Ontogeny of interxylary phloem and stem anatomy of the Cuban endemic Henleophytum (Malpighiaceae)

Lianas and vascular variants have long been correlated since it is in lianas where they are most commonly found. Structurally, these vascular variants mix soft and stiff tissues allowing lianas to twist without breaking when climbing up to the forest canopies. The study of vascular variants in Malpighiaceae lianas has a long history focusing on species-diverse taxa; however, interest in understanding them in understudied, smaller taxa has unraveled vascular variants previously unknown to the family, such as the interxylary phloem of Henleophytum, a Cuban endemic, monospecific genus. Interxylary phloem in Malpighiaceae had been previously recorded in the distantly related genera Dicella, Niedenzuella, and Tristellateia. Interxylary phloem in Henleophytum derives from a cambium that in certain periods and sites produces phloem both inward and outward, similar to the pattern described for Dicella. The phloem produced outward is not equal to that produced inward. Indeed, sieve tube elements of the inner phloem are always much wider and more abundant, something likely indicative of a division of labor between both phloem types, one specializing in conduction and the other in carbohydrate storage. This constitutes a new report of a vascular variant outside the large Tetrapteroid clade, where almost all the vascular variants have been reported for the family so far, and supports the idea of the independent, multiple origins of vascular variants in Malpighiaceae.