Palo verde is a tree that has green branches. In cacti, leaves have been eliminated and stipules at the base of leaves have become reduced spines that protect stem. As you know, cactus stems are green and handle all of the plant's photosynthesis. For example, orchids are tropical epiphytes, or plants that grow supported on the stems of other plants. Living high in a forest canopy can be dessicating, because these plants dont have a supply of soil water.
Plants increase water use efficiency through partial stomatal closure.
This draws down the CO2 level in the leaf and creates a wider CO2 gradient between the outside and inside of the leaf. This increased gradient leads to greater CO2 flux in. Several conifer species, such as pine trees, sink their stomates deep into the leaf surface. This has that effect of not exposing the stomatal pore directly to drier ambient humidity. The stomate boundary layer thickens considerably.