Hydraulic architecture of Monstera acuminata: evolutionary consequences ofthe hemiepiphytic growth form

The hydraulic architecture of the secondary hemiepiphyte Monstera acuminata was examined in native plants from Los Tuxtlas, Veracruz, Mexico, to deter mine how it compared to better-known growth forms such as trees, shrubs, li anas and primary hemiepiphytes. Monstera acuminata starts its life cycle as a prostrate herb. As it ascends a tree or other vertical support, the stem becomes thicker, produces larger leaves, and may die back from the base up wards until only aerial feeding roots serve to connect the stem to the soil . Unlike the pattern of vessel-size distribution along the stems of woody d icotyledons, M. acuminata has its wider vessels at the top of the stem, dec reasing in diameter towards the base. Also peculiar is the fact that Huber values (axis area/distal leaf area) tend to increase exponentially at highe r positions within the plant. Based on the hydraulic conductivity (k(h)) an d leaf-specific conductivity (LSC, K-h/distal leaf area), the base of the s tem potentially acts as a severe hydraulic constriction. This constriction is apparently not limiting, as aerial roots are produced further up the ste m. The plants have remarkably strong root pressures, up to 225 kPa, which m ay contribute to the maintenance of functional vessels by refilling them at night or during periods of very high atmospheric humidity, as in foggy wea ther and rain. In common with dicotyledonous plants, vessel length, vessel diameter, k(h), specific conductivity (k(s), k(h)/axis area) and LSCs were all positively correlated with axis diameter. The features of the hydraulic architecture of M. acuminata may be an evolutionary consequence of an anat omical constraint (lack of vascular cambium and therefore of secondary grow th) and the special requirements of the hemiepiphytic growth form.