Department of Ecology and Evolutionary Biology, University of Connecticut, Storrs, CT, USA.
Ann Bot. 2018 Mar 5;121(3):549-560. doi: 10.1093/aob/mcx171.
Much of morphological evolution in flowers has arisen from pollinator-mediated selection, often manifest as a match between the length of the pollinator's proboscis and the depth of tubular corollas or spurs. We investigate development, growth and homology of the unique nectar tube of Pelargonium, frequently described as 'a spur adnate to the pedicel'.
We focused on two species. The nectar tube of P. ionidiflorum is three times longer than that of P. odoratissimum. Light and scanning electron microscopy were carried out, and daily growth measurements were used to compare nectar tube development and vascular patterns.
Nectar tubes in both species are initiated centripetally to the dorsal sepal in a space created by lateral displacement of two antepetalous stamens. The cavity deepens through subsequent intercalary growth of the receptacle that proceeds at the same rate in both species until tubes reach approx. 10 mm in length. Differences in final nectar tube lengths arise via an increase in the rate and duration of growth of the receptacle that begins just before anthesis (floral opening) and continues for several days past anthesis in P. ionidiflorum but does not occur in P. odoratissimum. Epidermal cells of the dorsal surface of the nectar tube in P. ionidiflorum are approx. 1.6 times longer than those in P. odoratissimum. Histological sections show no evidence that the nectar tube is a spur that became evolutionarily fused to the pedicel.
Nectar tubes in Pelargonium are localized cavities that form in the receptacle via intercalary growth. Differences in the rate and duration of growth just prior to and following anthesis underlie differences in final tube lengths. Because differences in cell lengths do not fully account for differences in nectar tube lengths, evolutionary diversification must involve changes in both cell cycle and cell expansion.
花部形态的进化在很大程度上是由传粉者介导的选择引起的，通常表现为传粉者的喙长与管状花冠或距的深度之间的匹配。我们研究了天竺葵属特有的蜜腺管的发育、生长和同源性，该蜜腺管常被描述为“附生于花梗的距”。
我们专注于两个物种。香叶天竺葵的蜜腺管比香叶天竺葵长三倍。进行了光镜和扫描电子显微镜观察，并进行了每日生长测量，以比较蜜腺管的发育和脉管模式。
在两个物种中，蜜腺管都是从背萼的向心方向开始发育的，在两个侧生雄蕊侧向位移形成的空间中。随着受体的居间生长，腔加深，两个物种的生长速度相同，直到管长达到约 10 毫米。最终蜜腺管长度的差异是通过增加受体的生长速度和持续时间产生的，这种生长在开花（花开放）前开始，并在香叶天竺葵中持续数天，但在香叶天竺葵中不会发生。香叶天竺葵蜜腺管背表面的表皮细胞比香叶天竺葵长约 1.6 倍。组织切片显示没有证据表明蜜腺管是一个距，它在进化上与花梗融合在一起。
天竺葵的蜜腺管是通过居间生长在受体中形成的局部腔。在开花前后的生长速度和持续时间的差异是最终管长差异的基础。由于细胞长度的差异不能完全解释蜜腺管长度的差异，进化多样化必须涉及细胞周期和细胞扩张的变化。
