Stem anatomy of Urera caracasana and U. eggersii (Urticaceae) in relation to their water storage capacity. Water storage capacity (WSC) of stems is closely linked to their anatomy. During active transpiration, plants lose water quickly from their vessels, depending on the water stored in their living tissues for refilling in short-term periods. Bark and wood anatomy of U. eggersii (a liana) and U. caracasana (a tree) was analyzed, putting special emphasis on the relationship between parenchyma tissue and vessels in both contrasting life-forms. Water content of bark and wood of both species was measured in the rainy and dry season, to determine if the relative contribution to the WSC of the whole stem changed in time. The liana bark stored more water than that of the tree, and axial parenchyma in the tree and radial parenchyma in the liana occupied the greatest proportion of cross-sectional area. The greatest areas of vessel-parenchyma contact were found in the liana. Also, the liana showed the largest relative volume of water retained in the vessels both in the dry and rainy seasons. As a consequence of these anatomical differences, the liana was able to store more water than the tree on a tissue volume basis. These results suggest the hypothesis that by loosing supporting tissues during their evolution, lianas have been able not only to increase vessel dimensions but water storage parenchyma as well.

Key words: parenchyma, Urera caracasana, Urera eggersii, vessels water storage capacity, wood anatomy.