University of Life Sciences, Akademicka, Lublin, Poland.
Ann Bot. 2012 Sep;110(4):809-20. doi: 10.1093/aob/mcs158. Epub 2012 Jul 17.
A significant number of species assigned to the Neotropical orchid sub-tribe Oncidiinae reward insect pollinators with oil produced in floral glands termed elaiophores. The latter may be glabrous (epithelial elaiophores) or hirsute (trichomal elaiophores). Although the detailed anatomy and ultrastructure of epithelial elaiophores have been studied for a number of genera, such as Oncidium Sw., Gomesa R. Br. and Trichocentrum Poepp. & Endl., hitherto, trichomal elaiophores have been investigated only for a single species of Oncidiinae, Ornithocephalus ciliatus Lindl. Furthermore, this is the only representative of the Ornithocephalus clade to be investigated to date. Here, an examination is made of the elaiophore anatomy and ultrastructure of a further four species currently assigned to this clade (Ornithocephalus gladiatus Hook., Phymatidium falcifolium Lindl., Zygostates grandiflora (Lindl.) Mansf. and Zygostates lunata Lindl.) and the results compared with those obtained for other Oncidiinae.
Elaiophore structure was examined for all species at three stages of flower development: closed bud, first day of anthesis and final stage of anthesis, using light microscopy, fluorescence microscopy, scanning electron microscopy, transmission electron microscopy and histochemistry.
Elaiophores of O. gladiatus occur upon the lateral lobes of the labellum and display characters intermediate between those of typical epithelial and trichomal elaiophores, in that they are largely glabrous, consisting mainly of cuboidal epidermal cells, but bear short, unicellular hairs proximally. By contrast, the elaiophores of all the other species investigated occur on the callus and are of the trichomal type. In P. falcifolium, these unicellular hairs are capitate. In all species, oil secretion commenced at the closed floral bud stage. Ultrastructurally, the mainly trichomal elaiophores of the four representatives of the Ornithocephalus clade closely resembled the epithelial elaiophores of other Oncidiinae, in that their cells displayed an organelle complement typical of lipid-secreting cells. However, in some taxa, a number of noteworthy characters were present. For example, the elaiophore cuticle of O. gladiatus and P. falcifolium was bi-layered, the outer layer being lamellate, the inner reticulate. The cuticle of Z. grandiflora and Z. lunata was also lamellate, but here, a reticulate layer was absent. Accumulation of secreted oil resulted in the localized distension of the cuticle. Cuticular cracks and pores, however, were absent from all species. The walls of the secretory cells of Z. grandiflora were also atypical in that they had short protuberances or ingrowths, and contained cavities which are thought to be involved in the secretory process.
Of the species investigated, most displayed similar anatomical organization, their trichomal elaiophores occurring on the labellar callus. They, thus, differ from many other members of the Oncidiinae, where epithelial elaiophores are found either on the callus, or on the lateral lobes of the labellum. However, ultrastructurally, all elaiophores, whether those of representatives of the Ornithocephalus clade, or those of other oil-secreting Oncidiinae, possessed a similar complement of organelles, regardless of whether the elaiophores were trichomal or epithelial. In view of the latter, and the similar chemical composition of oils derived from all Oncidiinae investigated to date, it is probable that position and type of elaiophore, and possibly the structure of the overlying cuticle, play an important role in pollinator selection in these oil-secreting orchids.
许多被归为 Neotropical 兰花亚科 Oncidiinae 的物种用在称为油腺的花腺中产生的油来奖励传粉昆虫。后者可能是无毛的（上皮油腺）或多毛的（毛状油腺）。尽管已经对许多属的上皮油腺进行了详细的解剖和超微结构研究，例如 Oncidium Sw.、Gomesa R. Br. 和 Trichocentrum Poepp. & Endl.，但迄今为止，只有一个 Oncidiinae 物种的毛状油腺，即 Ornithocephalus ciliatus Lindl. 被研究过。此外，这也是迄今为止唯一被研究过的 Ornithocephalus 分支的代表。在这里，检查了目前归属于该分支的另外四个物种的油腺解剖结构和超微结构（Ornithocephalus gladiatus Hook.、Phymatidium falcifolium Lindl.、Zygostates grandiflora (Lindl.) Mansf. 和 Zygostates lunata Lindl.），并将结果与其他 Oncidiinae 的结果进行了比较。
使用光显微镜、荧光显微镜、扫描电子显微镜、透射电子显微镜和组织化学方法，在花发育的三个阶段（闭合花蕾、第一天开花和开花末期）检查所有物种的油腺结构。
O. gladiatus 的油腺位于唇瓣的侧裂片上，其特征介于典型的上皮油腺和毛状油腺之间，因为它们主要是无毛的，主要由立方表皮细胞组成，但在近端有短的单细胞毛。相比之下，研究的所有其他物种的油腺都位于愈伤组织上，并且是毛状类型的。在 P. falcifolium 中，这些单细胞毛是头状的。在所有物种中，油分泌始于闭合的花蕾阶段。超微结构上，四个代表 Ornithocephalus 分支的主要毛状油腺与其他 Oncidiinae 的上皮油腺非常相似，因为它们的细胞显示出典型的脂质分泌细胞的细胞器组成。然而，在某些分类群中，存在一些值得注意的特征。例如，O. gladiatus 和 P. falcifolium 的油腺表皮具有双层结构，外层为板层状，内层为网状。Z. grandiflora 和 Z. lunata 的表皮也是板层状的，但这里没有网状层。分泌的油的积累导致表皮的局部膨胀。然而，所有物种的表皮都没有裂缝和孔。Z. grandiflora 分泌细胞的壁也不典型，它们有短的突起或内陷，并且含有空腔，据认为这些空腔与分泌过程有关。
在所研究的物种中，大多数显示出相似的解剖组织，它们的毛状油腺位于唇瓣的愈伤组织上。因此，它们与许多其他 Oncidiinae 成员不同，上皮油腺位于愈伤组织上，或位于唇瓣的侧裂片上。然而，超微结构上，所有的油腺，无论是代表 Ornithocephalus 分支的油腺，还是其他分泌油的 Oncidiinae 的油腺，都具有相似的细胞器组成，无论油腺是毛状还是上皮。鉴于后者，以及迄今为止从所有研究过的 Oncidiinae 中提取的油的化学成分相似，油腺的位置和类型，可能还有覆盖的表皮的结构，在这些分泌油的兰花中对传粉者的选择起着重要作用。
