Nearly all orchid species are surface growers. Epiphytic forms (the majority) can be found high in the rain forest canopy, their roots stuck to the bare bark of tree branches. A few species grow on rock faces (lithophytes), their roots growing into the cracks in the rocks (similar to the way the epiphytic species grow on bark). And a handful of fairly popular groups grow on the ground as terrestrials. Even though the terrestrials grow on the ground, they do not generally grow "in" the ground like normal plants. Their thick roots run underneath the leaf litter, but do not penetrate the dirt. As with everything in nature, there are exceptions...some orchids grow in watery bogs, and some in forest dirt, but these are the exception, not the rule.
In these surfacial root environments, conditions are very transient. Liquid water is available only from periodic rain showers. At this time water running down branches, or through leaf litter leaches out very dilute amounts of nutrients (probably from such sources as insect and bird droppings). The orchid roots soak up as much of this water as possible, as fast as possible to get the nutrients before they wash past. The thick roots have an skin, or velamen, that when dry prevents water loss and allows the roots to store the water for use during the dry times. As the plant uses the stored water, the dry roots will shrivel, only to quickly swell up again at the next rain.
Although orchid roots are in a high humidity environment (rain forest tree branch, forest leaf litter etc), they are not in contact with liquid water most of the time. A normal plant has very fine roots that do not store water, and are in continuous contact with liquid water (at a microscopic level). In the soil, bacterial breakdown of organic matter creates low oxygen conditions. Very fine roots with many many root hairs allow for enough uptake of oxygen, even under nearly anoxic conditions, for root survival. Thick orchid roots quickly suffocate under low oxygen conditions, and either die out right, or become infected with pathogenic bacteria or fungi, which can spread to the rest of the plant.
The little tags that often come with orchids say "water weekly". This kills a lot of plants. Why?
As previously stated, an orchid's roots when dry and dehydrated, will absorb water quickly on watering day but any remaining liquid water not absorbed by the roots, or into the pieces of potting mix, will promote bacterial decomposition of the potting mix. This is accompanied by compaction of the medium and the low oxygen conditions that lead to root death and rot.
Orchid roots put out very tiny root hairs that grow into the pieces of potting mix, so the roots appear to hang on to the pieces of bark, charcoal etc. Even though the outside of the individual pieces of the potting mix is dry, the root hairs are able to pull out the absorbed water until the interior of the pieces is dry. Up until the very end of this time, the humidity inside the pot is still very high and is healthy for the roots.
The rate of drying inside a pot is reliant on several factors independent of the plant itself; temperature, humidity, airflow, and pot permeability. The plant itself pulls water out of the potting mix for the process of transpiration. Transpiration rate is dependent on rate of photosynthesis. The plant does not use this water directly, but instead allows it to simply pass out of tiny poors in the leaf surface called stomata. Stomata open to allow carbon dioxide (CO2) into the interior of the leaf where it can then enter moist cells. Within the cells the CO2 is used as the carbon source for sugar molecules (light energy "photo" used to build "synthesize" sugar molecules = photosynthesis). When the stomata are open the evaporation rate of water from the interior cell surfaces is very high. In most cases the number of open stomata is in direct proportion to light level. Higher temperature, and airflow across the leaf increase the speed at which water vapor passes through the stomata.
...So a large plant in a small clay pot full of well established roots in a warm bright green house with fans running all day will dry out a pot much much faster than a small plant that has just been repotted (roots have not grown root hairs into the new medium) and is being kept in a shaded area in your kitchen. The tag that said water weekly didn't tell us this did it?
Different types of pots and potting mixes seek to replicate those conditions found on a branch high in the canopy (namely a short period in contact with liquid water followed by a few days of humid conditions). Plastic and glazed ceramic pots keep the humidity around the roots high longer, while unglazed clay pots allow for faster drying of the mix. Lava rock holds almost no water, bark chunks hold a little, coconut husk products more, and Sphagnum moss holds a lot. What combinations of mix and pot you use must be based on the rate of evaporation and transpiration in your growing area.
Conclusion:
--Most orchids can have dry roots for a few days without root damage, longer if the humidity is higher.
--For most orchids, water when the potting mix has lost all of its free liquid water, and much of its absorbed water. You can judge this by comparing the weight of a fully hydrated pot to a very light pot of completely dry bark. When in doubt, wait a day.
--In general, watering one day too soon (consistently) is much worse than watering two days late.
--In order to water weekly, you have to know your plants and growing conditions, and create a combination of pot and potting mix that will time water loss just right.
--Watering schedule changes with the seasons as temperatures, humidities, and day length changes. Watering every three days in mid summer can change to every three weeks in mid winter.