Recent data shows that this traditional picture is more complicated. Vessels all start out at more or less the same diameters in the leaves, and then widen gradually down the stem. This widening maintains hydraulic resistance constant as the tree grows and the conductive path lengthens. This means that a small plant in a rainforest will have similar sized vessels as a similar-sized desert plant. Plants in the desert on average are smaller than those in the rainforest. Given the dependence of vessel diameter on plant size, this is why vessels in desert plants are on average narrower than in rainforest plants. What this means is that we need to take plant size into account in understanding how vulnerable a given plant is to embolism.
This is what UNAM postdoc Diana Soriano is going to test. Using a low genetic diversity line of Moringa olefiera bred here at the Collection, Diana is planting seeds once a month to generate a population of individuals of different sizes to see how plant size affect vulnerability to embolism. The fast growth of moringa makes the experiment possible. Whereas the growth of most woody plants is measured on a scale of years, moringa trees will grow to over 3 meters tall in a matter of months. Diana has already planted three crops of seeds, and is getting her scientific gear ready to take out to the Collection to start measurements.