clothing for tropical rainforest

Well, one reason is that you will perspire heavily in this environment and bodyodors quickly make synthetics smell pretty rank. That being said, I have used polyester/cotten blend shirts and pants in SE Asia that were pretty comfortable, held their shape well, and dried out faster than pure cotten garments. Garments whose fabric has some texture to it, tends to help hold the garment away from the skin for better circulation and comfort. Make sure clothing is not skintight, you'll want them loose—again for comfort.

All that Kevin said.

Sometimes the dogma that you learn about survival needs a little reasoning. In the rainforest and any place where the temperature is nearly always hot (deserts can get very cold) there is little fear of getting hypothermia. Quite the opposite, in fact. The disadvantage of cotton in cold environments is actually a plus in extreme muggy or always hot environments; by holding water cotton cools you down and keeps you from overheating. Nylon and polyester actually make you much hotter and that can be at best uncomfortable, at worst dangerous. Here in Japan many companies are now designing linen/ polyester blends precisely because of linen's properties for cooling you down. The summers in the lower altitudes here are so humid and hot that wearing a purely polyester or nylon fabric often feels like you are sweltering in a sauna. Many outdoor stores don't carry purely wool underwear for this reason, and even when they do they don't sell well. I bought a wool/ polyester blend shirt three years ago, called "sport wool", that has been perfect for the heat. In a place like the rainforest in Ecuador, though, it would be way too warm. Cotton is definitely better. Just look at the locals who live in the rainforest; all of them wear cotton.

Echoing the above, I too highly recommend a cotton/polyester blend. Mine are 85 poly / 15 cotton — where you have most of the benefits of wicking and quick drying, plus the comfy feel and better smell of cotton.

I've worn Duofold tees either alone or as a base layer in all kinds of climates and realy like them. Don't let their low prices fool you. I can wear the same one for five days straight — come back to civilization and have lunch at a restaurant, then off to visit mom — and she didn't notice anything different! YMMV, of course.

Duofold short sleeves – Campmor Item 25950

Duofold long sleeves – Campmor Item 25951

Cotton and cotton blends are definitely more comfortable once conditions are regularly up over the mid 20's (Celsius..), and especially moreso when humidity is well over 50% …
Other natural fibres – linen, hemp or bamboo, can also work well under these conditions..
Adding polyester in a blend may add benefits to wear-and-tear, how it hangs, drying a bit quicker/better 'wicking', but at the same time it will probably be a little warmer (or less cool..), not "breathe" quite as well, and hold more odours. Adding nylon to a blend may also make for a tougher fabric.

Under tropical and sub-tropical conditions I would prefer to wear as little as possible, but that might not always be possible on the more organised trips..