The first thing that everybody notices about the cactus is that it does not have any leaves but has spines, needle-like shaped features, on its skin. Depending on the cactus type, these spines can be arranged in many ways and have different shapes.
Cacti survive in difficult conditions of heat and dryness. They reduce water loss by having spines instead of leaves, which significantly reduces the surface area, and by having a waxy coating on the body. Moreover, cacti can collect water in many different ways. Like most plants, the cactus has roots, which are spread over a large area and placed close to the soil surface to take advantage of light rains. The water, which is taken in through the roots, is stored in the spongy or hollow stems of the cactus. Their waxy-skinned bodies hold moisture very efficiently as it contains the slow evaporation stomata with microscopic pores on the surface, which can open and close in the right moment in order to prevent air and water escaping from the cactus. The outer skin of the cactus is often ribbed, like an accordion. They expand as they fill up with water and fold together as the water in the stem is used up. Additionally, cacti have the ability to absorb water through their outer skin, allowing them to capture moisture in the air. The surface covered by spines helps to pick up water as it works as drip-tip from fog, dew or light rain. The dense needle-like surface can trap fog droplets, and shed them on the ground below, which is absorbed then by roots and later transported into their spongy interior tissue. Now one can see that the function of spines is not only protection against animals and sunlight (giving shadow). If you check the cactus structure under a microscope you see that spines are not smooth, they have some roughness or channels to aid in trapping water which can be slowly absorbed by the cactus skin, specifically through the epidermis (upper part of a skin).
The cactus in my office - Rebuta Muscula, grey images represent the surface of cacti’ spikes.
The collection of water by cacti has been an inspiration for people. Water from fog can be collected via mesh of wires/fibres or fabric, which is called a fog fence or fog collector. The fog collector should face the prevailing winds so the droplet can settle on a mesh and wind just flows around it. Droplets slowly coalesce and run to the bottom of the mesh, where they are collected. A similar thing happens with dew, which is a process known as condensation. In nature, atmospheric water vapour condenses on cold surfaces into droplets of liquid water on plant leaves, blades of grass or spider webs as you can see in the picture below.
Water collected by a spider web in the mountains surrounding Santa Barbara in California, August 2005.
In principle the spider web in nature works like the man-made fog collector here. If you want to see in detail how this brilliant idea works, please see the movie from FogQuest:
