andmaybegayer:

There is a reason why desert plants are so short.

In a well-watered environment, the vast majority of the force needed to move water and nutrients around inside their stems comes from transpiration pressure. Transpiration pressure is a pull created inside the fluid system of a plant by water evaporating off the leaves. In high rainfall areas, larger plants can lose upwards of 95% of the water they take in through this method, because they have to: there is simply no other way for the largely immobile creature of a tree to move water around other than using evaporating water to create a vacuum in its leaves.

Desert plants, obviously, cannot afford this. Where most plants have wide open stomata and thin leaves designed to pull water out of the moist soil and hurl it into the sky, desert plants grow thick, waxy cuticles and dense, meaty leaves through which little water escapes. Desert plants snap their scant stomata shut against the sun and only open them at night, when the bare minimum of water will drag the necessary materials for life up into the tips of the stems. They grow an absolute bare minimum of leaves, favouring chunky green stems and diminished thorns. And, of course, when you want to avoid pumping water around, one way to do this is to never get tall in the first place.

(Someone should tell some of the more heavy duty cacti about this technique, although really they seem to get along just fine. They know better than you or I about these matters.)

There are other sources of force for moving water inside a plant. Root pressure, the force created by drawing water in to the roots and letting it all pile up, can support smaller plants and is essential if your plant, say, has very few leaves because they have all been adapted into thorns. Capillary action, the natural tendency of some fluids like water to climb narrow tubes because of surface tension, can be exploited by the narrow tubes of the xylem and phloem systems to shuttle materials around. But you need transpiration pressure to grow tall. Nothing else comes close. The internal pressure in a well-watered plant can exceed one megapascal, the equivalent of 10kg worth of force pressing out on every square centimeter, or 145 lbs per square inch, for you Imperials. If you have to do without this formidable force, you have to make compromises.

(I was at a plant nursery today.)