside and outside of the staminal cuculli, it appears difficult for pollen to be released from the inner to the outer side. (G1, G2) On the side distant from the pistil, the filament of the outer stamen is fused with the filaments of the two inner stamens (black asterisk in G2), and the stamens cannot be separated without making an incision along the orange dotted line in G2. When the three separated stamens are observed from the pistil side (B2) and from the opposite side (G3, G4), the interior of the staminal cuculli is filled with spiral anthers and white, powdery pollen (black arrow in B3). Cochliostema means “spiral stamens,” and during development the distal end of the anther elongates and coils spirally four to five times (Hardy and Stevenson 2000; S1, S2). The orange arrow in S2 indicates a filament extending into the interior of the staminal cuculli. The anthers dehisce longitudinally along their entire length. The orange hairs arise from the base of the outer stamens.
Cochliostema odoratissimumの送粉昆虫や送粉様式は報告されていないが（Hardy and Stevenson 2000)、開花するだけでは花粉がフード状雄ずい束の外側に放出されないことから、ハチ類がフード状雄ずい束に振動を与えることで花粉が放出される振動送粉（buzz-pollination）である可能性が指摘されている（Vogel 1978, Endress 1994, p. 155)。振動送粉は、無駄な花粉損失を防ぎ、適切な振動を生じさせることができる送粉者を選択できる利点があり（Mesquita-Neto aet al. 2018)、少なくとも2万種の被子植物で見られる（Vallejo-Marin and Russell 2024, Woodrow et al. 2024)。これらの花は、孔開葯 poricidal antherと呼ばれる、細長く、全体が裂開するのではなく先端の孔から花粉が徐々に放出される葯を持つことがほとんどである（Vallejo-Marin and Russell 2024, Woodrow et al. 2024)。
一方、C. odoratissimumは３本の雄ずいが合成して２枚貝状の構造を形成する点でこれらと異なる。葯は孔開ではなく、全長にわたって縦裂開する。通常の振動送粉花では、黄色い葯自体がハチ類を誘引するが、C. odoratissimumでは葯はフード状雄ずい束の内部にあり外部からは見えない。その代わりに、外側雄ずいの基部に葯に類似した橙色の毛（白黒写真および右上写真の橙色矢印）が存在し、これがハチ類を誘引していると考えられている（Vogel 1978)。種小名が示すように花弁は芳香を発するが、それが誘引に寄与するかどうかは明らかでない。また、蜜は分泌されない。左上の写真は正面から、下の写真は斜めから撮影したものであり、フード状雄ずい束は花の正面から見て左側に曲がっているので、花の右奥に橙色の毛が見える。
振動送粉をするハチ類は、葯に到達すると体をC字状に曲げて葯の周囲に巻き付き、胸部にある間接飛翔筋 indirect flight muscleの動きによって胸部、脚、あるいは頭部に振動を生じさせ、葯から花粉を放出させる（Vallejo-Marin and Russell 2024, Woodrow et al. 2024)。
Pollinating insects and the pollination system of Cochliostema odoratissimum have not been reported (Hardy and Stevenson 2000). However, because pollen is not released to the outside of the staminal cuculli simply upon anthesis, it has been suggested that this species may be buzz-pollinated, with bees inducing pollen release by applying vibrations to the staminal cuculli (Vogel 1978; Endress 1994, p. 155). Buzz pollination has the advantages of preventing wasteful pollen loss and enabling the selection of pollinators capable of generating appropriate vibrations (Mesquita-Neto et al. 2018), and it occurs in at least 20,000 angiosperm species (Vallejo-Marín and Russell 2024; Woodrow et al. 2024). In most such flowers, the anthers are poricidal (poricidal anthers), being elongate and releasing pollen gradually through apical pores rather than by dehiscing along their entire length (Vallejo-Marín and Russell 2024; Woodrow et al. 2024).
In contrast, C. odoratissimum differs in that three stamens are fused to form a bivalve-like structure. The anthers are not poricidal but dehisce longitudinally along their entire length. In typical buzz-pollinated flowers, the yellow anthers themselves attract bees, whereas in C. odoratissimum the anthers are enclosed within the staminal cuculli and are not visible externally. Instead, orange hairs resembling anthers are present at the base of the outer stamens (orange arrows in the black-and-white and upper right photographs), and these are thought to attract bees (Vogel 1978). As indicated by the specific epithet, the petals emit a fragrance, although its role in attraction is unknown. Nectar is not secreted. The upper left photograph shows a frontal view and the lower photograph an oblique view; because the staminal cuculli are curved to the left when viewed from the front, the orange hairs are visible at the back right of the flower.
In buzz pollination, bees reaching the anthers curl their bodies into a C-shape around them and, through the action of the indirect flight muscles in the thorax, generate vibrations of the thorax, legs, or head that cause pollen to be released from the anthers (Vallejo-Marín and Russell 2024; Woodrow et al. 2024).
Cochliostema odoratissimumのフード状雄ずい束をピンセットで揺らしても、花粉はほとんど放出されない。
Even when the staminal cuculli of Cochliostema odoratissimum are shaken with forceps, little pollen is released.
ハチ類の振動は100から400 Hz程度であることから（De Luca et al. 2019, De Luca et al. 2012)、256 Hzの音叉を用いてフード状雄ずい束に振動を与えた。
Because bee vibrations are on the order of 100–400 Hz (De Luca et al. 2012, 2019), we applied vibrations to the staminal cuculli using a 256 Hz tuning fork.
Pollen release from the staminal cuculli of Cochliostema odoratissimum induced by a 256 Hz tuning fork (first trial)
Pollen release from the staminal cuculli of Cochliostema odoratissimum induced by a 256 Hz tuning fork (second trial)
フード状雄ずい束の雌しべ側の面に音叉を接触させると、花粉が勢いよく放出された。約10秒後、同じフード状雄ずい束に再び音叉を当てると、１回目よりも少量ではあるが、さらに花粉が放出された。3回目以降は花粉放出は観察されなかった。
When the tuning fork was brought into contact with the pistil-side surface of the staminal cuculli, pollen was released vigorously. Approximately 10 seconds later, when the same staminal cuculli were stimulated again with the tuning fork, additional pollen was released, although in a smaller amount than in the first trial. No pollen release was observed after the third and subsequent stimulations.
別な花でも音叉で振動を与えると花粉放出が観察された。通常の振動送粉では、葯の孔から花粉が徐々に放出されるのに対し、Cochliostema odoratissimumのフード状雄ずい束からは花粉が勢いよく飛び出す。この動力はどこから生じるのだろうか。
Pollen release from another staminal cucullus of Cochliostema odoratissimum induced by a 256 Hz tuning fork
In another flower as well, pollen release was observed when vibrations were applied with a tuning fork. In typical buzz pollination, pollen is released gradually from apical pores of the anthers, whereas in Cochliostema odoratissimum pollen is ejected forcefully from the staminal cuculli. What is the source of this driving force?
Vibration of the spiral anthers of Cochliostema odoratissimum induced by a 256 Hz tuning fork
Vibration of the spiral anthers of Cochliostema odoratissimum induced by a 256 Hz tuning fork (magnified)
フード状雄ずい束を裂開して螺旋状葯を露出させ、256 Hzの音叉で振動を与えた。その結果、葯が小刻みに振動することがわかった。フード状雄ずい束の内部には６個の螺旋状葯が密に配置されており、これらが共振することで、花粉を勢いよく放出する動力を生み出している可能性がある。
The staminal cuculli were opened to expose the spiral anthers, and vibrations were applied using a 256 Hz tuning fork. As a result, the anthers were observed to vibrate finely. Because six spiral anthers are densely packed within the staminal cuculli, their resonance may generate the driving force that ejects pollen forcefully.
引用文献 References
De Crane LPR. 2010. Floral Diagrams. (Cambridge Univ. Press: Cambridge).
De Luca PA, Buchmann S, Galen C, Mason AC, and Vallejo-Marín M. 2019. Does body size predict the buzz-pollination frequencies used by bees? Ecol Evol. 9: 4875–4887. https://doi.org/10.1002/ECE3.5092
De Luca PA, Bussière LF, Souto-Vilaros D, Goulson D, Mason AC, and Vallejo-Marín M. 2012. Variability in bumblebee pollination buzzes affects the quantity of pollen released from flowers. Oecologia 172: 805–816. https://doi.org/10.1007/S00442-012-2535-1
Endress PK. 1994. Diversity and Evolutionary Biology of Tropical Flowers (Cambridge Univ. Press: Cambridge).
Evans TM, Faden RB, Simpson MG, Sytsma KJ. 2000. Phylogenetic relationships in the Commelinaceae: I. A cladistic analysis of morphological data. Syst. Bot. 25: 668–691.
Hardy, C.R. and Faden, R.B. 2004. Plowmanianthus, a new genus of Commelinaceae with five new species from tropical america. Syst Bot. 29: 316–333. https://doi.org/10.1600/036364404774195511
Hardy, C. and Stevenson, D. 2000. Development of the gametophytes, flower, and floral vasculature in Cochlioptema odoratissimum (Commelinaceae). Bot J Linn Soc. 134: 131–157.
Mesquita-Neto JN, Blüthgen N, and Schlindwein C. 2018. Flowers with poricidal anthers and their complex interaction networks—Disentangling legitimate pollinators and illegitimate visitors. Funct Ecol. 32: 2321–2332. https://doi.org/10.1111/1365-2435.13204
Vallejo-Marin M and Russell AL. 2024. Harvesting pollen with vibrations: towards an integrative understanding of the proximate and ultimate reasons for buzz pollination. Ann Bot. 133: 379–398. https://doi.org/10.1093/AOB/MCAD189