Surface area in the cactus house

This is the Cactus House on the Threave Estate. Notice the extra fins on the heating pipes. I think that the mode of heating is actually by conduction from the metal to the air molecules. This extra energy makes the molecules move further apart, becoming less dense. The less dense air floats up spreading thermal energy by convection. It helps for the pipes to be given an extra large surface area because the thermal energy leaves through the surface. The more surface there is touching the air, the more air can be heated at one go. The cacti employ a different approach. They would lose water through their leaves by evaporation. So the leaves are rolled up into spines to give them a much smaller surface area. Less water vapour is able to escape so they don't dry up. The leaves can't do photosynthesis because they have such a tiny surface area. Hence the stem has to be full of chlorophyll and green so that it can do the photosynthesis.