Amphicribral (ectophloic, hadrocentric) vascular bundles are bundles having a central xylem region surrounded by phloem. Such vascular bundles were reported earlier in primitive plants like rhizomatous ferns such as Selaginella and Pteris. The logical explanation is that both in Selaginella and Pteris, the plant body grows prostate in moist environment and there is no need for much xylem which is primarily a mechanical as well as a water conducting tissue. Stems mostly rhizomatous in nature and thus food has to be stored in such stems and that may be the reason of more phloem tissue that too outside the xylem. There are no reports of such vascular bundles in Gymnosperms. In Angiosperms amphicribal vascular bundles are located in an aquatic monocot, Hydrilla, a plant which does not need much xylem. . But of late amphicribral vascular bundles are reported in placenta of numerous taxa of angiosperms like Papaveraceae, Leguminosae, Winteraceae, Solanaceae, Gesneriaceae, Buxaceae, Annonaceae, Actinidiaceae, and Magnoliaceae etc and Kiwi fruit (Guo, X. et al.,2013). These taxa cover the whole scope of angiosperms from the basal clade Magnoliids to the terminal eudicot lineage in the phylogenetic tree of APG (2009),( Guo, X. et al, 2017). In dicots such as Begonia, Mesembryanthemum, Rheum, and Rumex, amphicribral bundles occur as medullary bundles, which run through the pith (Mauseth, 1988). The presence of this feature in placenta, which is a reproductive structure, indicates it’s ancestry.
In our recent studies (please see my story on 5thMay, 2022)we have come across a number of plants of Zingiberaceae having amphicribal vascular bundles in rhizomes (unpublished results). Amphicribral bundles are definitely a primitive feature because of their occurrence in pteridophytes and primitive monocots and their presence in placenta of higher plants. The presence of amphicribral vascular bundles in placenta of many plants indicates their utility in conducting large amounts of food to the developing ovules (to seeds) in fruits. As the phloem tissues are placed at the outer parts of the vascular bundles, food from these cells can directly be channelled to the seeds. It should be for the same reason the underground storage organs such as such as rhizomes of Curcuma and Zingiber retained this primitive character. This is also an example of a primitive feature recapitulated in advanced monocots like Zingiberaceae.
References:
Guo, X, Yu. Y, Bai, L and R. Gao (2017) Dianthus chinensis L.: The Structural Difference between Vascular Bundles in the Placenta and Ovary Wall Suggests Their Different Origin Front. Plant Sci., 30 November 2017 | https://doi.org/10.3389/fpls.2017.01986
Guo, X., Xiao, X., Wanh, G and R.Gao (2013) Vascular Anatomy of Kiwi Fruit and its Implications for the origin of carpels, Frontiers in Plant Science, 4:391
Mammen Daniel
