| Rattans have the longest aerial stems of any plant, yet lack secondary growth. As vines, they must convey water to a large area of leaves through a narrow stem. The unusual vascular structure of rattans relative to other palms suggests adaptation to these hydraulic demands. But unlike most other vines, which are optimized for transport, rattans have a modified vascular model that would seem to increase their hydraulic resistance. Their large axial vessels are interconnected only by small transverse vessels. In addition, water is forced to flow through a layer of living parenchyma to reach the leaves. The work presented here examined the consequences of rattans' vascular model for the efficiency and safety of water transport in their stems, based on experiments in Malaysia and Australia on Calamus caesius, Daemonorops sparsiflora, and Calamus caryotoides.;Single-vessel and whole stem measurements in the rattan Daemonorops sparsiflora showed that the small transverse vessels separating axial vessels increase the contribution of resistance between vessels in the rattan stem compared to this resistance in most other plants. However, all three study species had sufficiently wide vessels that their stems' area-normalized conductances were as high as those of other tropical vines. The parenchyma layer separating leaf and stem xylem contributed little to the resistance between stem and leaves. Water potential measurements showed that a smaller proportion of the hydraulic resistance between soil and atmosphere occurred in the crown of the rattan Calamus caryotoides than previously measured in the palm Rhapis excelsa. The large area of overlap between metaxylem and protoxylem in the leaf trace may allow a low resistance in spite of the high resistivity of the parenchyma barrier.;Because rattans have no vascular cambium, their vascular system must be robust against cavitation, spontaneous vaporization of water in the xylem. I explored whether the rattan vascular system remains functional by successfully avoiding cavitation or through some mechanism removing gas bubbles, or emboli, caused by cavitation. Patterns in stem embolism in Calamus caryotoides in Queensland suggested that rattans may be unable to reverse embolism in their stems. |
