Stem and Leaf Anatomy of <i>Aragoa</i> (Plantaginaceae): In Search of Lost Rays
2021
Alexei Oskolski | Nathi Vuza | Alexey Shipunov
<i>Aragoa</i> is a shrubby genus endemic to páramo in the northern Andes representing the sister group to <i>Plantago</i> and <i>Limosella</i>. Stem and leaf structure of <i>Aragoa corrugatifolia</i> were studied to clarify the evolutionary pathways and ecological significance of their anatomical traits. <i>Aragoa</i> and <i>Plantago</i> share a non-fascicular primary vascular system, rayless wood and secondary phloem, and anomocytic stomata. <i>Aragoa</i> is distinctive from most Plantaginaceae in the presence of cortical aerenchyma and of helical thickenings in vessels. Its procambium emerges in the primary meristem ring as a continuous cylinder. The view on the ring meristem and procambial strands as developmental stages in the formation of a primary vascular system is not relevant for <i>Aragoa</i>, and probably for other Plantaginaceae. The raylessness is synapomorphic for the crown clade of Plantaginaceae comprising <i>Aragoa</i>, <i>Littorella</i>, <i>Plantago</i>, <i>Veronica</i>, <i>Picrorhiza</i>, <i>Wulfenia</i>, and <i>Veronicastrum</i>. The loss of rays is thought to be predetermined by procambium rather than by the vascular cambium. The extremely narrow vessels with helical thickenings are presumably adaptive to hydric and thermic conditions of páramo. Cortical aerenchyma is thought to be a response to the local hypoxia caused by the water retained by ericoid leaves. Trichomes on juvenile leaves are expected to be the traits of considerable taxonomic importance.
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